I wonder either my antimicrobial susceptibility test on Muller-Hinton agar is contaminated or a subpopulation of my bacteria is resistant to those antibiotics. The thing is, when I was measuring the size of the inhibition zones diameter I noticed multiple colonies grown within the zone of inhibition. They looked identical to each other, and grew in different antibiotic disks.
Here in the picture happened this phenomenon in my first attempt with EPEC only within the chloramphenicol's inhibition zone, however in my second attempt didn't happen. Otherwise, in my first attempt with EAEC this didn't happened, only after the second trial within different antibiotic inhibition zones, but it did not modify the diameter size.
I'd really appreciate your comments and possible solutions! Thanks.
It is hard to distinguish just from looking whether those are contaminants or whether they are drug resistant mutants that arose from the lawn. You can just streak a few out for single colonies and then determine whether they are the same as your starting strain or not.
Michael J. Benedik Thank you, today I accomplished Gram stain to those colonies, the result was Gram negative rods. I think if I streak out for a single colony like you say, it would support the existence of mutant subpopulation. Thank you so much! I really appreciate your answer.
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