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Antibiotic Sensitivity Test

Antibiotic Sensitivity Test - Microbiology Forum

Antibiotic Sensitivity Test - Discuss Microbiology Science and Protocols here. Post questions on the study of viruses, fungi, parasites and bacteria here. Microbiology Forum.


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Old 10-14-2011, 09:10 AM
Pipette Filler
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Question Antibiotic Sensitivity Test



Hi, i'm an fresh undergraduate in microbiology field. I have encountered a doubt in doing antibiotic sensitivity test using Mueller-Hilton agar. There are clear zones that appear as double layer. Some the inner layer appear as thin layer of swarming growth, some can see tiny dots.
I understand there are 3 exceptions:-
1. With sulfonamides and co-trimoxazole, slight growth occurs within the inhibition zone; such growth should be ignored.

2. When b-lactamase producing staphylococci are tested against penicillin, zones of inhibition are produced with a heaped-up, clearly defined edge; these are readily recognizable when compared with the sensitive control, and regardless of the size of the zone of inhibition, they should be reported as resistant.

3. Certain Proteus spp. may swarm into the area of inhibition around some antibiotics, but the zone of inhibition is usually clearly outlined and the thin layer of swarming growth should be ignored.

How should I take the measurement? Measure until outer layer or inner layer?
Thank you.
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Old 10-30-2011, 01:01 PM
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Default Re: Antibiotic Sensitivity Test

Ignore the thin veil of Proteus swarm - measure from thr defind margin.

Also be aware of a rare observation where the bacterial isolate - to my experience, enteric isolates - grows primarily within the zone of diffusion for streptomycin. Such streptomycin-deoendency has been expresed by E. coli and Salmonella isolates.
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Old 01-20-2012, 02:46 AM
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Default Re: Antibiotic Sensitivity Test

I would measure both. For disc diffusion tests we have three possible results: resistant, intermediate and sensitive. When there's an inner ring we normally report them as intermediate results. That is, they are not completely sensitive to the antibiotic but are sensitive at a certain concentration (since the antibiotic diffuses through the agar over time, the zone is just an indication of the point where the concentration is low enough to be overcome by the bacteria or where the antibiotic concentration is just enough to inhibit the bacteria).

So yes, I would report both rings.
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