a research lab is to begin experimentation with a bacteria

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Lab Culture: How do Scientists Grow and Study Bacteria?

Bacteria are the microorganisms all around us—on our bodies, in our food, and in the environment. Some bacteria are helpful, but others can cause disease. To learn how bacteria protect or hurt us, researchers usually grow the bacteria in their labs so that they have many of them to study. In this article, you will learn about how we “culture” bacteria in the lab and how different culture methods can affect bacterial behavior. We will tell the story of a new culture system that our research lab developed for studying interactions between different types of bacteria. Finally, you will learn about the clever ways that scientists separate out different types of bacteria, using a method called selective plating. Learning these methods is a fundamental first step for researchers who study how to nurture bacteria that are good for us and fight disease-causing bacteria!

Bacteria Everywhere

Bacteria are the microorganisms all around us—they are on our bodies, in the food we eat, and in the air we breathe. Some bacteria are helpful, but others can cause disease. Scientists want to understand how disease-causing pathogens work so they can help make better treatments for patients. Scientists also want to know how to nurture the good bacteria that keep us healthy. Most researchers study bacteria in a laboratory. Studying bacteria in a speck of soil or from wiping your skin can be difficult [ 1 ]. Those samples only have a limited number of bacteria to study and they generally contain a mixture of many types of bacteria. Growing “farms” of separate types of bacteria in the lab is much easier. Researchers can simply grow a lot of a certain bacteria when they want to do an experiment with them. However, “farming” bacteria does not involve green fields or pig pens! Instead, researchers grow bacteria in many kinds of media .

Oh, The Places You Will Grow (Bacteria)

When you hear the word “media,” you might think of TV, movies, or social media. But when microbiologists say “media,” they are talking about how they grow bacteria. Just like farmers feed their chickens different foods than they feed their fish, different kinds of bacteria also need different nutrients to grow. We can make media with various ingredients or recipes to feed specific types of bacteria. Also, media for growing bacteria can be a liquid or solid ( Figure 1 ). Compare chicken farms and fish farms: chickens live on grass (a solid substrate ) and fish live in the water (a liquid substrate). Researchers make liquid media into a solid by adding an ingredient called agar. Agar is similar to Jello—it dissolves into hot liquids and turns them into a wiggly solid when cooled down. Scientists use liquid or solid cultures for different purposes. Liquid cultures are like a big bacterial soup—everything is well-mixed. Solid cultures let bacteria grow in structured communities, like cities: what a single bacterium eats and how it talks to its neighbors changes based on where it lives [ 2 ].

• Figure 1 - In the lab, bacteria can be grown on several different substrates.
• In liquid cultures, the media is continually shaken to mix the bacteria. In solid cultures, bacteria grow in groups. If you think of solid cultures as a city, you can imagine that the bacteria living in the basement of a building have very different lifestyles than the bacteria living in a penthouse! The differences in how much food and oxygen the bacteria receive affects how they grow and behave in experiments.

How Do Scientists Choose Their Media?

So, how do scientists choose which media to use for their experiments? It depends on what questions they want to answer. Scientists who study pathogens that cause skin diseases might want to grow bacteria on solid substrates to imitate how bacteria grow on the skin. Researchers interested in bacterial genes might not worry about how the bacteria grow—a bacterium’s DNA should stay the same no matter how it is cultured. Scientists also choose how nutritious to make the media. Bacteria in the soil are surrounded by nutrients, but skin pathogens often live in nutrient-limited environments. Deciding which media to use is an important first step that scientists take when designing experiments.

Real-World Experiment: Good vs. Evil Escherichia coli

Let us explore a real experimental design process from research done on the bacterial species Escherichia coli ( E. coli ). You may have heard of this species because E. coli can cause food-borne illness when it grows on beef or lettuce. However, some types of E. coli are actually good for us! These probiotic E. coli can help fight off pathogenic E. coli in the gut. For our research, we wanted to study how a probiotic E. coli strain called Nissle stops another strain of E. coli from surviving drug treatment [ 3 ]. First, we grew a model pathogenic E. coli in low-nutrient, liquid media. The minimal amount of nutrients represents an infection environment. We chose liquid culture so that the drug we added would mix in evenly and reach all bacteria. After a few hours, we removed the drug and measured how many bacteria were still alive. Even with a high drug dose, some of the bacteria always survived. We tried adding probiotic Nissle to kill the drug-treated bacteria—and it worked! Fewer pathogens survived when Nissle was added to the culture.

If we could learn how Nissle works, we could try to make it even better at killing pathogens. So, the important question is: how does Nissle kill the other bacteria? Nissle might directly touch the other bacteria to kill them [ 4 ]. Or, Nissle could send a chemical attack through the liquid to the other bacteria. It would take a lot of time and money to directly watch our bacteria under the microscope to see if they are touching or sending tiny molecules through the media. Instead, we chose to design a new type of culture system to answer this question.

Designing a New Culture System

We call our new system the H-Cell because the two chambers (or “cells”) look like the letter H ( Figures 2A , B ). A bridge with a filter in the middle connects the two chambers. The filter has microscopic openings that are too small for bacteria to go through, but big enough to allow liquid and chemical signals to flow between the chambers. Therefore, the filter keeps the probiotic and the pathogen from directly touching. When we repeated our experiment in the H-Cell, we grew probiotic Nissle in one side and the drug-treated cells in the other ( Figure 2C ). Unlike experiments in a test tube, the Nissle did not kill the drug-treated bacteria in the H-Cell. This result means that Nissle must have physical contact to kill the pathogenic bacteria!

• Figure 2 - (A) In our H-Cell system, there is a filter that divides two chambers.
• The filter keeps bacteria from crossing over but allows chemical signals (small circles) from the bacteria to pass through. (B) This picture of an H-Cell shows the glass chambers and their connecting bridge. The cross-sectional view shows the filter in the middle of the bridge. (C) In our experiment, drug-treated pathogens were cultured in one side of the H-Cell, with probiotic Nissle on the other side.

The H-Cell allows us to ask other interesting questions about bacterial interactions. Unlike the Nissle experiments, not all interactions between bacteria are harmful. When certain bacterial species are together, drugs can be less effective against them. Many researchers are interested in how these bacteria survive as partners [ 5 ]. To study bacterial relationships, researchers need a way to count each species separately to measure how well each type of bacteria survives ( Figure 3 ). One method to do this is called selective plating . We first use one liquid culture to grow all of the species together. Then, we take samples of that multi-species culture and transfer them to various types of solid media, each containing a unique combination of nutrients that “selects” for specific species. The number of bacteria that grow on each type of solid media tells us how many of one species survived vs. the others. This method does not work if the various bacterial species have the same diets. In other words, this method does not work if there is no “selective” nutrient to use to grow one species vs. another. This problem could be solved by using the H-Cell. Researchers can study how bacteria interact because the two species can communicate through the liquid media; but, when it is time to count the survivors, the species are physically separate and can be easily measured.

• Figure 3 - (A) If two bacterial species are cultured together and then transferred onto non-selective agar, it is impossible to know whether the bacteria grew from species A or B, or a mix of both.
• (B) If the two species need different nutrients or are killed by different drugs, we can tell them apart using selective plating. When the bacteria are transferred to agar plates, they will only survive under the conditions that support their growth.

Which System Is Best?

As you can see, there are many ways to grow and study bacteria, but each system has its ups and downs. Test tubes with liquid media are easy to work with, but they are not very similar to human skin, soil, or other places where bacteria normally live. Scientists have built microchips that simulate the environment or human organs, but they can be expensive and tricky to use. No system is perfect, but that is why scientific questions are studied in different ways by multiple researchers! Researchers conduct experiments then share the details of their methods and results with other researchers, so that everyone can compare their systems. Altogether, each experiment helps paint a more complete picture of how bacteria work and how we can stop disease-causing pathogens.

Pathogen : ↑ Bacteria or other microorganisms that cause disease.

Media : ↑ A mix of specific nutrients used for growing bacteria.

Microbiologist : ↑ A scientist that studies microscopic organisms like bacteria, fungi, and viruses.

Substrate : ↑ What an organism grows in/on; examples include agar gels, liquid media, or solid surfaces like plastic.

Culture : ↑ A batch of bacteria grown in the laboratory (noun); or, to grow bacteria in the laboratory (verb).

Probiotic : ↑ A bacterial species that helps promote a healthy balance of bacteria in our bodies.

H-Cell : ↑ A custom glassware co-culture system. The filter between each bacterial population prevents them from physically interacting, but they can communicate to each other by transferring small molecules across the filter.

Selective Plating : ↑ A method for separating out certain bacteria from a mixture of bacterial species.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Original Source Article

↑ Hare, P. J., Englander, H. E., and Mok, W. W. K. 2022. Probiotic Escherichia coli Nissle 1917 inhibits bacterial persisters that survive fluoroquinolone treatment. J. Appl. Microbiol . 132:4020–32. doi: 10.1111/jam.15541

[1] ↑ Fournier, P. E., Drancourt, M., Colson, P., Rolain, J. M., La Scola, B., and Raoult, D. 2013. Modern clinical microbiology: new challenges and solutions. Nat. Rev. Microbiol . 11:574–85. doi: 10.1038/nrmicro3068

[2] ↑ Manner, C., Dias Teixeira, R., Saha, D., Kaczmarczyk, A., Zemp, R., Wyss, F., et al. 2023. A genetic switch controls Pseudomonas aeruginosa surface colonization. Nat. Microbiol . doi: 10.1038/s41564-023-01403-0. [Epub ahead of print].

[3] ↑ Hare, P. J., Englander, H. E., and Mok, W. W. K. 2022. Probiotic Escherichia coli Nissle 1917 inhibits bacterial persisters that survive fluoroquinolone treatment. J. Appl. Microbiol . 132:4020–32. doi: 10.1111/jam.15541

[4] ↑ Sonnenborn, U. 2016. Escherichia coli strain Nissle 1917-from bench to bedside and back: history of a special Escherichia coli strain with probiotic properties. FEMS Microbiol. Lett . 363:fnw212. doi: 10.1093/femsle/fnw212

[5] ↑ Orazi, G., Ruoff, K. L., and O'Toole, G. A. 2019. Pseudomonas aeruginosa increases the sensitivity of biofilm-grown staphylococcus aureus to membrane-targeting antiseptics and antibiotics. mBio . 10:e01501–19. doi: 10.1128/mBio.01501-19

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Science Projects > Chemistry Projects > How To Grow Bacteria and More  

How To Grow Bacteria and More

If learning how to grow bacteria in a petri dish interests you, read on.

How Can Bacteria Help Us? How Can Bacteria Harm Us? What Are Antibacterial Agents? Experiment #1: Cheek Cell Swab Experiment #2: Testing Antibacterial Agents Experiment #3: Soap Survey Experiment #4: Bacteria in the Air Experiment #5: Homemade Yogurt More Experiment Ideas

Bacteria Overview

Bacteria are one-celled, or unicellular, microorganisms . They are different from plant and animal cells because they don’t have a distinct, membrane-enclosed nucleus containing genetic material. Instead, their DNA floats in a tangle inside the cell.

Individual bacteria can only be seen with a microscope, but they reproduce so rapidly that they often form colonies that we can see. Bacteria reproduce when one cell splits into two cells through a process called binary fission. Fission occurs rapidly in as little as 20 minutes. Under perfect conditions a single bacterium could grow into over one billion bacteria in only 10 hours! (It’s a good thing natural conditions are rarely perfect, or the earth would be buried in bacteria!)

Growing and testing bacteria is a fun any-time project or a great science fair project. Bacteria are everywhere, and since they reproduce rapidly they are easy to study with just a few simple materials. All you need are some petri dishes , agar, and sterile swabs or an inoculating needle . Agar is a gelatinous medium that provides nutrients and a stable, controlled environment for bacteria growth . Most bacteria will grow well using nutrient agar , but some more fastidious bacteria (those with more complex nutrient requirements like Bacillus stearothermophilus , Branhamella catarrhalis , and Bacillus coagulans ) prefer tryptic soy agar .

You also need a source for bacteria, and this is not hard to find! You can swab your mouth or skin, pets, soil, or household surfaces like the kitchen sink or toilet bowl. If you want to study a particular type of bacteria, you can also purchase live cultures . Keep reading to see four experiments using bacteria, and many more ideas for science projects (also consider this hands-on Bacteria Growing Kit )! Adult supervision is recommended when working with bacteria.

How Can Bacteria Help Us?

Where would we be without bacteria? Well, we might not be getting bacterial diseases, but we would still be a lot worse off! Bacteria perform all sorts of very important functions, both in our bodies and in the world around us. Here are just a few.

Digestion. Our large intestines are full of beneficial bacteria that break down food that our bodies can’t digest on their own. Once the bacteria break it down, our intestines are able to absorb it, giving us more nutrients from our food.

Vitamins. Bacteria in our intestines actually produce and secrete vitamins that are important for our health! For example, E. coli bacteria in our intestines are a major source of vitamin K. (Most E. coli is good for us, but there is a harmful type that causes food poisoning.)

Food. Bacteria are used to turn milk into yogurt, cheese, and other dairy products.

Oxygen. Cyanobacteria (which used to be called blue-green algae) live in water and perform photosynthesis, which results in the production of much of the oxygen we need to breathe.

Cleanup. Oil spills, sewage, industrial waste — bacteria can help us clean all of these up! They ‘eat’ the oil or toxins and convert them into less harmful substances.

Bacteria are amazing creatures, aren’t they? They can be so dangerous and yet so important at the same time. Keep reading to see an experiment that uses good bacteria!

How Can Bacteria Harm Us?

Some types of bacteria cause disease and sickness. These kinds of bacteria are called pathogens. They reproduce very rapidly, like all bacteria. These come in many forms and can cause illnesses from an ear infection to strep throat to cholera. They can get into our bodies via our mouth and nose, or through cuts and scrapes. Some are airborne, others are found in food, resulting in food poisoning. Bacteria are also the cause of plaque buildup on our teeth, which can lead to cavities and gum disease.

Before the discovery of antibiotics, many severe bacterial diseases had no cure and usually resulted in death. Antibiotics work by destroying bacteria or inhibiting their reproduction while leaving the body’s own cells unharmed. After a time, some bacteria develop resistance to an antibiotic, and it will no longer be effective against them. Because of this, scientists are always researching new antibiotics. (Many diseases, such as chicken pox, hepatitis, or polio, are caused by viruses rather than bacteria. Antibiotics have no effect against these diseases.)

Bacterial infections are common, but many of them can be avoided by good cooking, cleaning, and hand-washing practices.

What Are Antibacterial Agents?

How do people stop bacteria from growing and spreading? They control it in two ways: by killing the bacteria cells, and by stopping the bacteria from reproducing. An agent is a solution or method which either kills or stops reproduction. Bactericides are agents that kill bacteria cells. Static agents inhibit cell growth and reproduction.

There are a variety of ways to kill bacteria or keep it from reproducing.

Physical methods:

• Sterilization. The application of heat to kill bacteria. Includes incineration (burning), boiling, and cooking.
• Pasteurization. The use of mild heat to reduce the number of bacteria in a food.
• Cold temperatures. Refrigeration and freezing are two of the most common methods used in homes, for preserving food’s life span.

Chemical methods:

• Antiseptics. These agents can be applied directly to living tissues, including human skin.
• Disinfectants. These agents are not safe for live tissues. Disinfectants are used to clean toilets, sinks, floors, etc.
• Some food preservatives are: sodium benzoate, monosodium glutamate (MSG), sulfur dioxide, salts, sugar, and wood smoke.
• Amoxycillin and Ampicillin—inhibit steps in cell wall synthesis (building)
• Penicillin—inhibits steps in cell wall synthesis
• Erythromycin—inhibits RNA translation for protein synthesis

SAFETY NOTE

While most environmental bacteria are not harmful to healthy individuals, once concentrated in colonies, they can be hazardous.

To minimize risk, wear disposable gloves while handling bacteria, and thoroughly wash your hands before and after. Never eat or drink during bacteria studies, nor inhale or ingest growing cultures. Work in a draft-free room and reduce airflow as much as possible. Keep petri dishes with cultured mediums closed—preferably taped shut—unless sampling or disinfecting. Even then, remove the petri dish only enough to insert your implement or cover medium with bleach or 70% isopropyl alcohol.

When finished experimenting, seal dishes in a plastic bag and dispose. Cover accidental breaks or spills with bleach or alcohol for 10 minutes, then carefully sweep up, seal in a plastic bag, and discard.

Preparing Culture Dishes

Before you can grow bacteria, you’ll need to prepare sterile culture dishes. A 125ml bottle of nutrient agar contains enough to fill about 10 petri dishes.

Water Bath Method – Loosen the agar bottle cap, but do not remove it completely. Place the bottle in hot water at 170-190 °F until all of the agar is liquid. To prevent the bottle from tipping, keep the water level even with the agar level.

• Let the agar cool to 110-120 °F (when the bottle still feels warm but not too hot to touch) before pouring into petri dishes.
• Slide open the cover of the petri dish just enough to pour agar into the dish. Pour enough agar to cover 1/2 to 2/3 of the bottom of the dish (about 10-13ml). Don’t let the bottle mouth touch the dish. Cover the dish immediately to prevent contamination and tilt it back and forth gently until the agar coats the entire bottom of the dish. (Fill as many dishes as you have agar for: you can store extras upside down until you’re ready to use them.)
• Let the petri dishes stand one hour for the agar to solidify before using them.

Experiment #1: Cheek Cell Swab

Make a culture dish using the instructions above. Once the culture dish is prepared, use a sterile cotton swab or inoculating needle and swab the inside of your cheek. Very gently rub the swab over the agar in a few zigzag strokes and replace the lid on the dish. You’ll need to let the dish sit in a warm area for 3-7 days before bacteria growth appears. Record the growth each day with a drawing and a written description. The individual bacteria are too tiny to see without a high-power microscope, but you can see bacteria colonies. Distinguish between different types of bacteria by the color and shape of the colonies.

Experiment #2: Testing Antibacterial Agents

Preparing Sensitivity Squares

One method for testing the antibacterial effectiveness of a substance is to use ‘sensitivity squares.’ Cut small squares of blotter paper (or other absorbent paper) and then soak them in whatever substance you want to test: iodine, ethyl alcohol, antibacterial soap, antiseptics, garlic, etc. Use clean tweezers to handle the squares so you don’t contaminate them. Label them with permanent ink, soak them in the chosen substance, and blot the excess liquid with a paper towel.

Collecting Bacteria

Decide on a source for collecting bacteria. For using sensitivity squares, make sure there is just one source, and keep each dish as consistent as possible. Sources could include a kitchen sink, bathroom counter, cell phone, or another surface you would like to test. Rub a sterile swab across the chosen surface, and then lightly rub it across the prepared agar dish in a zigzag pattern. Rotate the dish and repeat.

Setting Up an Experiment

Each experiment should have a control dish that shows bacteria growth under normal conditions and one or more test dishes in which you change certain variables and examine the results. Examples of variables to test are temperature or the presence of antiseptics. How do these affect bacteria growth?

• Label one dish ‘Control.’ Then in your test dish, use tweezers to add the sensitivity squares that have been soaked in a substance you wish to test for antibacterial properties. It’s a good idea to add a plain square of blotter paper to see if the paper by itself has any effect on bacteria growth. For best results, use multiple test dishes and control the variables so the conditions are identical for each dish: bacteria collected from the same place, exposed to the same amount of antibacterial substance, stored at the same temperature, etc. The more tests you perform, the more data you will collect, and the more confident you can be about your conclusions.
• Place all the dishes in a dark, room-temperature place like a closet.

Wait 3-7 days and examine the bacteria growth in the dishes, without removing the lids. You will see multiple round dots of growth; these are bacteria colonies. Depending on where you collected your bacteria samples, you may have several types of bacteria (and even some mold!) growing in your dishes. Different types of colonies will have different colors and textures. If you have a compound or stereo microscope, try looking at the colonies up close to see more of the differences.

Compare the amount of bacteria in the control dish to the amount in the test dishes. Next, compare the amount of bacteria growth around each paper square. Which one has bacteria growing closest to it? Which one has the least amount of bacteria growing near it? If you did more than one test dish, are the results similar in all the test dishes? If not, what variables do you think might have caused the results to be different? How does this affect your conclusions?

For a variation on this experiment, test the effect of temperature on bacterial growth instead of using sensitivity squares. Put a control dish at room temperature, and place other dishes in dark areas with different temperatures.

Experiment #3: Soap Survey

Every time you touch something you are probably picking up new bacteria and leaving some behind. This is how many infectious diseases spread — we share our bacteria with everyone around us! Even bacteria that lives safely on our skin can make us sick if it gets inside our bodies through our mouths or cuts and scrapes. This is one reason why it is so important that we wash our hands frequently and well.

What kind of soap works best for cutting down on the bacteria on our hands? You can test this by growing some bacteria cultures using agar and petri dishes.

• Two (or more)  petri dishes
• Sterile swabs
• Blotter paper  or other absorbent paper
• Forceps  or tweezers
• Different kinds of hand cleaners: regular soap, antibacterial soap, dish soap, hand sanitizer

1. Prepare the agar according to the directions on the label, then pour enough to cover the bottom of each petri dish. Cover the dishes and let them stand for about an hour until the agar has solidified again. (If you aren’t going to use them right away after they have cooled, store them upside down in the refrigerator.)

2. When your petri dishes are ready, collect some bacteria from your hand or the hand of a volunteer. (Make sure the person hasn’t washed his or her hands too recently!) Do this by rubbing the sterile swab over the palm in a zigzag pattern.

3. Remove the cover from the petri dish and lightly rub the swab back and forth in a zigzag pattern on the agar. Turn the dish a quarter turn and zigzag again. Cover the dish and repeat steps two and three for the other dish, using a new sterile swab. Label the dishes “Test” and “Control.” (You may want to do more than one test dish, so you can compare the results.)

4. Cut the blotter paper into small “sensitivity squares.” Use permanent ink to label the squares for the different types of hand cleaners you are going to test, e.g., “R” for regular soap, “A” for antibacterial soap, and “S” for hand sanitizer. Using tweezers, dip each square into the appropriate cleaner. Blot the excess cleaner on a paper towel and then place the squares on the agar in the “Test” dish. (Spread the squares out so there is distance between them.) Add one square of plain blotter paper to test if blotter paper by itself has any effect. Don’t put any squares in the “Control” dish – this one will show you what the bacterial growth will look like without any soap.

5. Put the dishes in a dark, room-temperature place like a closet and leave them undisturbed for a few days.

What Happened

The rate of bacteria growth in your dishes will depend on temperature and other factors. Check your cultures after a couple of days, but you’ll probably want to wait 5-7 days before recording your data. You will see multiple round dots of growth; these are bacteria colonies. There may be several types of bacteria growing in the dishes. Different types of colonies will have different colors and textures.

For each soap test, count and record the number of bacteria colonies in each dish. To see how effective each soap was, divide the number of colonies in the test dish by the number of colonies in the control dish, then subtract the result from 1 and write the answer as a percentage. For example, if your control dish had 100 colonies and your soap test dish had 30, the soap eliminated 70% of the bacteria: 1 — (30 ÷ 100) = .7 = 70%

According to your results, which type of soap was the most effective at eliminating bacteria?  Does “antibacterial” soap really work better than regular soap? How well did washing hands in water without soap work? What further tests could you do to determine which soaps and hand washing methods are most effective at eliminating bacteria?

Experiment #4: Bacteria in the Air

You need two culture dishes for this experiment, in which you’ll demonstrate how antibacterial agents (such as antibiotics and household cleaners) affect bacteria growth.

Leave the dishes with their lids off in a room-temperature location. Leave the culture dishes exposed for about an hour.

While you wait, cut small squares of paper (blotter paper works well), label them with the names of the antibacterials you’re going to test (e.g. ‘L’ for Lysol, ‘A’ for alcohol, etc.), and soak each in a different household chemical that you wish to test for antibacterial properties. If you have time, you might also experiment with natural antibacterial agents, such as tea tree oil or red pepper. Wipe off any excess liquid and use tweezers to set each of the squares on a different spot in one of the culture dishes. The second culture dish is your ‘control.’ It will show you what an air bacteria culture looks like without any chemical agents.

Store the dishes (with lids on) in a dark place like a closet where they will be undisturbed for a few days. After 3-7 days, take both culture dishes and carefully observe the bacteria growth in each dish, leaving the lids on. The bacteria will be visible in small, colored clusters. Take notes of your observations and make drawings. You could also answer the following questions. In the control culture, How much of the dish is covered with bacteria? In the sensitivity square test culture, Have the bacteria covered this dish to the same extent as the control culture? What effect have each of the chemicals had on the bacteria growth? Did a particular chemical kill the bacteria or just inhibit its growth?

• For further study you could use an  antibiotic disc set  to see what different antibiotics can do against bacteria.
• For a  more advanced project , learn how gram staining relates to the use of antibiotics.

Experiment #5: Homemade Yogurt

Generally when people think of ‘bacteria,’ they think of harmful germs. However, not all forms of bacteria are bad! You can enjoy a tasty product of good bacteria by making a batch of yogurt at home.

You’ll need to use a starter (available at grocery or health food stores), or else one cup of plain, unflavored yogurt that has live cultures in it. (If it contains live cultures, it will say so on the container.)

Slowly heat four cups of milk until it is hot, but not boiling or scalding. The temperature should be around 95-120 degrees to kill some of the harmful bacteria. Cool slightly, until milk is warm, and then add one cup of active yogurt or the starter.

Put the mixture in a large bowl (or glass jars) and cover. Make sure that the bowl or jars are sterilized before using by either running them through the dishwasher or washing them with very hot water.

There are two different methods for culturing the yogurt mixture: You can put the covered bowl or jars into a clean plastic cooler, and fill the cooler with hot water to just below the top of the culture containers. With this method, you will need to occasionally refill the cooler with hot water, so that the temperature of the yogurt stays consistent. The other method is to wrap the containers in a heating pad and towels, setting the heating pad on low to medium heat.

Check the mixture after heating for 3 1/2 to 4 hours. It should be ‘set up,’ having a smooth, creamy consistency similar to store-bought yogurt. If the mixture is not set up yet, heat it for another 1-2 hours. When it is the right consistency, add some flavoring—such as vanilla extract, chocolate syrup, or berries—and store the yogurt in the refrigerator. It should keep for a couple of weeks. For safety, we suggest that you do not eat any yogurt that has separated or has a non-typical consistency.

More Bacteria Experiment Ideas

Here are some other project ideas for you to try on your own or use as a basis for a bacteria science fair project:

• Mouthwash . Swab your teeth and gums and see how well toothpaste or mouthwash work against the plaque-causing bacteria on your teeth.
• Dog’s mouth : Have you heard people say that dogs’ mouths are cleaner than humans’? Design an experiment to test whether this is really true!
• Band-aid . Some band-aids are advertised as being antibacterial. Test to see if they really work better than regular band-aids at inhibiting bacteria.
• Water bottle . Is it safe to keep refilling a water bottle without washing it? Test a sample of water from the bottom of a water bottle that has been used for a couple days and compare it to a sample from a freshly-opened, clean water bottle. You can also test to see if a bottle gets more bacteria in it if you drink with your mouth or with a straw.
• Shoes . Do bacteria grow in your shoes? Is there a difference in bacteria growth between fabric shoes and leather? Do foot powders work to cut down on bacteria?
• Toothbrush . Do bacteria grow on your toothbrush? What are some ways you could try to keep it clean? Mouthwash? Hot water?
• Makeup . Some people recommend getting new mascara every six weeks because bacteria can grow in the tube. Test this by comparing bacteria growth from old mascara and new, unused mascara. You can also test how much bacteria is on other kinds of makeup.

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Methodology

• Guide to Experimental Design | Overview, Steps, & Examples

Guide to Experimental Design | Overview, 5 steps & Examples

Published on December 3, 2019 by Rebecca Bevans . Revised on June 21, 2023.

Experiments are used to study causal relationships . You manipulate one or more independent variables and measure their effect on one or more dependent variables.

Experimental design create a set of procedures to systematically test a hypothesis . A good experimental design requires a strong understanding of the system you are studying.

There are five key steps in designing an experiment:

• Consider your variables and how they are related
• Write a specific, testable hypothesis
• Design experimental treatments to manipulate your independent variable
• Assign subjects to groups, either between-subjects or within-subjects
• Plan how you will measure your dependent variable

For valid conclusions, you also need to select a representative sample and control any  extraneous variables that might influence your results. If random assignment of participants to control and treatment groups is impossible, unethical, or highly difficult, consider an observational study instead. This minimizes several types of research bias, particularly sampling bias , survivorship bias , and attrition bias as time passes.

Table of contents

Step 1: define your variables, step 2: write your hypothesis, step 3: design your experimental treatments, step 4: assign your subjects to treatment groups, step 5: measure your dependent variable, other interesting articles, frequently asked questions about experiments.

You should begin with a specific research question . We will work with two research question examples, one from health sciences and one from ecology:

To translate your research question into an experimental hypothesis, you need to define the main variables and make predictions about how they are related.

Start by simply listing the independent and dependent variables .

Then you need to think about possible extraneous and confounding variables and consider how you might control  them in your experiment.

Finally, you can put these variables together into a diagram. Use arrows to show the possible relationships between variables and include signs to show the expected direction of the relationships.

Here we predict that increasing temperature will increase soil respiration and decrease soil moisture, while decreasing soil moisture will lead to decreased soil respiration.

Prevent plagiarism. Run a free check.

Now that you have a strong conceptual understanding of the system you are studying, you should be able to write a specific, testable hypothesis that addresses your research question.

The next steps will describe how to design a controlled experiment . In a controlled experiment, you must be able to:

• Systematically and precisely manipulate the independent variable(s).
• Precisely measure the dependent variable(s).
• Control any potential confounding variables.

If your study system doesn’t match these criteria, there are other types of research you can use to answer your research question.

How you manipulate the independent variable can affect the experiment’s external validity – that is, the extent to which the results can be generalized and applied to the broader world.

First, you may need to decide how widely to vary your independent variable.

• just slightly above the natural range for your study region.
• over a wider range of temperatures to mimic future warming.
• over an extreme range that is beyond any possible natural variation.

Second, you may need to choose how finely to vary your independent variable. Sometimes this choice is made for you by your experimental system, but often you will need to decide, and this will affect how much you can infer from your results.

• a categorical variable : either as binary (yes/no) or as levels of a factor (no phone use, low phone use, high phone use).
• a continuous variable (minutes of phone use measured every night).

How you apply your experimental treatments to your test subjects is crucial for obtaining valid and reliable results.

First, you need to consider the study size : how many individuals will be included in the experiment? In general, the more subjects you include, the greater your experiment’s statistical power , which determines how much confidence you can have in your results.

Then you need to randomly assign your subjects to treatment groups . Each group receives a different level of the treatment (e.g. no phone use, low phone use, high phone use).

You should also include a control group , which receives no treatment. The control group tells us what would have happened to your test subjects without any experimental intervention.

When assigning your subjects to groups, there are two main choices you need to make:

• A completely randomized design vs a randomized block design .
• A between-subjects design vs a within-subjects design .

Randomization

An experiment can be completely randomized or randomized within blocks (aka strata):

• In a completely randomized design , every subject is assigned to a treatment group at random.
• In a randomized block design (aka stratified random design), subjects are first grouped according to a characteristic they share, and then randomly assigned to treatments within those groups.

Sometimes randomization isn’t practical or ethical , so researchers create partially-random or even non-random designs. An experimental design where treatments aren’t randomly assigned is called a quasi-experimental design .

Between-subjects vs. within-subjects

In a between-subjects design (also known as an independent measures design or classic ANOVA design), individuals receive only one of the possible levels of an experimental treatment.

In medical or social research, you might also use matched pairs within your between-subjects design to make sure that each treatment group contains the same variety of test subjects in the same proportions.

In a within-subjects design (also known as a repeated measures design), every individual receives each of the experimental treatments consecutively, and their responses to each treatment are measured.

Within-subjects or repeated measures can also refer to an experimental design where an effect emerges over time, and individual responses are measured over time in order to measure this effect as it emerges.

Counterbalancing (randomizing or reversing the order of treatments among subjects) is often used in within-subjects designs to ensure that the order of treatment application doesn’t influence the results of the experiment.

Finally, you need to decide how you’ll collect data on your dependent variable outcomes. You should aim for reliable and valid measurements that minimize research bias or error.

Some variables, like temperature, can be objectively measured with scientific instruments. Others may need to be operationalized to turn them into measurable observations.

• Ask participants to record what time they go to sleep and get up each day.
• Ask participants to wear a sleep tracker.

How precisely you measure your dependent variable also affects the kinds of statistical analysis you can use on your data.

Experiments are always context-dependent, and a good experimental design will take into account all of the unique considerations of your study system to produce information that is both valid and relevant to your research question.

If you want to know more about statistics , methodology , or research bias , make sure to check out some of our other articles with explanations and examples.

• Student’s  t -distribution
• Normal distribution
• Null and Alternative Hypotheses
• Chi square tests
• Confidence interval
• Cluster sampling
• Stratified sampling
• Data cleansing
• Reproducibility vs Replicability
• Peer review
• Likert scale

Research bias

• Implicit bias
• Framing effect
• Cognitive bias
• Placebo effect
• Hawthorne effect
• Hindsight bias
• Affect heuristic

Experimental design means planning a set of procedures to investigate a relationship between variables . To design a controlled experiment, you need:

• A testable hypothesis
• At least one independent variable that can be precisely manipulated
• At least one dependent variable that can be precisely measured

When designing the experiment, you decide:

• How you will manipulate the variable(s)
• How you will control for any potential confounding variables
• How many subjects or samples will be included in the study
• How subjects will be assigned to treatment levels

Experimental design is essential to the internal and external validity of your experiment.

The key difference between observational studies and experimental designs is that a well-done observational study does not influence the responses of participants, while experiments do have some sort of treatment condition applied to at least some participants by random assignment .

A confounding variable , also called a confounder or confounding factor, is a third variable in a study examining a potential cause-and-effect relationship.

A confounding variable is related to both the supposed cause and the supposed effect of the study. It can be difficult to separate the true effect of the independent variable from the effect of the confounding variable.

In your research design , it’s important to identify potential confounding variables and plan how you will reduce their impact.

In a between-subjects design , every participant experiences only one condition, and researchers assess group differences between participants in various conditions.

In a within-subjects design , each participant experiences all conditions, and researchers test the same participants repeatedly for differences between conditions.

The word “between” means that you’re comparing different conditions between groups, while the word “within” means you’re comparing different conditions within the same group.

An experimental group, also known as a treatment group, receives the treatment whose effect researchers wish to study, whereas a control group does not. They should be identical in all other ways.

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SOLUTION: A research laboratory is to begin experimentation with a bacterium that doubles every 3 hours. At the start, there are 200 bacteria. How many bacteria will be present at the end of

Experimentation in Scientific Research: Variables and controls in practice

by Anthony Carpi, Ph.D., Anne E. Egger, Ph.D.

Listen to this reading

Did you know that experimental design was developed more than a thousand years ago by a Middle Eastern scientist who studied light? All of us use a form of experimental research in our day to day lives when we try to find the spot with the best cell phone reception, try out new cooking recipes, and more. Scientific experiments are built on similar principles.

Experimentation is a research method in which one or more variables are consciously manipulated and the outcome or effect of that manipulation on other variables is observed.

Experimental designs often make use of controls that provide a measure of variability within a system and a check for sources of error.

Experimental methods are commonly applied to determine causal relationships or to quantify the magnitude of response of a variable.

Anyone who has used a cellular phone knows that certain situations require a bit of research: If you suddenly find yourself in an area with poor phone reception, you might move a bit to the left or right, walk a few steps forward or back, or even hold the phone over your head to get a better signal. While the actions of a cell phone user might seem obvious, the person seeking cell phone reception is actually performing a scientific experiment: consciously manipulating one component (the location of the cell phone) and observing the effect of that action on another component (the phone's reception). Scientific experiments are obviously a bit more complicated, and generally involve more rigorous use of controls , but they draw on the same type of reasoning that we use in many everyday situations. In fact, the earliest documented scientific experiments were devised to answer a very common everyday question: how vision works.

• A brief history of experimental methods

Figure 1: Alhazen (965-ca.1039) as pictured on an Iraqi 10,000-dinar note

One of the first ideas regarding how human vision works came from the Greek philosopher Empedocles around 450 BCE . Empedocles reasoned that the Greek goddess Aphrodite had lit a fire in the human eye, and vision was possible because light rays from this fire emanated from the eye, illuminating objects around us. While a number of people challenged this proposal, the idea that light radiated from the human eye proved surprisingly persistent until around 1,000 CE , when a Middle Eastern scientist advanced our knowledge of the nature of light and, in so doing, developed a new and more rigorous approach to scientific research . Abū 'Alī al-Hasan ibn al-Hasan ibn al-Haytham, also known as Alhazen , was born in 965 CE in the Arabian city of Basra in what is present-day Iraq. He began his scientific studies in physics, mathematics, and other sciences after reading the works of several Greek philosophers. One of Alhazen's most significant contributions was a seven-volume work on optics titled Kitab al-Manazir (later translated to Latin as Opticae Thesaurus Alhazeni – Alhazen's Book of Optics ). Beyond the contributions this book made to the field of optics, it was a remarkable work in that it based conclusions on experimental evidence rather than abstract reasoning – the first major publication to do so. Alhazen's contributions have proved so significant that his likeness was immortalized on the 2003 10,000-dinar note issued by Iraq (Figure 1).

Alhazen invested significant time studying light , color, shadows, rainbows, and other optical phenomena. Among this work was a study in which he stood in a darkened room with a small hole in one wall. Outside of the room, he hung two lanterns at different heights. Alhazen observed that the light from each lantern illuminated a different spot in the room, and each lighted spot formed a direct line with the hole and one of the lanterns outside the room. He also found that covering a lantern caused the spot it illuminated to darken, and exposing the lantern caused the spot to reappear. Thus, Alhazen provided some of the first experimental evidence that light does not emanate from the human eye but rather is emitted by certain objects (like lanterns) and travels from these objects in straight lines. Alhazen's experiment may seem simplistic today, but his methodology was groundbreaking: He developed a hypothesis based on observations of physical relationships (that light comes from objects), and then designed an experiment to test that hypothesis. Despite the simplicity of the method , Alhazen's experiment was a critical step in refuting the long-standing theory that light emanated from the human eye, and it was a major event in the development of modern scientific research methodology.

Comprehension Checkpoint

• Experimentation as a scientific research method

Experimentation is one scientific research method , perhaps the most recognizable, in a spectrum of methods that also includes description, comparison, and modeling (see our Description , Comparison , and Modeling modules). While all of these methods share in common a scientific approach, experimentation is unique in that it involves the conscious manipulation of certain aspects of a real system and the observation of the effects of that manipulation. You could solve a cell phone reception problem by walking around a neighborhood until you see a cell phone tower, observing other cell phone users to see where those people who get the best reception are standing, or looking on the web for a map of cell phone signal coverage. All of these methods could also provide answers, but by moving around and testing reception yourself, you are experimenting.

• Variables: Independent and dependent

In the experimental method , a condition or a parameter , generally referred to as a variable , is consciously manipulated (often referred to as a treatment) and the outcome or effect of that manipulation is observed on other variables. Variables are given different names depending on whether they are the ones manipulated or the ones observed:

• Independent variable refers to a condition within an experiment that is manipulated by the scientist.
• Dependent variable refers to an event or outcome of an experiment that might be affected by the manipulation of the independent variable .

Scientific experimentation helps to determine the nature of the relationship between independent and dependent variables . While it is often difficult, or sometimes impossible, to manipulate a single variable in an experiment , scientists often work to minimize the number of variables being manipulated. For example, as we move from one location to another to get better cell reception, we likely change the orientation of our body, perhaps from south-facing to east-facing, or we hold the cell phone at a different angle. Which variable affected reception: location, orientation, or angle of the phone? It is critical that scientists understand which aspects of their experiment they are manipulating so that they can accurately determine the impacts of that manipulation . In order to constrain the possible outcomes of an experimental procedure, most scientific experiments use a system of controls .

• Controls: Negative, positive, and placebos

In a controlled study, a scientist essentially runs two (or more) parallel and simultaneous experiments: a treatment group, in which the effect of an experimental manipulation is observed on a dependent variable , and a control group, which uses all of the same conditions as the first with the exception of the actual treatment. Controls can fall into one of two groups: negative controls and positive controls .

In a negative control , the control group is exposed to all of the experimental conditions except for the actual treatment . The need to match all experimental conditions exactly is so great that, for example, in a trial for a new drug, the negative control group will be given a pill or liquid that looks exactly like the drug, except that it will not contain the drug itself, a control often referred to as a placebo . Negative controls allow scientists to measure the natural variability of the dependent variable(s), provide a means of measuring error in the experiment , and also provide a baseline to measure against the experimental treatment.

Some experimental designs also make use of positive controls . A positive control is run as a parallel experiment and generally involves the use of an alternative treatment that the researcher knows will have an effect on the dependent variable . For example, when testing the effectiveness of a new drug for pain relief, a scientist might administer treatment placebo to one group of patients as a negative control , and a known treatment like aspirin to a separate group of individuals as a positive control since the pain-relieving aspects of aspirin are well documented. In both cases, the controls allow scientists to quantify background variability and reject alternative hypotheses that might otherwise explain the effect of the treatment on the dependent variable .

• Experimentation in practice: The case of Louis Pasteur

Well-controlled experiments generally provide strong evidence of causality, demonstrating whether the manipulation of one variable causes a response in another variable. For example, as early as the 6th century BCE , Anaximander , a Greek philosopher, speculated that life could be formed from a mixture of sea water, mud, and sunlight. The idea probably stemmed from the observation of worms, mosquitoes, and other insects "magically" appearing in mudflats and other shallow areas. While the suggestion was challenged on a number of occasions, the idea that living microorganisms could be spontaneously generated from air persisted until the middle of the 18 th century.

In the 1750s, John Needham, a Scottish clergyman and naturalist, claimed to have proved that spontaneous generation does occur when he showed that microorganisms flourished in certain foods such as soup broth, even after they had been briefly boiled and covered. Several years later, the Italian abbot and biologist Lazzaro Spallanzani , boiled soup broth for over an hour and then placed bowls of this soup in different conditions, sealing some and leaving others exposed to air. Spallanzani found that microorganisms grew in the soup exposed to air but were absent from the sealed soup. He therefore challenged Needham's conclusions and hypothesized that microorganisms suspended in air settled onto the exposed soup but not the sealed soup, and rejected the idea of spontaneous generation .

Needham countered, arguing that the growth of bacteria in the soup was not due to microbes settling onto the soup from the air, but rather because spontaneous generation required contact with an intangible "life force" in the air itself. He proposed that Spallanzani's extensive boiling destroyed the "life force" present in the soup, preventing spontaneous generation in the sealed bowls but allowing air to replenish the life force in the open bowls. For several decades, scientists continued to debate the spontaneous generation theory of life, with support for the theory coming from several notable scientists including Félix Pouchet and Henry Bastion. Pouchet, Director of the Rouen Museum of Natural History in France, and Bastion, a well-known British bacteriologist, argued that living organisms could spontaneously arise from chemical processes such as fermentation and putrefaction. The debate became so heated that in 1860, the French Academy of Sciences established the Alhumbert prize of 2,500 francs to the first person who could conclusively resolve the conflict. In 1864, Louis Pasteur achieved that result with a series of well-controlled experiments and in doing so claimed the Alhumbert prize.

Pasteur prepared for his experiments by studying the work of others that came before him. In fact, in April 1861 Pasteur wrote to Pouchet to obtain a research description that Pouchet had published. In this letter, Pasteur writes:

Paris, April 3, 1861 Dear Colleague, The difference of our opinions on the famous question of spontaneous generation does not prevent me from esteeming highly your labor and praiseworthy efforts... The sincerity of these sentiments...permits me to have recourse to your obligingness in full confidence. I read with great care everything that you write on the subject that occupies both of us. Now, I cannot obtain a brochure that I understand you have just published.... I would be happy to have a copy of it because I am at present editing the totality of my observations, where naturally I criticize your assertions. L. Pasteur (Porter, 1961)

Pasteur received the brochure from Pouchet several days later and went on to conduct his own experiments . In these, he repeated Spallanzani's method of boiling soup broth, but he divided the broth into portions and exposed these portions to different controlled conditions. Some broth was placed in flasks that had straight necks that were open to the air, some broth was placed in sealed flasks that were not open to the air, and some broth was placed into a specially designed set of swan-necked flasks, in which the broth would be open to the air but the air would have to travel a curved path before reaching the broth, thus preventing anything that might be present in the air from simply settling onto the soup (Figure 2). Pasteur then observed the response of the dependent variable (the growth of microorganisms) in response to the independent variable (the design of the flask). Pasteur's experiments contained both positive controls (samples in the straight-necked flasks that he knew would become contaminated with microorganisms) and negative controls (samples in the sealed flasks that he knew would remain sterile). If spontaneous generation did indeed occur upon exposure to air, Pasteur hypothesized, microorganisms would be found in both the swan-neck flasks and the straight-necked flasks, but not in the sealed flasks. Instead, Pasteur found that microorganisms appeared in the straight-necked flasks, but not in the sealed flasks or the swan-necked flasks.

Figure 2: Pasteur's drawings of the flasks he used (Pasteur, 1861). Fig. 25 D, C, and B (top) show various sealed flasks (negative controls); Fig. 26 (bottom right) illustrates a straight-necked flask directly open to the atmosphere (positive control); and Fig. 25 A (bottom left) illustrates the specially designed swan-necked flask (treatment group).

By using controls and replicating his experiment (he used more than one of each type of flask), Pasteur was able to answer many of the questions that still surrounded the issue of spontaneous generation. Pasteur said of his experimental design, "I affirm with the most perfect sincerity that I have never had a single experiment, arranged as I have just explained, which gave me a doubtful result" (Porter, 1961). Pasteur's work helped refute the theory of spontaneous generation – his experiments showed that air alone was not the cause of bacterial growth in the flask, and his research supported the hypothesis that live microorganisms suspended in air could settle onto the broth in open-necked flasks via gravity .

• Experimentation across disciplines

Experiments are used across all scientific disciplines to investigate a multitude of questions. In some cases, scientific experiments are used for exploratory purposes in which the scientist does not know what the dependent variable is. In this type of experiment, the scientist will manipulate an independent variable and observe what the effect of the manipulation is in order to identify a dependent variable (or variables). Exploratory experiments are sometimes used in nutritional biology when scientists probe the function and purpose of dietary nutrients . In one approach, a scientist will expose one group of animals to a normal diet, and a second group to a similar diet except that it is lacking a specific vitamin or nutrient. The researcher will then observe the two groups to see what specific physiological changes or medical problems arise in the group lacking the nutrient being studied.

Scientific experiments are also commonly used to quantify the magnitude of a relationship between two or more variables . For example, in the fields of pharmacology and toxicology, scientific experiments are used to determine the dose-response relationship of a new drug or chemical. In these approaches, researchers perform a series of experiments in which a population of organisms , such as laboratory mice, is separated into groups and each group is exposed to a different amount of the drug or chemical of interest. The analysis of the data that result from these experiments (see our Data Analysis and Interpretation module) involves comparing the degree of the organism's response to the dose of the substance administered.

In this context, experiments can provide additional evidence to complement other research methods . For example, in the 1950s a great debate ensued over whether or not the chemicals in cigarette smoke cause cancer. Several researchers had conducted comparative studies (see our Comparison in Scientific Research module) that indicated that patients who smoked had a higher probability of developing lung cancer when compared to nonsmokers. Comparative studies differ slightly from experimental methods in that you do not consciously manipulate a variable ; rather you observe differences between two or more groups depending on whether or not they fall into a treatment or control group. Cigarette companies and lobbyists criticized these studies, suggesting that the relationship between smoking and lung cancer was coincidental. Several researchers noted the need for a clear dose-response study; however, the difficulties in getting cigarette smoke into the lungs of laboratory animals prevented this research. In the mid-1950s, Ernest Wynder and colleagues had an ingenious idea: They condensed the chemicals from cigarette smoke into a liquid and applied this in various doses to the skin of groups of mice. The researchers published data from a dose-response experiment of the effect of tobacco smoke condensate on mice (Wynder et al., 1957).

As seen in Figure 3, the researchers found a positive relationship between the amount of condensate applied to the skin of mice and the number of cancers that developed. The graph shows the results of a study in which different groups of mice were exposed to increasing amounts of cigarette tar. The black dots indicate the percentage of each sample group of mice that developed cancer for a given amount cigarette smoke "condensate" applied to their skin. The vertical lines are error bars, showing the amount of uncertainty . The graph shows generally increasing cancer rates with greater exposure. This study was one of the first pieces of experimental evidence in the cigarette smoking debate , and it helped strengthen the case for cigarette smoke as the causative agent in lung cancer in smokers.

Figure 3: Percentage of mice with cancer versus the amount cigarette smoke "condensate" applied to their skin (source: Wynder et al., 1957).

Sometimes experimental approaches and other research methods are not clearly distinct, or scientists may even use multiple research approaches in combination. For example, at 1:52 a.m. EDT on July 4, 2005, scientists with the National Aeronautics and Space Administration (NASA) conducted a study in which a 370 kg spacecraft named Deep Impact was purposely slammed into passing comet Tempel 1. A nearby spacecraft observed the impact and radioed data back to Earth. The research was partially descriptive in that it documented the chemical composition of the comet, but it was also partly experimental in that the effect of slamming the Deep Impact probe into the comet on the volatilization of previously undetected compounds , such as water, was assessed (A'Hearn et al., 2005). It is particularly common that experimentation and description overlap: Another example is Jane Goodall 's research on the behavior of chimpanzees, which can be read in our Description in Scientific Research module.

• Limitations of experimental methods

Figure 4: An image of comet Tempel 1 67 seconds after collision with the Deep Impact impactor. Image credit: NASA/JPL-Caltech/UMD http://deepimpact.umd.edu/gallery/HRI_937_1.html

While scientific experiments provide invaluable data regarding causal relationships, they do have limitations. One criticism of experiments is that they do not necessarily represent real-world situations. In order to clearly identify the relationship between an independent variable and a dependent variable , experiments are designed so that many other contributing variables are fixed or eliminated. For example, in an experiment designed to quantify the effect of vitamin A dose on the metabolism of beta-carotene in humans, Shawna Lemke and colleagues had to precisely control the diet of their human volunteers (Lemke, Dueker et al. 2003). They asked their participants to limit their intake of foods rich in vitamin A and further asked that they maintain a precise log of all foods eaten for 1 week prior to their study. At the time of their study, they controlled their participants' diet by feeding them all the same meals, described in the methods section of their research article in this way:

Meals were controlled for time and content on the dose administration day. Lunch was served at 5.5 h postdosing and consisted of a frozen dinner (Enchiladas, Amy's Kitchen, Petaluma, CA), a blueberry bagel with jelly, 1 apple and 1 banana, and a large chocolate chunk cookie (Pepperidge Farm). Dinner was served 10.5 h post dose and consisted of a frozen dinner (Chinese Stir Fry, Amy's Kitchen) plus the bagel and fruit taken for lunch.

While this is an important aspect of making an experiment manageable and informative, it is often not representative of the real world, in which many variables may change at once, including the foods you eat. Still, experimental research is an excellent way of determining relationships between variables that can be later validated in real world settings through descriptive or comparative studies.

Design is critical to the success or failure of an experiment . Slight variations in the experimental set-up could strongly affect the outcome being measured. For example, during the 1950s, a number of experiments were conducted to evaluate the toxicity in mammals of the metal molybdenum, using rats as experimental subjects . Unexpectedly, these experiments seemed to indicate that the type of cage the rats were housed in affected the toxicity of molybdenum. In response, G. Brinkman and Russell Miller set up an experiment to investigate this observation (Brinkman & Miller, 1961). Brinkman and Miller fed two groups of rats a normal diet that was supplemented with 200 parts per million (ppm) of molybdenum. One group of rats was housed in galvanized steel (steel coated with zinc to reduce corrosion) cages and the second group was housed in stainless steel cages. Rats housed in the galvanized steel cages suffered more from molybdenum toxicity than the other group: They had higher concentrations of molybdenum in their livers and lower blood hemoglobin levels. It was then shown that when the rats chewed on their cages, those housed in the galvanized metal cages absorbed zinc plated onto the metal bars, and zinc is now known to affect the toxicity of molybdenum. In order to control for zinc exposure, then, stainless steel cages needed to be used for all rats.

Scientists also have an obligation to adhere to ethical limits in designing and conducting experiments . During World War II, doctors working in Nazi Germany conducted many heinous experiments using human subjects . Among them was an experiment meant to identify effective treatments for hypothermia in humans, in which concentration camp prisoners were forced to sit in ice water or left naked outdoors in freezing temperatures and then re-warmed by various means. Many of the exposed victims froze to death or suffered permanent injuries. As a result of the Nazi experiments and other unethical research , strict scientific ethical standards have been adopted by the United States and other governments, and by the scientific community at large. Among other things, ethical standards (see our Scientific Ethics module) require that the benefits of research outweigh the risks to human subjects, and those who participate do so voluntarily and only after they have been made fully aware of all the risks posed by the research. These guidelines have far-reaching effects: While the clearest indication of causation in the cigarette smoke and lung cancer debate would have been to design an experiment in which one group of people was asked to take up smoking and another group was asked to refrain from smoking, it would be highly unethical for a scientist to purposefully expose a group of healthy people to a suspected cancer causing agent. As an alternative, comparative studies (see our Comparison in Scientific Research module) were initiated in humans, and experimental studies focused on animal subjects. The combination of these and other studies provided even stronger evidence of the link between smoking and lung cancer than either one method alone would have.

• Experimentation in modern practice

Like all scientific research , the results of experiments are shared with the scientific community, are built upon, and inspire additional experiments and research. For example, once Alhazen established that light given off by objects enters the human eye, the natural question that was asked was "What is the nature of light that enters the human eye?" Two common theories about the nature of light were debated for many years. Sir Isaac Newton was among the principal proponents of a theory suggesting that light was made of small particles . The English naturalist Robert Hooke (who held the interesting title of Curator of Experiments at the Royal Society of London) supported a different theory stating that light was a type of wave, like sound waves . In 1801, Thomas Young conducted a now classic scientific experiment that helped resolve this controversy . Young, like Alhazen, worked in a darkened room and allowed light to enter only through a small hole in a window shade (Figure 5). Young refocused the beam of light with mirrors and split the beam with a paper-thin card. The split light beams were then projected onto a screen, and formed an alternating light and dark banding pattern – that was a sign that light was indeed a wave (see our Light I: Particle or Wave? module).

Figure 5: Young's split-light beam experiment helped clarify the wave nature of light.

Approximately 100 years later, in 1905, new experiments led Albert Einstein to conclude that light exhibits properties of both waves and particles . Einstein's dual wave-particle theory is now generally accepted by scientists.

Experiments continue to help refine our understanding of light even today. In addition to his wave-particle theory , Einstein also proposed that the speed of light was unchanging and absolute. Yet in 1998 a group of scientists led by Lene Hau showed that light could be slowed from its normal speed of 3 x 10 8 meters per second to a mere 17 meters per second with a special experimental apparatus (Hau et al., 1999). The series of experiments that began with Alhazen 's work 1000 years ago has led to a progressively deeper understanding of the nature of light. Although the tools with which scientists conduct experiments may have become more complex, the principles behind controlled experiments are remarkably similar to those used by Pasteur and Alhazen hundreds of years ago.

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Lactic Acid Bacteria

Methods and Protocols

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About this book

This updated volume presents experimentation-based approaches to lactic acid bacteria (LAB) research. Split into three parts, the book explores techniques for analyzing lactic acid bacteria metabolism and characteristics, applications for food-related industries, such as yogurt production, beer, and wine making, and functions of LAB in human health. Written for the highly successful Methods in Molecular Biology series, chapters include introduction to their respective topic, lists of the necessary materials and reagents, step-by-step and readily reproducible laboratory protocols, and tips on troubleshooting and avoiding known pitfalls.

Authoritative and up-to-date, Lactic Acid Bacteria: Methods and Protocols, Second Edition serves as an ideal guide for improving research into this vital area of nutrition and health science.

• Food production
• Bacterial metabolism
• Immunoglobulin immunobiotics

Table of contents (19 protocols)

Front matter, metabolism of lactic acid bacteria, isolation and identification of lactic acid bacteria from environmental samples.

• Akihito Endo, Yasuhiro Tanizawa, Shintaro Maeno, Masanori Arita

Introduction of Spontaneous Mutations Using Streptomycin as a Method for Lactic Acid Bacteria Breeding

• Fu Namai, Keita Nishiyama, Haruki Kitazawa, Takeshi Shimosato

The Chromosomal Gene Manipulation Method for Lactobacillus delbrueckii subsp. bulgaricus Using a Conjugative Shuttle Vector pGMβ1

• Hiromu Kudo, Yasuko Sasaki

Preparation and Structural Analysis of Lipoteichoic Acid on Cell Membranes Derived from Lactic Acid Bacteria

• Tsukasa Shiraishi, Shin-ichi Yokota

Applications of Lactic Acid Bacteria in Food Industries

Yogurt production.

• Takefumi Ichimura

Applications of the Third-Generation DNA Sequencing Technology to the Identification of Spoilage Microorganisms in the Brewing Industry

• Yohanes Novi Kurniawan, Yuji Shinohara, Koji Suzuki

Assay Analysis of Tannase from Lactobacillus plantarum

Makoto Kanauchi

Isolation of Lactic Acid Bacteria Eliminating Trimethylamine (TMA) for Application to Fishery Processing

• Satoshi Mouri, Makoto Kanauchi

Hydroxylation of Fatty Acids by Lactic Acid Bacteria

Assaying d-alanine racemase in lactic acid bacteria using nadh oxidoreduction enzymic system, assaying d-amino acid in japanese sake using l-amino acid derivatizing agent.

• Hinako Kato, Makoto Kanauchi

Healthy Functions of Lactic Acid Bacteria

Inhibition of advanced glycation end products (ages) by fermented foods using lactic acid bacteria.

• Yuki Nakashima, Hideki Kinoshita

Chemical Dephosphorylation of Phosphorylated Polysaccharide

• Junko Nishimura

Lactic Acid Bacteria in the Human Oral Cavity: Assessing Metabolic Functions Relevant to Oral Health and Disease

• Jumpei Washio, Yuki Abiko, Takuichi Sato, Nobuhiro Takahashi

Biosorption of Histamine by Lactic Acid Bacteria for Detoxification

• Tomoyuki Hibi, Hideki Kinoshita

Eliminating Lipopolysaccharide (LPS) Using Lactic Acid Bacteria (LAB)

Editors and affiliations, bibliographic information.

Book Title : Lactic Acid Bacteria

Book Subtitle : Methods and Protocols

Editors : Makoto Kanauchi

Series Title : Methods in Molecular Biology

DOI : https://doi.org/10.1007/978-1-0716-4096-8

Publisher : Humana New York, NY

eBook Packages : Springer Protocols

Copyright Information : The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature 2024

Hardcover ISBN : 978-1-0716-4095-1 Published: 31 August 2024

Softcover ISBN : 978-1-0716-4098-2 Due: 14 September 2025

eBook ISBN : 978-1-0716-4096-8 Published: 30 August 2024

Series ISSN : 1064-3745

Series E-ISSN : 1940-6029

Edition Number : 2

Number of Pages : X, 228

Number of Illustrations : 66 b/w illustrations, 25 illustrations in colour

Topics : Food Science , Microbiology

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Snapsolve any problem by taking a picture. Try it in the Numerade app?

The optimal value is 18.

The core claim of the question is to determine the optimal value of the given linear programming problem.

The optimal value of the linear programming problem can be found by solving the system of inequalities and maximizing or minimizing the objective function.

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• High School

a research lab is to begin experiments with a bacteria that doubles every 4 hours. the lab starts with 200 bacteria. a. how many bacteria will be present at the end of 12th hour? b. how many bacteria will be present at the end of one day? please answer this​

Hazelpescador is waiting for your help., expert-verified answer.

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A) 1600 bacteria

B) 12800 bacteria

Step-by-step explanation:

We are told that the bacteria doubles every 4 hours.

Now, if the amount of bacteria is X, it means that for 4 hours, it becomes 2X and after 8 hours it becomes 2(2X)

For 12 hours, it becomes; 2 × (2(2X))

This means that for 12 hours, it becomes 2 × (2(2X)) = 8X

A) Since the lab starts with 200 bacteria, after 12 hours, it has;

N_12 = 8 × 200.

N_12 = 1600 bacteria

B) From above, we have seen that after t hours, where t is a multiple of 4, the number of bacteria is;

N_t = [2^(¼t - 1)] × (2X)

Where X is initial amount of bacteria.

Thus, after a day which is 24 hours, we have;

N_24 = 2^(¼(24) - 1) × 2X

N_24 = 2^(5) × 2X

Where X is 200, N_24 = 32 × 2(200) = 12800 bacteria

Still have questions?

Get more answers for free, you might be interested in, new questions in mathematics.

What is a research lab and how to start a career in one?

Understand the types of research labs, their main characteristics and get smart tips on how to become a lab researcher.

Research laboratories, or “ labs ” for the intimates, are spaces indicated to execute experimental tasks which may aim for new discoveries and advances in science . They are also used to perform quality control and optimization of processes prior to industrial implementation.

There are many laboratory types and areas. Depending on both the objective and needs of the research, each lab is supplied according to the sort of research to be performed, including equipment and environment control, such as light, temperature and pressure.

This article will take you through the types and main characteristics of research labs and provide you some insights on how to start your career in a research lab.

What do Research Labs do?

As the name says: research. And that means lots and lots of experimentation about diseases, cancers, and other factors that impact human or animal health.

Even before the term “science” was used by mankind, the need for experimentation already existed. Around the 5th century, the famous Greek philosopher and mathematician, Pythagoras de Samos, supposedly managed the oldest known laboratory in history. In it, Pythagoras headed studies about different instruments and objects’ sonority, drawing conclusions known today as frequencies.

Do as Pythagoras and start drawing your science

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Types of Research Labs

We can divide them according to their objectives and characteristics . It’s important to emphasize that even labs that share the same field of knowledge or specialization may have subtle but necessary differences between them. Take a look at these types of research labs:

1. Quality Control Labs

Quality Control labs are mostly used to run tests in which both components and objects of study are crucial to the analysis. This type of laboratory is often associated with chemical practices, physics or biological sciences, such as microbiology .

2. Biosafety Labs

In biological research, scientists often deal with pathogens that could represent a serious risk to public health outside of the laboratory environment, like viruses and bacteria. These labs are classified into 4 levels of biosecurity where level 1 represents the lowest and is designated for organisms of little danger, such as Saccharomyces cerevisiae.

In contrast, level 4 is where scientists study the effect of biological agents that are very harmful to individual life and with high spreading skills, like the Ebola virus.

 3. Clinical Labs

Clinical laboratories are those dedicated to the analysis of various biological samples, such as blood and urine. Also known as medical laboratories, they are essential to assist in the diagnosis, treatment and prevention of certain diseases. In such places, science is applied to improve the quality of treatment for patients, not necessarily to develop scientific knowledge.

4. Production Labs

Production Laboratories are fundamental to assure the perfect transition from research to industrial production, whereas some processes may not work well when transitioning from small to large scale, and vice versa.

Normally, the main objective of this kind of lab is the study and design of a process that works well in different technologies. Production labs are very common in industries such as biotechnology, technology and pharmaceuticals, for example.

5. Research and university Labs

Research and university laboratories focus on either science or humanities. The role of the professionals in such labs is to work alongside post-doctorates and principal investigators. It’s not unusual to see university laboratories turning research and teaching labs into places where students can practice and test their knowledge.

Solution for Labs visual creations

Talking about Labs, Mind the Graph’s Teams & Labs subscription is an awesome solution for those who like to co-create. Besides unlimited start-from templates and science illustrations , subscribers can also share creations with up to 10 simultaneous users. 

But if you haven’t started your career yet, a Researcher subscription might be better to start creating visually appealing infographics and attract attention to your science paper.

How to start a career in a research lab?

If you are interested in becoming a Lab Researcher, follow the steps below:

1. Pursue higher education

Your first objective is to gain the credentials needed to pursue your career goals. It’ll depend on the kind of Lab Researcher you want to become, but most careers start with a bachelor’s degree in the selected field of study. 

2. Gain relevant experience

After or while completing your degree program, consider finding opportunities to gain relevant work experience. Volunteer opportunities are a great gateway.

Another option is to pursue an internship during your degree program or after completing your education. Internships give you the chance to work under the supervision of an experienced professional, which could grant you much knowledge and recognition.

3. If required, obtain a license

Some countries require medical lab researchers to have licensure before they can practice. Conquering it means that you’ve reached a high standard of professional qualifications for performing your work.

If you plan to earn licensure, you’ll need a certain quantity of practical training hours and, in some cases, pass an exam. But that should be easy after years and years dedicated to scientific discoveries.

In or out of a Research Lab, communicating science visually is essential

Since humans are visual creatures, counting on the support of infographics is a great start for reaching a wider audience. Make your science greater with Mind the Graph . According to Cactus Communication studies, articles with graphical abstracts have 3x more downloads in comparison with those without it.

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