How do oxidase tests assist in differentiating Vibrio species from many Enterobacteriaceae?

• A. Enterobacteriaceae are oxidase-positive; Vibrio are oxidase-negative
• B. Vibrio are oxidase-positive; most Enterobacteriaceae are oxidase-negative
• C. Oxidase tests are not useful
• D. Both groups are oxidase-positive

Answer: (B)

The main concept here is how the oxidase test differentiates groups of Gram-negative bacteria by detecting the enzyme cytochrome c oxidase. Vibrio species are oxidase-positive, meaning they produce this enzyme and give a positive, color-changing reaction with the oxidase reagent. Most Enterobacteriaceae lack this enzyme, so they produce a negative oxidase test and do not change color. This makes the test useful for separating Vibrio from Enterobacteriaceae in the lab: a positive result supports Vibrio, while a negative result supports Enterobacteriaceae. While there are exceptions in the broader bacterial world, the general pattern is Vibrio oxidase-positive and most Enterobacteriaceae oxidase-negative, which is why that choice is the best fit.