For a long time, I have been curious about some of the inhibition
zones we get in our laboratory when testing unknown antimicrobial
substances in fermentation broths for activity in well-agar diffusion
assays. A cross-section of the agar looks like this:
WWWIIIIIIGGGIIIIBBBBBBBBBBB WHERE
W = Sample site of Application (well)
I = Inhibition Zone
G = Zone of Growth
B = Background Lawn of Growth
We usually refer to these types of inhibition zones as 'bullseye'-type
zones or halo-zones. I had thought that the inner growth zone
represented some diffusible component (nutrient?) that protected the
test organism from the toxic effects of the antimicrobial agent. More
recently, a colleague suggested that this may be a manifestation of
the postantibiotic effect in agar (as opposed to the 'skips' seen in
broth dilution MIC testing), but a quick review of the literature on
this subject failed to give me any references regarding this effect in
agar. Any verifiable explanations or references on this are welcome.
Thanks, SWM
mamber at synapse.bms.com
