Science Buddies: "Ask an Expert"

I am confused, in the procedure for which acne medication can really zap that zit. The supply list says to use TSA blood agar plates for the disc diffusion, instead of Mueller Hinton plates which are commonly used for the Kirby Bauer test. How will you be able to measure your zone of inhibition on a blood plate? A blood plate is opaque. The organism I will be using is P. acnes, can this organism grow on a Mueller Hinton plate. Please advise.

A disk diffusion test is used to measure the effect of different materials on microbial growth. These materials may be newly synthesized chemicals or extracts from microbes, microbial metabolites or various tissues.

Some microbes associated with warm blooded animals may require a component of mammalian blood for optimum growth and so the requirement for TSA blood plates.

A blood agar plate is opaque, but the disk diffusion test is used to measure the zone of inhibition of bacterial growth around a paper disk containing the test chemical. You will be measuring bactrial growth on the surface of the plate not looking through the plate.

The following sites may some of your questions -

http://www.rlc.dcccd.edu/mathsci/reynol ... otics.html

https://www.hardydiagnostics.com/catalo ... tonMed.htm

http://en.wikipedia.org/wiki/Agar_plate

If you have any further questions please ask.

Matthew W. Mulanax, Ph.D.
Plant Pathologist

Just to add to what the other expert said, don't worry--you'll be able to tell the difference between the bacterial colony and the agar, which is all you really need to be able to do to measure the zone of inhibition.