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What conditions are favorable for bacteria growth or control?

What conditions are favorable for bacteria growth or control?

Introduction: (Initial Observation)


If we don’t brush our teeth, next day we will have a bad breath. In our mouth, bacteria will find food, water and warm environment that they need to live and reproduce. By learning about the favorable conditions for bacteria growth and control, we will be able to keep ourselves safe from invading harmful bacteria. In this project we will investigate on favorable conditions for bacteria growth.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Information Gathering:

Find out about bacteria and the environment that they grow. Study about different types of bacteria. Not all bacteria are bad and harmful. Read books, magazines or ask professionals who might know in order to learn about good bacteria and bad bacteria or pathogen. Keep track of where you got your information from.

Search the Internet for “Good Bacteria” to learn about helpful and beneficial bacteria. For many years humans have used these good bacteria, such a lactobacillus, to ferment and preserve milk and plant product. Good bacteria in our body digest our food and fight with bad bacteria.

Search the Internet for “Bad Bacteria”. Contamination of food and drinking water by ‘bad’ bacteria can result in serious illness and even death, especially in children and senior citizens. Examples of bad bacteria, include strains of E. coli, Salmonella and Campylobacter species.

Finally search for “bacteria growth” or “bacteria growth conditions”.

In order to grow, a bacterium must have an energy source, a source of carbon and other required nutrients, and a permissive range of physical conditions such as food, temperature, and pH. Sometimes bacteria are referred to as individuals or groups based on their patterns of growth under various chemical (nutritional) or physical conditions. For example, phototrophs are organisms that use light as an energy source; anaerobes are organisms that grow without oxygen; thermophiles are organisms that grow at high temperatures.

In the laboratory, bacteria are grown in culture media which are designed to provide all the essential nutrients in solution for bacterial growth. Picture on the right, shows a petri-dish with agar culture media and bacteria colonies on that.

Question/ Purpose:

The purpose of this project to find out under what conditions do the bacteria grow best.

Identify Variables:

Variables that may affect the growth of bacteria are food or nutrients, light, temperature, PH, Moisture and other possible environmental variables. In this project we will investigate on the effect of Food, light, temperature and PH on bacteria growth.


Since bacteria are live creatures and exist in the nature, a neutral PH (7) is possibly the best PH for bacteria growth. Also since they can easily live and reproduce in human body, warm temperature or body temperature must be the good temperature for bacteria growth. Light does not exist inside our body, where bacteria grow, so light may have a negative affect on bacteria growth. Finally food is needed for bacteria growth as it is needed for any other organism.

Experiment Design:

In order to test the effect of food, light, PH and temperature (Or any other possible factors) in bacteria growth, we must make some preparations as described here:

    1. We need a nutritious culture media, where bacteria can grow. In laboratories agar based culture media is being used. Varieties of culture media are available from laboratory suppliers. These culture media consists of agar (A gelatinous material derived from certain marine algae) and some nutrients such as sugar, minerals and more. We may as well invent our own culture media. Fresh milk for example is very nutritious and bacteria may easily grow on that. Also gelatin that can almost be found in every supermarket may be used as a culture media. You may need to add some more nutrients to that to get the best result.
    2. We need some bacteria to grow. Fortunately bacteria can be found everywhere. On your hand, in your mouth, on the door handle and many other places you may find samples of bacteria to grow. Just use a clean Q-tip to collect them and transfer them to your culture media.
    3. We need an incubator. Incubator is a warm cabinet with adjustable temperature. A few empty cardboard box can be used with similar results. You can simply place each box in a different place in your house with different temperature. You may use a desk lamp to keep your sample warm. Covering the box with black paper will make it to absorb more light energy and become warmer.
    4. After a few days (1 to 5 days) we need to see the growth rate of bacteria in our samples. If we use good quality culture media, we can visually observe bacteria colonies. Otherwise we may need some test methods or other indications for bacteria growth. One that may work is using hydrogen peroxide. Hydrogen peroxide will fizz when added to an infected media.
      The reason for this fizz is that the hydrogen peroxide decomposes into water and oxygen when in contact with anaerobic bacteria. The bubbles you see are oxygen bubbles. We may use the rate of bubbles as an indication for the rate of bacteria growth.

Special Safety Considerations:

    • Caution: Keep lids on streaked petri dishes at all times. Some of this growth may be harmful.
    • When finished with petri dishes, collect and destroy them.

Experiment 1: (effect of temperature on bacteria growth)

Cook some chickpeas, empty any excess water and let them cool in a tray for a few hours. Don’t cover the peas so bacteria from the air will get to them. Now take 5 samples of the peas in five cups. Try to put same amount of peas in each cup. Cover each cup with a small peace of paper towel and secure the paper towel with the rubber band. Place each cup in a different location in your home with a different temperature.

I suggest to put one in the freezer, one in refrigerator, one in a normal room temperature, one in a slightly warm place and the last one in a warm place. Place a thermometer next to each cup so you will know the temperature in each location. Visit your samples every day and smell them. Bacteria grow will cause chickpeas to create bad odors. Write the results of your experiment in a table like this.

Bacteria growth spoiled the peas after this hours
Sample in Freezer
Sample in refrigerator
Sample in room temperature
Sample in a little warm place
Sample in a very warm place

Experiment 2: (Effect of light in bacteria growth)

Repeat the previous experiment with 4 cups of chick peas. Place two cups under a bright light (From window or a desk lamp), place the other two in the dark, but under exact same conditions (For example same room temperature). If you want to use a desk lamp, use fluorescent bulbs or modern energy saving lamps that do not create heat. Smell all cups every day and record the results in a table like this:

Bacteria growth spoiled the peas after this hours
First sample under light
Second sample under light
First sample in the dark
Second sample in the dark

Experiment 3: (Effect of pH on bacteria growth) 

Repeat the experiment 1 with 3 sample cups of chickpeas. Before you cover the cups with paper towel, add 10 drops of vinegar to one cup and mix it toughly so the chickpeas will become slightly acidic. In the other cup add 10 drops of a saturated solution of baking soda and mix it toughly so the chickpeas will become slightly basic (Alkali). Place them somewhere in room temperature and normal light. Smell the 3 samples every day and record the results in a table like this.

Bacteria growth spoiled the peas after this hours
No pH change (control)
Acidic Sample
Basic sample

Experiment 4: (Effect of food on bacteria growth)


In this experiment we will use three regular sponges as a bacteria growth media.

We will use plain regular sponges, not those that have anti-bacteria chemicals.

Cook some chicken wings (or any other part of chicken) with some chickpeas. When it is well cooked filter the broth and use it as a liquid food for bacteria.


Place three sponges in 3 different plastic plates and number them from 1 to 3.

Let sponge number 1 remain dry as a control. Add water to sponge number 2 until it is saturated and absorbs no more water. Add some liquid food to the sponge number 3 until it is saturated and absorbs no more liquid.

Leave all dishes exposed to the air (uncovered) for about 1 hour. This will give enough time for bacteria to get to the sponges.

Cover the plates with 3 other plastic plates.

Inspect and smell all plates every day for up to 2 weeks. Bacteria growth will create a bad odor that can be identified by smelling the sponges.

Record the results in a table like this:

Plate 1 (Control) Plate 2 (water) Plate 3 (broth)
Smelled after

………. hours

………. hours

………. hours

More Advanced Bacteria Growth

Procedures: (using agar or gelatin culture media)

You need to be prepared and conduct each step accurately to get results. Keep in mind bacterium live everywhere and most strains can grow easily under a favorable conditions already exist in your home, given water and food. Once you grow them, you can easily cross contaminate your environment if care is not taken. Consult an adult prior to conducting these experiments.

Prior to collecting bacterium sample you need to prepare the culture media. Culture media is where bacteria can grow. In previous experiments we used chickpeas and sponges with broth as culture media, but in advanced level and in chemical or biology laboratories, we can not use those as a culture media.

A good culture media must be clear and solid, so we can grow the bacteria on it’s surface and observe the bacteria colony from different angles and also we must be able to take samples from bacteria without them being mixed with chickpeas or any other things.

Gelatin can be a good culture media for starters. It is available as a dessert with different flavors such as strawberry and Lemon. There are some problem with gelatin however that makes it unusable for serious laboratory use. One problem is that it is solid only in cold temperature. So if you place it in a warm location, it will change to liquid again. The other problem is that manufacturers add some preservatives and flavors that slows down or prevents growing bacteria.

In laboratories another gelatinous substance is used called “Agar Agar”. Even though it’s name is “agar agar”, we usually call it agar. Agar gel will remain solid in warm temperatures too.

You can buy agar culture media from laboratory suppliers (Expensive). Or by gelatin from supermarkets.

Dissolve the gelatin or agar in hot water according to its instruction on the package and pure about 30 ml (few table spoons) into each petri dish until the bottom of the dish is covered with liquid. Notice that agar is not a food for most bacteria. So you normally add sugar and broth to it as foods.

Two different types of mold grow in two of our dishes (above pictures, left and right). In one dish we have indications of some bacteria growth (above middle picture). Images are taken about 2 weeks after the preparation of culture media.

For pH experiment you can add a few drops of vinegar to lower PH or add a few drops of baking soda solution to increase the PH. Add these in a separate test tube or small beaker, mix it with hot media (agar or gelatin) and then pure it into the petri dish.

Prepare 3 identical petri dishes, one neutral PH (7), one acid PH and one basic PH.

If you just want to experiment growing bacteria, Take some clean Q-tips or cotton swabs, and start collecting bacterial sample by swabbing against:

A) Your kitchen table or sink, B) Your refrigerator’s door handle, C) Your own arm pit, D) Your own mouth, E) Soil sample. (Throw away each Q-tip/cotton swabs after one use).

Use the Q-tip you collected the bacterium on, and smear the surface of the solidified culture media in the dish and cover the plate. Label the dish with the source of bacteria you collected i.e., if the bacteria was collected from your fridge, label it as fridge bacteria and so on….

Swab the clean Q-tip under around you armpit, or inside your mouth then smear the agar and cover the plate. Remember to only swab one location at a time. (Do not swab your mouth, using the same Q-tip, if you used it to swan your armpit)

You can also sneeze or cough in one plate to collect bacteria without touching it. But always remember to cover the plate as soon as you are done with smearing.

You can dissolve some of the soil in water and pure the contaminated water on top of the agar you prepared and cover the plate.

For more acurate tests, you can not do any of the above methods, because there is no guarantee that the amount of bacteria in all of your test samples are the same. So do this instead.

To test the effect of PH, temperature, light or any other factor on bacteria growth, you need to make sure that the amount of bacteria entered in all your test samples for each experiment are the same. To do that collect the bacteria with previous method in a cup of clean water or distilled water. Then stir it for a few minutes so the bacteria will be distributed in the water evenly. Then use a dropper to drop one drop of your polluted water to your culture media.

Once you inoculate all your plates, you need to cover them store them upside down. It is important to store them up side down because the culture media is moist and will lose moisture quickly and becomes dry if placed face up. If you keep it upside down, the moisture will not evaporate so quickly and media will remain moist for a longer time. (If you used gelatin, just cover them. Gelatin will remain moist while covered. )

For the temperature experiment you have to use agar and gelatin can not be used. For this experiment place one sample at room temperature (remember always store all your plates upside down), but somewhere that no body can easily touch it. It should not be disturbed during the incubation period. Place another plate in a plastic zip log bag, seal it and place it in the fridge, in one corner isolated from other food items. Place another one in the bag and put in the freezer. Finally, place one in a warm location (beside the water hater, behind the fridge, inside the warmed up oven….)

After 2 days, collect them all and open them and write down your observation from each plate in your note book. Count the colony of bacteria in each place. Compare the plates, type of bacterial, growth rate, size of the colonies, etc….


NOTE: You eventually need to dispose of your all plates. Collect them all and place them in aluminum or metal container, cover and place in oven. Heat them up for 2-3 hours above 350F. Ideally, you want to kill all the bacteria with heat prior to disposing them.

Materials and Equipment:

Material that you can use for this experiments are:

    • Perti dishes (MiniScience Product Code: PD_1)
    • clean Q-tips or cotton swabs
    • gelatin (A solidifying agent)
    • soil sample (good source of bacteria)
    • water
    • vinegar (Acidic pH)
    • baking soda (Alkaline pH)
    • paper pH indicator
    • ziplock bag
    • Agar (A solidifying Agent like gelatin)
    • broth (contains nutrients)
    • filter paper

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Write down your observation from ach plate. Some plates have more growth than others. Some plates have no growth, which one would you think are they?


If you do any calculations, write them in your report.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.