A microbiologist inoculates Staphylococcus epidermidis and Escherichia coli into a culture medium. Following incubation, only E. coli grows in the culture. What is the most likely explanation?

The culture medium must be selective

Explanation:

Selective media are the media that have those ingredients which favor the growth of a particular bacterium and inhibit the growth of unwanted bacteria. So a selective media is used to selectively grow a particular micro-organism.

For example, MacConkey agar is a selective media that is used to isolate gram-negative enteric bacteria based on lactose fermenting ability. It contains ingredients like bile salt and crystal violet which inhibits the growth of gram-positive bacteria therefore it only allows the growth of gram-negative bacteria.

Staphylococcus epidermidis is a gram-positive bacteria and E. coli is gram-negative enteric bacteria so MacConkey agar can be used to select E. coli over Staphylococcus epidermidis. So the correct answer is the culture media must be selective.