An organism suspected of being one of the Vibrio spp. grew on MacConkey agar. To determine whether it was a vibrio cell or in the Enterobacteriaceae family, an oxidase test was performed on the colony. The results of the oxidase test, however, were negative. Which one of the follow-ing statements explains this discrepancy?

Question

a. The oxidase test should be performed on TCBS agar, not MacConkey agar.
b. The oxidase test should be performed on a sheep blood agar plate to avoid false-negative results.
c. The pH of the MacConkey agar is most likely elevated to alkaline conditions.
d. No discrepancy is present. All vibrio cells are oxidase-negative.

Answer 1

B
The oxidase test must be performed on 5% sheep blood or on another medium without a fer-mentable sugar (e.g., lactose in MacConkey agar, sucrose in TCBS agar). The reason for this is that fermentation of a carbohydrate results in acidification of the medium, and a false-negative oxidase test may result if the surrounding pH is below 5.1.