Within six months of effectively using methicillin to treat S. aureus infections in a community, all new S. aureus infections were caused by MRSA. How can this best be explained?
A) A patient must have become infected with MRSA from another community.
B) In response to the drug, S. aureus began making drug-resistant versions of the protein targeted by the drug.
C) Some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency.
D) S. aureus evolved to resist vaccines.

Question

Quizplus · Accepted Answer

C) This scenario is a classic example of natural selection in action. Before the use of methicillin, there were likely a few S. aureus bacteria that were naturally resistant to the drug (MRSA). Once methicillin was introduced, these resistant bacteria had a survival advantage and could reproduce without competition from non-resistant strains, eventually becoming the predominant type of S. aureus in the community.

Within Six Months of Effectively Using Methicillin to Treat S