A microbiologist inoculates Staphylococcus aureus into a culture medium. Following incubation, both Staphylococcus aureus and Staphylococcus epidermidis are determined to be growing in this culture. What is the most likely explanation?

A. The microbiologist used too much inoculum.

B. The culture is contaminated.

C. The incubation temperature was incorrect.

D. The culture medium must be selective.

E. The culture medium must be differential.

B. The culture is contaminated.

Explanation:

The only plausible explanation for this result is that the initial inoculum is either a mixture of both Staphylococcal species or the culture is contaminated during the culturing procedures.

Too much inoculum will not lead to contamination except the inoculum is not a pure culture.

Incorrect incubation temperature is not the cause because both species require the same growth condition. Either they grow together or they do not grow at all.

Using selective or differential culture medium has nothing to do with contamination as long as pure culture is used during inoculation.

The correct option is B.