Question

You perform a Kirby-Bauer assay, but you neglect to use aseptic technique. Your plate becomes contaminated with a bright red organism. You can easily tell this organism apart from the one you intended to inoculate (which has grown into a nice lawn all over the plate). In order to conserve time, you measure the zone of inhibition for your intended organism, ignoring the contaminants. Are you assured to get the correct answer using this method?

A. Yes, but you have to factor in the zone of inhibition for the contaminant as well.
B. No, the contaminant adds too many variables to make this assay usable.
C. Yes, but you would have to subtract 1–2 mm from the intended organism’s zone of inhibition to account for the presence of the contaminant.
D. Yes, you can easily tell the two organisms apart, ensuring that you measure only the zone of inhibition for the correct organism.

Answer 1

When contaminants are present due to a failure to use aseptic technique in a Kirby-Bauer assay, the added variables make the assay unreliable, and the results cannot be assured as correct.

Step-by-step explanation:

No, measuring the zone of inhibition for the intended organism and ignoring the contaminants does not assure correct results in a Kirby-Bauer assay. The presence of an additional, unforeseen organism due to a failure to use aseptic technique introduces too many variables into the assay, which can impact the diffusion and activity of the antibiotic in unplanned ways. Therefore, the answer is B: No, the contaminant adds too many variables to make this assay usable. Even though you can distinguish between the two organisms based on appearance, the assay's valid interpretation relies on a standardized procedure and uniform conditions, which have been compromised by the contamination.