Growth Within Inhibition Zones-Explanation? (CORRECTED)

Stephen_W._Mamber_at_~PRIWFB05 at Stephen_W._Mamber_at_~PRIWFB05 at
Mon Oct 10 13:53:22 EST 1994

     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:
     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 PARADOXICAL 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  
     **I'm sorry about the initial error - it was late friday and my 
     fingers were ahead of my brain.  Anyway, for those who are interested 
     in the paradoxical effect, two key references are Eagle and Musselman, 
     J. Experimental Medicine 88:99-131, 1948 and Lorian et al., J. 
     Antimicrobial Chemotherapy 5:613-616.

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