Expert: Walter Hintz - 9/27/2005

Question
Hello Walter,
I am taking a microbiology class and we performed a lab experiment where we licked an agar plate.  Then we rinsed our mouths with Listerine for 30 sec. and licked another plate.  After incubating for 2 days at 37 deg C, both plates had approx. the same number of cultures.  We are to assume that Listerine does what it says it will do (kill 99% of the germs).  Why then are the plates the same?  My theory is that bacteria live in our saliva and in our throat so, once we produce more saliva and/or breathe we replace the killed bacteria.  Any thoughts?
I appreciate any direction you can give me.
sincerely,
Connie Leslie

Answer
Hi Connie
 I think we can assume that the Listerine claims are false.  Apparently the Listerine did not kill the mouth bacteria.  Fresh produced saliva does not contain bacteria. Another explanation is the bacteria on the tongue are not necessarily washed of with a mouthwash. There are grooves and papillae on the tongue that can trap bacteria. I must comment about the technique.  As a former bacteriologist and a college micro teacher I have never heard of innoculating media by licking it.  A sterile swab is usually used.  I do not consider licking media to be a wise practice.