Science Buddies: "Ask an Expert"

I'm making a research proposal and it's on a study of the efficacy salt water mouthwashes on oral bacteria particularly S. mutans and Lactobacilli, how do I go about the methodology?

Do I culture the bacteria on mannitol salt agar and see if they survive? But it could be argued that the bacteria in the salt agar is bathed in salt as opposed to say a 30second rinse in the mouth with salt water. If possible a methodology that makes use of bacterial cultures instead of making someone use mouthwash and then swabbing after. Thank you!

There are a couple of ways you can do this:

(1) Agar plates
Probably the way to go with this is to use salt water to make up your agar. I would not use use MSA since it is selective and already contains a high salt concentration. You're better off using TSA, and using salt water to dissolve the TSA before sterilization. Since this medium is not really selective, you need to use pure cultures of the bacteria (you can get the from Carolina Biological Supply for about 12 bucks each. Grow the cultures until they are turbid, then use a sterile loop to inoculate the plates. After 24-28 hours of inoculation it should be easy to determine the susceptibility of each bacteria.

(2) Liquid cultures
Again, starting with pure cultures, grow each to turbidity in TSB. Prepare TSB tubes with increasing concentrations of salt, and incubate as above. If you have access to a Spec 20, that's even better because you follow growth of each bacteria in real time (say, hourly).

If it a particular mouthwash you are interested in, or are comparing mouthwashes, just substitute mouthwash for the salt described above.