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Name: Tammy
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I am working with my daughter on her Science Fair Project. We are testing daily items that we come in contact with to see how many germs and bacteria it has. How can we differentiate between the types of bacteria? How can we decide which one has the most? We are using the growth medium Agar in petri dishes. Where can I find more scientific info as to why this happens so we can write up the project?

These are complex questions. First, the agar medium is used as a solid phase so that one can see colonies formed. These are round mounds of growth because bacteria multiply in all directions, but they cannot normally move in or on a solid phase so they remain at the site of multiplication. Every bacterial cell can multiply into a colony. Thus, the number of colonies is a measure for the number of cells present, if you have taken quantitative samples. If you want to quantitate, you should try to standardize your samples (for example, use 1 ml liquid to wash surfaces, food particles, 1 ml of liquids, etc. and add of this one drop (with a micropipette would be more accurate) per agar plate and let the drop form a tear on the plate. The number of colonies that grow in this tear are a measure for the original number of bacteria present in the drop, because each colony is derived from a single bacterial cell.

You should realize that the agar you use may not be optimal to allow growth for all kinds of bacteria that are present. However you may notice differences in morphology, such as colony size, shape, color etc. This indicates that there are different kinds of bacteria growing. Try to count the colonies per species and count the number of different species you see.

I strongly recommend you spend some time browsing in the Virtual Museum of Bacteria, in which you will find basic information on what bacteria are, how they grow, how they are contaminating our food, what bacteriologists do, etc. Have a look at If you have further questions, or difficulties getting around in the musuem, please contact me.

Dr. Trudy Wassenaar

There are many types of agar. What specific kind are you using? What exactly is the question you are trying to answer and what is the hypothesis of this experiment? Are you just counting bacteria in general or are you looking for a specific kind of bacteria? If you are just trying to count numbers, you can just count the colonies (dots) of bacteria. If you are looking for a particular type, you can look on the web for a picture if you know the name and spelling. There are many microbiology books out there but they don't always have pictures in them. Good luck.

Van Hoeck

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