Question

Imagine that you have set up a Kirby Bauer experiment on Mueller Hinton agar (non-selective agar) in order to test if Escherichia coli is susceptible or resistant to Drug X. After 24 hours, you notice a zone of inhibition (clear zone) around your antibiotic disc and determine that E. coli is susceptible to Drug X. You then take a cotton swab and subculture a sample from this zone of inhibition (where no growth was apparent) onto a new agar plate without antibiotics and after 24 hours you see growth of E.coli. What can you conclude about Drug X? Explain your answer for full credit.

Answer 1

In this case, you took a small sample of bacterial cells from the zone of inhibition. Generally, if the antibiotic is effective, you would not expect to find growth in a subculture from the zone of inhibition (in fact, it should not be any bacterial cell in the zone of inhibition). You observed growth because in the zone of inhibition were a small amount of resistant cells of E coli. Remember that antibiotics may promote mutation in the bacteria, producing resistant strains. So, we can suspect that the drug X promoted the formation of resistant E.coli strains.