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

Question

Quizplus · Accepted Answer

Some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency. This suggests that the original population of S. aureus contained some individuals that were resistant to methicillin. As these resistant individuals survived the antibiotic treatment and reproduced, their resistance genes became more frequent in the population and eventually led to the emergence of MRSA as the dominant pathogen in the community.

Within Six Months of Effectively Using Methicillin to Treat S