Science Buddies: "Ask an Expert"

Hello,

My name is Mona and I am a junior at PV Peninsula High School. I am doing a project on the effects of xylitol on reducing the risk of obtaining oral cavities. I would like to take an oral bacterial count during my experimentation, and have two questions about my procedure:
1. How do I obtain a sample for bacteria in the mouth (accurate enough to be used to make a bacteria count)?
2. Which method of finding the bacterial count of the sample will be most effective: a Standard Plate Count or a Spectrophotometer?

Thank you for you help,
Mona

Hi Mona,

There's a great thread about measuring bacterial in your the original post https://www.sciencebuddies.org/mentorin ... php?t=2864 You should continue your question in there to help others follow up on your related question.

How you obtain a sample for bacteria in the mouth depends on the equipment you are going to use for a bacteria count.

Among the two methods you listed, a standard plate count is the best one unless you were planning to take a lot of samples. If you wanted to use a spectrophotometer, you would still need to use a standard plate count to create a calibration curve to help you measure a bacteria count.

Hi Mona,

You may find it easiest to use a sterile cotton Q-tip and take a swab of a person's inner cheek or underneath the tongue and run the cotton tip on a bacterial growth medium (LB agar plates).

However, I highly suggest that you take a gander into the literature on mouth flora. I think you will be surprised at the number of bacteria in the mouth and where they really reside. This may help you design your experiment better.

James

Mona,

Plating bacteria on agar and counting the number of colonies that result should work fine. However, if you plate the bacteria straight from the mouth, you will probably end up with a "lawn". I would suggest swirling the Q-tip in sterile water or LB broth and then diluting it down before plating. If you keep track of how much you have diluted the solution, you will still have a pretty good idea of the total number of bacteria. You may have to do some trial platings before you come up with a dilution that gives you a countable number of colonies.

Post back if you have more questions,
Sonia

Thank you so much for your responses. They have been extremely helpful.
I have a few new questions:
1. Is it necessary to use a colony counter to do the standard plate method of counting bacteria?
2. If I am testing the carries risk factor, should I take the bacteria sample from one of the teeth?

Thanks again,
Mona

You are welcome to use a colony counter if you would like, but it is not necessary. Your bacterial colonies should be easily visible if you want to count them by hand. If you're worried about counting colonies twice or missing some, you can mark the colonies you've counted with a pen on the bottom of the petri dish. I've also found it helpful to occasionally write down the number I'm on.

Sonia

2. If I am testing the carries risk factor, should I take the bacteria sample from one of the teeth?

Hi Mona,

I liked your idea of sampling the bacteria directly from the teeth. This could help you target the effects of xylitol on the oral bacteria that actually hang out on people's teeth (and cause cavities).

Depending on how much time you have, it might be fun to look at oral swabs vs. tooth swabs, to see if there is any difference in the amount of bacteria that grow. However, that would be an extension, and not necessary to answer your original question.

Your project sounds great. Good luck!

Cheers,
Heather