So I made a plate with wild type K12 strain E.coli and wild type S. epidermidis and did a basic disk diffusion antibiotic susceptibility test on it using various antibiotics. But for some reason, antibiotics that are not supposed to produce inhibition zones like ampicillin or penicillin for s. epi (due to it being β-lactamase positive) end up producing visible zones of inhibition. Then other antibiotics like ampicillin for E. coli that should produce zones of inhibition do not. Does anyone know why this may be? I've repeated the plates numerous times and get the same results. Thank you in advance!
Are you sure?
Can you explain in more details?
I am almost sure that I know what happened there.
Keep in touch please.
Regards
Alireza
Hello john,
If that is the case, i suggest you to stop repeating the assay. Please check your culture Or discard the culture and prepare another subculture from the main stock.
From your statement i think there has been a cross contamination of another bacteria which would have been sensitive to the antibiotics.
Please let me know if you get this issue sorted.
Hope this was helpful
Alireza Mordadi So I basically streak plated two plates with the different cultures, split it into quadrants, then used different antibiotic disks to do sensitivity tests for both of them. But then after doing this for 5 different trials, I kept getting results such as zones being produced by tetracycline for E. coli and no zone for penicillin. Then for S. epi there were zones produced for both ampicillin and penicillin. Is it potentially stock contamination?
@John Locke
Yes it can be contamination but sometimes this issue happens in co-culture or combination of antibiotics causing change in gene expression.
So I am interested in changing of expression because in staphylococcus family resistance to beta lactam are induction and if you find a way that stop gene expression is very nice.
Sarath, NO: beta-lactam ARE active on many Staphylococci. This being said, a high frequecy of S.epidermidis are resistant.
Article:
Beta-lactam Resistance in Staphylococcus Aureus: The Adaptive Resistance of a Plastic Genome
Alain Rambach !...yes..you are absolutely right ! I had a little confusion there. I am sorry about my previous statement. I just remembered the discovery of penicillin against staphylococcus which also is a beta lactam antibiotic.
Thank you for correcting my mistake sir!!
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