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?