I’m working with a presumptive Vibrio cholerae isolate and noticed a curious pattern during antibiotic susceptibility testing on Mueller-Hinton Agar (MHA). At 12 hours of incubation, there was a clear zone of inhibition around the ampicillin disk. However, after 24 hours, colonies appeared within the previously inhibited area, suggesting regrowth.
To rule out media or disk issues, I repeated the test using freshly prepared MHA and newly opened antibiotic disks. The same regrowth pattern occurred, but only with ampicillin. The isolate remained consistently inhibited by Cefotaxime, Ceftazidime, Chloramphenicol, Ciprofloxacin, and Tetracycline throughout the full 24-hour period.
Has anyone encountered similar regrowth behavior in V. cholerae or other bacteria with ampicillin? Could this indicate tolerance, persistence, or an early stage of resistance? I’d appreciate any insights, references, or suggestions on how to further investigate this observation.
This bacterial growth may indicate the presence of antibiotic-resistant bacteria. Why does it take 24 hours for the bacteria to grow, while the bacteria far from the disc (i.e., the antibiotic) only take 12 hours to grow? It is normal that a bacteria, even if it is resistant to an antibiotic, can have slowed growth in the presence of this antibiotic.
The second reason is that the antibacterial agent (ampicillin) has been degraded.
Billy James Encallado yes all the time.
AM is a chemically sensitive molecule. When you put the disc on the plate and the AM difuses, there will be some persisters in the zone.
During prolonged time, Am breaks down and concentration decreases and allows persisters to ,,wake up'' and grow.
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