Why choose an antibiotic which produces a smaller zone of inhibition?

Posted on April 28th, 2009 in Antibiotics

lisse_aranel asked:

Hi, I have to answer this question for my medical microbio lab - Why would a doctor choose to use an antibiotic that produces a smaller zone of inhibition against a pathogen than one which produces a larger ZI?

This isn't related to the use of wide spectrum vs narrow spectrum antibiotic, is it? Does it have anything to do with the prevention of resistant strains from emerging? Help, please. Thanks in advance!

Tags: Medical Doctor, Narrow Spectrum

This post has one comment

Sarah T
April 30th, 2009
The zone of inhibition is inversely related to the MIC (minimal inhibitory concentration). This means that the bigger the zone, the lower the concentration of antibiotic required to kill it.

Choosing an antibiotic with a smaller zone will not help prevent resistance emerging. In fact the reverse is sometimes the case - a smaller zone means the pathogen is already closer to being resistant, and may require a smaller step to become so.

You’re right - this has nothing to do with spectrum of an antibiotic. The spectrum relates to the number of different organisms a particular antibiotic can kill. A broad spectrum antibiotic is therefore active against a wider range of bugs that a narrow spectrum one. You question seems to be referring to the different antibiotics that can be used against one bug, whereas the spectrum is the complete other way round.

Despite this, there are a couple of reasons why you may pick an antibiotic with a small zone of inhibition:

1) Remember that you cannot directly compare zone sizes between different antibiotics. Some antibiotics naturally produce small zone sizes, but this doesn’t mean they are less sensitive or useful compared to others. For example, the antibiotic vancomycin gives a small zone even when sensitive, but you may still choose to use it, as it is a good antibiotic.

2) The patient may be allergic to the antibiotic with the larger zone of inhibition, so the doctor may be forced to use one with a smaller zone.

3) As well as the sensitivity to an antibiotic you must also take into account whether the antibiotic can penetrate into the site of infection. There is no point giving a antibiotic with a large zone if it will not be able to diffuse into the infected area.