Zone of Inhibition and Antibiotics
Date: Summer 2013
Does the mere presence of a zone of inhibition mean bacteria were susceptible to that antibiotic?
Thanks for the question. Usually, but not always, the presence of a zone of inhibition implies that the bacteria are susceptible to the antibiotic. Exceptions to the above statement can occur when the bacteria are not plated correctly or when there is no control plate used. These are the simplest exceptions that come to mind, but one can think of more elaborate reasons for exceptions to the rule.
I hope this helps. Please let me know if you have more questions.
Not totally. There is a minimum inhibitory concentration that is determined. Each antibiotic has a minimum concentration that will begin to kill or inhibit the bacteria. The size of the zone is important and is different for each antibiotic. Obviously, if there is a zone at all, some of the bacteria are being inhibited. But even on the same plate of agar, different sized zones are considered ideal for different antibiotics. So, if for instance penicillin and cephalosporin have the same sized zone, it might still not be considered large enough for one or the other. Also, some antibiotics are considered best to use for different infections because of the way they metabolize in the body, for instance a urinary tract infection vs a skin infection, and also whether the bacteria are gram positive or gram negative. There is actually a lot to consider.
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Update: November 2011