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How to grow bacteria and fungi (mold and yeast) for a science project.
(Copyright - Science Enterprises 2008)

Safety First:

  • Get an adult to help you with your project.
  • Be very careful with boiling water and hot surfaces.
  • Always wear safety glasses.
  • Be aware that you will be growing a culture that may contain bacteria or other organisms that can make you or others sick.
  • Completely wash your hands each time you handle your experiment.
  • Do not eat or drink anything while you are handling your cultures.
  • When you are finished with your experiment, dispose of it safely. (See Disposing of Your Experiment at the end of this article)

Where can I grow my culture?

The best place to successfully and safely grow bacteria, mold and yeast are in specially shaped glass or plastic dishes called Petri Dishes. Cultures grow best with gentle heat in the range between 70 degrees to 95 degrees Fahrenheit. Cultures will grow at lower temperatures but at a much slower rate.


What is a Petri Dish?

Petri Dishes are flat bottomed dishes with straight sides and have two parts, a top and bottom. The top dish slides over and covers the bottom dish allowing easy access but it also keeps out unwanted contaminated air. Petri dishes are designed so that they will contain your cultures, allow you to watch them grow and safely allow any gasses generated by your cultures to escape safely.


What size and type of Petri Dish should I use?

  • There are different sizes and are made from either plastic or glass.
  • The most common size in use is 100 mm to 90 mm in diameter x 15 mm high (3.937 inches to 3.543 inches). The other common size is 60 mm in diameter (2.362 inches).
  • If you are unsure about the size you need select the 100 mm to 90 mm diameter size dish. This size is the most often used and is often the easiest to find. It will also give you ample room for your culture to grow. Matching these exact dimension recommendations is not necessary. Find a size that will work for you. 
  • Petri Dishes are made from either glass or plastic.
    • Plastic dishes are the more convenient as they most often come pre-sterilized and are less expensive than the glass dishes. They are usually the best choice unless you want to be able to re-use your dishes. They are designed to be only used once (although they can be reused, see below)
    • Glass Petri Dishes can be reused and are available in different types of glass.
      • Flint - an inexpensive type of glass most often used in windows and jars. This type of glass works well but it is more easily broken (than the borosilicate type) when you quickly heat or cool it. You just have to treat it a bit more carefully when heating and cooling and not shock it with rapid temperature changes.
      • Borosilicate - a more expensive type of glass (often 4-5 times the cost of Flint) that doesn't break as easily as Flint type when the glass is quickly heated and cooled. People often refer to this type of glass as Pyrex. Pyrex actually is a brand name used by Corning Glass Co. for the borosilicate glass items that they sell.

Do I have to use sterile Petri Dishes?

Yes, make sure you use only sterile Petri Dishes. Remember, microorganisms are all around us every where. You want to use a sterile dish so that you will not have any unwanted microorganisms in the dish that will contaminate your project.

Do I have to sterilize plastic Petri Dishes and can I re-use them? Well - Yes, sort of…

  • Plastic Petri Dishes - They are most often sold pre-sterilized, sealed in plastic bags and are designed for only one use. If you are using these, no further sterilization will be needed for the first use and you are ready to go.  If however you want to reuse them, they will have to be sterilized so they will not contaminate the next culture you grow.
  • Most plastic Petri Dishes cannot be sterilized by boiling because the heat will make the plastic bend and warp.
    • If they are not pre-sterilized or need to be reused, plastic dishes can be partially sterilized by washing them in hot soapy water, thoroughly rinsing them and then soaking them in a bleach and water solution {15 ml Household Bleach (about 1 Tablespoon) per Liter (about one Quart of water)} for 30 minutes.
    • Rinse thoroughly, shake off excess water and assemble the bottoms and covers. You have to remember this technique might still leave some hearty microorganisms that might still contaminate your project.
How can I sterilize glass Petri Dishes?
  • Glass Petri Dishes - Must be sterilized before each use and can be reused.  Most glass Petri Dishes should be sterilized before use even when they are new as most glass Petri Dishes come from the factory unsterilized.  Sterilized dishes will be sealed in wrapper (usually plastic) and will say that they are sterile on the packaging.
  • Sterilizing Petri Dishes - Washing with soap or detergent will not completely sterilize your Petri Dishes. There will still be some bacteria, fungi, etc. present that could contaminate your culture.
    • After washing and thoroughly rinsing your Petri Dishes, boil them in water for at least 15 to 20 minutes. Hold the hot Petri Dishes with tongs and shake off any excess water. Use tongs to be sure you do not burn your fingers. Assemble the bottoms and covers and allow them to cool.
  • More complete sterilization (this is not required for most home based experiments) can be achieved by placing your Petri Dishes in a pressure cooker or autoclave and heat them to 121 degrees C (250 degrees F) at 103 kPa (15 psi) pressure for 15 to 20 minutes

Do I have to feed my cultures as they are growing?

  • Bacteria, mold and yeast are like us, they all need some sort of food source. Agar is the standard growth medium for the culturing of bacteria, mold or yeast. Agar is extracted from type of red algae and is a polysaccharide (a type of carbohydrate) which is an ideal growth medium for these microorganisms. Additional food or nutrients can be added to the agar to make your cultures grow more successfully. When agar is mixed up it resembles Jell-O. It is available usually in a powder form that is mixed with water and nutrients to make a gel that you can use to grow microorganisms. It also can come premixed in gel form.
    Using agar alone will support microorganism growth. Adding additional nutrient (an additional type of food) will make your culture grow more successfully. Microorganisms are similar to you and I, we all like some types foods and not other types.
  • You can add a small amount of chicken, beef or vegetable broth to your agar mix. Note: Be aware that salt and preservatives will kill or slow the growth of microorganisms. Broths labeled "organic" and/or "low sodium" are the best to use because they most often have the lowest amount of salt and preservatives. Add only add a small amount of broth, about 5 ml (just under a teaspoon) of each 250 ml (just under one cup) of mixed agar. Add any broth to the agar / water mixture before you boil the agar.  This will sterilize the broth when you sterilize the agar / water mixture.  You can also add small amounts of sugar instead of the broth if you want to promote yeast growth.   Again always add any nutrients to your mixture before you boil it.
  • You can also use pre-mixed nutrient mediums. There are thousands of premixed culture mediums in use by the biomedical industry today and are mixed to grow specific types of microorganisms. These nutrient mediums are often referred to as broth mediums. One type that is especially effective at growing a bacteria culture is Blood Agar. This is an Agar that had dried animal blood (usually from sheep) added to promote vigorous bacteria growth.
How do I mix my agar growth medium?
  • Most mediums designed for growing cultures will have instructions. Prepare the medium following the instructions.
  • Most mediums are sold in powder form that will need to be mixed with water and then boiled before adding to Petri Dishes. This is easy to do and is very similar to making Jell-O.
  • You can also buy Agar premixed in bottles that can be heated to turn the agar back into a liquid so that it can be poured into Petri Dishes.
  • There are some Petri Dishes available that have culture mediums all ready in them. These are harder to find and are most often only used by larger laboratories.

How much agar medium should I put in my Petri Dishes and how long do I have to wait to use them?
  • Pour a thin layer of your liquid growth medium into the bottom of the Petri Dish. Note: for microorganism growth, only the thinnest layer is required. Pour your agar medium into the dish and tilt from side to side to coat the bottom. Replace the lid promptly to avoid bacteria and molds settling out of the air onto the growth medium and creating contamination.
  • Allow the medium to harden. This may take 30 to 120 minutes depending on room temperature and type of medium. When the medium has completely set up (it will resemble Jell-O at this point), It is best to store the plates upside down in a refrigerator until you are ready to use them. Allowing your plates to fully harden in the refrigerator overnight will make the agar harder and prevent gouging or tearing the surface when you wipe or transfer your samples.
  • You can store your prepared dishes in the refrigerator for weeks or up to a couple of months. Keeping them in the refrigerator helps to prevent the water in the agar from evaporating.
  • If you do store your dishes in a refrigerator, be sure to allow your prepared dishes to come back to room temperature before you add your samples.
  • As soon as the agar is set up to a hardness similar to Jell-O, they are ready to use.

My agar dishes are fogged up and I cannot see inside.  What can I do?

  • If your dishes are fogged up inside, storing them in the refrigerator overnight before you add your cultures will usually clear enough of the fogging so you can see. Remember, most bacteria, molds and yeast grow well in a moist environment so if there is some drops of moisture inside the dishes this is fine. You just have to clear enough of the fogged areas so you can see your cultures. If you have a lot of water droplets on the cover, just open the dish and shake some of the excess water off of the top cover. Replace the top quickly to prevent any contamination.
  • If you grow your culture with the dishes upside down, much of the moisture will go back into the agar or collect in small drops on the dish cover.  Remember if you are having problems seeing inside, DO NOT wipe the inside of the dish with a paper towel or dish rag as these are not sterile and you will contaminate your cultures.  You can use a sterile cotton swab to soak up some of the drops but it is best just leave the moisture in the dishes unless they are really preventing you seeing inside.

How do I collect samples and get them into the dishes?

  1. Collect a sample of what you want to grow using a sterile cotton swab (the type that you can buy at most drug stores is great for this). You can also use a sterilized inoculating needle or some other sterile collecting instrument (tongue depressor, etc.). Note: Dampening your cotton swab with sterile distilled water will help you collect samples from dry surfaces such as faucet handles, counter tops etc.
  2. Wipe or drag your sample across the surface of the set up agar. Replace the cover of the dish and record information about each sample.
Where and how do I grow my cultures?
 
Cultures will most often grow the fastest when there is a steady source of warmth. This can be on top of a hot water heater, in warm room or closet.
Note: Avoid window sills and in front of heaters as these locations will cycle from hot to cold rapidly and will not provide ideal growing conditions.
  1. Make sure that the area is not too hot or too cold. Temperatures from 20 degrees C (about 70 degrees F) to body temperature 37 degrees C (98.6 degrees F) are ideal. Colder temperatures will work but your colonies will grow slower. The simplest test you can do to find out if an area is too warm is to place your hand where you want to put your Petri dishes to grow your cultures. If it is warm and comfortable for you and you can keep your hand there, it isn't too hot and your culture should grow well there.
  2. You can also make an incubator (a type of warm growing chamber) using a heating pad or a light bulb for a heat source. Scientists growing cultures in laboratories often use incubators to carefully control growing temperatures. Always have an adult help you when setting up a heat source and test it before adding your cultures to be sure it isn't too hot. When ever you are using an electrical heat source, be sure it is safe. Again check with an adult to get help.
  3. Some cultures will grow more successfully in the dark, so experiment by placing your sealed dishes in a paper bag. Wrapping the Petri dishes in Aluminum foil or putting them in a paper bag or cardboard box will also work well.
  4. Make sure your plates will not accidentally come open.
    • Tape the tops and bottoms together
    • Put rubber bands around them
    • Wrap aluminum foil around the plates.
  5. It is best to grow your samples with your plates upside down.
    Your samples will grow just fine this way. This trick (regularly used by labs) will prevent any excess condensed water droplets from spreading your samples over the agar. This spreading of the sample could give you inaccurate results if you are using the size of the colonies as a measure of how fast your samples are growing.

How often do I record my results and how long will it take?

  • Check you cultures at least once a day and record your results.
  • Allow at least 4-6 days to allow enough time for your cultures to properly grow.
  • Cultures will grow at different rates depending on growing conditions. Microorganisms will start growing immediately. Remember that they are so small that you have to wait until you have a large number of them (often millions and billions) before there will be enough of them for you to be able to see the colonies. Some colonies will look fuzzy, they may have different colors. Some will change shape and color as they grow. Record your results.
  • You can record how much your colonies of your sample grow by using a marker pen such as a Sharpie and trace with the pen and measure the size of each colony on the bottom of the dish. Trace the growth over a number of days showing the growth with a series of concentric rings. Be sure to keep good records so you can understand your results.

What to do with my cultures when I am finished with my experiment?

  • When your experiment is complete, always dispose of your cultures properly.
  • Always destroy the microorganisms before disposing of dishes in the trash.
  • Pour a small amount of a strong disinfectant such as household bleach over the colonies while holding the Petri dish over a sink.
  • Caution - never allow any strong disinfectant such as bleach touch your skin, eyes or clothes. It may burn! Get help from an adult.

One last very important note:
Nothing described here is meant to replace good safety practices.

  • Always work with an adult to get their help and supervision.
  • You never know exactly what you might grow. Most microorganisms will not make you sick. There are some that will, so handle your cultures with care.
  • Always wash your hands after you handle your cultures.
  • Always wash and clean (with a good disinfectant) all surfaces (counters, etc.), containers (plates, dishes, etc.) and utensils (spoons, etc.) that have come in contact with your cultures.
  • It is best not to open you dishes while you are growing your cultures. You can let unwanted bacteria in from the air to contaminate your experiment. You can also let some of your culture out. It is best to keep everything sealed safely in the dishes.


What are some ideas for some science projects?

Who has the cleanest/dirtiest mouth??
Collect samples from your mouth, and compare to samples taken from a pet’s (dog or cat) mouth. Remember to always use a new clean swab to collect samples. DO NOT reuse swabs.

Bacteria are everywhere, but what part of the house or school has the most bacteria??
Collect samples from remote control buttons, sink sponges, sink faucets, kitchen counters, the floor, door handles or any where you want to test for bacteria.

Which hand soaps or disinfectants work the best??
Collect bacteria samples from a known source (see above for ideas). Swab the dishes and then put a drop of the disinfectant in one area for the Petri dish. See how well it prevents growth. You can also grow a culture for a few days and then place your test disinfectant on the colonies that grew and see what happens. Does it kill the growth?

Learn more about science fair projects and about bacteria and fungi (molds and yeast) and how to grow them by going to the following web based information sources:

How to set up a successful science fair project from:
Neuroscience For Kids at the University of Washington:
http://faculty.washington.edu/chudler/fair.html

What actually are bacteria, molds and yeast?
From Wikipedia, the free encyclopedia
On Bacteria
http://en.wikipedia.org/wiki/bacteria
On Mold
http://en.wikipedia.org/wiki/mold
On Yeast
http://en.wikipedia.org/wiki/yeast

More information
From Cells alive:
http://www.cellsalive.com/ecoli.htm

More information about microorganisms
From the University of California at Berkeley Museum of Paleontology
http://www.ucmp.berkeley.edu/bacteria/bacterialh.html

Here are some more project ideas
From Free Science Fair Project
http://www.freesciencefairproject.com/biology/how_to_grow_bacteria.html
 



 
Where can I get Petri Dishes and agar?

 

Petri Dishes and agar can be found on our web site ScienceEnterprises.com at:
http://www.scienceenterprises.com/search.aspx?find=petri

Have fun exploring the fascinating and very cool world of microorganisms.

 

 





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