Science Buddies: "Ask an Expert"

I'm collecting oral bacteria from people (asking them to swab their teeth with a a sterile swab), and I'm not sure on how to measure the bacteria. I want to use agar and perform a viable plate count, but I'm very confused on how to measure dilution. To calculate dilution, I believe you have to know how much bacteria collected first, but I don't know how to measure the amount of bacteria on the sterile swab. Help!

Thanks, Rubi

Hi Rubi,

Welcome to Science Buddies! This is a very interesting idea for a science project. Your question is a good one because it is not easy to quantitate bacteria from an unknown sample.

It is possible to count colonies on an agar plate when the number ranges from about 30 to 300 colonies. If, for example, your sample contained 1,000,000 bacteria, you would need to make a 1:10,000 dilution to get a countable plate with 100 colonies. Since the number is unknown, you will need to make several 10-fold dilutions and incubate the plates and count the plate that has between 30 and 300 colonies and multiply by the dilution factor.

You will need to use a consistent technique to collect your samples, and control the time between sample collection and plating. If you use a sterile swab, you can immerse it in a tube containing one milliliter of sterile water and call the first tube, your undiluted sample.

I definitely recommend that you do a pilot experiment with one or two samples so you can narrow the range of dilutions that you have to do for each sample.

The Science Buddies website has great technical and troubleshooting information on microbiology techniques:

https://www.sciencebuddies.org/science- ... ques.shtml

Oral samples will contain a mixture of aerobic and anaerobic bacteria. Are you planning to count aerobic organisms only?

Please post again if you need any additional explanation.

Donna Hardy