Expert: Walter Hintz - 9/25/2010

Question
QUESTION: I'm helping my son with this project.  How should we set up the nutrient agar petri dishes?  Apply bacteria to entire dish first, and then swab the dogs mouth and rub on one side of the dish?  Or, wait for bacteria to grow and then apply dog saliva?  Will the bacteria disappear if the saliva kills it, or will it just shrink and decrease in the number of spores/colonies?

ANSWER: Hi Justice
Inoculate the entire plate with the bacteria and apply saliva to half the plate. If you wish you can grow an entire plate with bacteria and then apply saliva to half the plate.
I believe you will find that saliva will not kill the bacteria. Dog saliva has just as many bacteria as human saliva

---------- FOLLOW-UP ----------

QUESTION: So, it will be okay to put the saliva directly on top of the bacteria?  Like immediately after I've placed the bacteria?  BTW...we are using staphylococcus epidermal bacteria.  What do you think will happen?  Also, if the saliva does kill the bacteria, how will I be able to tell?

Answer
You should inoculate the plate with the bacteria first and then add the saliva. What you are looking for is a lack of bacterial colony growth. The staph bacteria should be in suspension before plating them.
Just for the heck of it repeat the experiment using human saliva. Let me know the results