[Campbell Biology P.534] Within six months of effectively using methicillin to treat S. aure... | Practice Question

Within six months of effectively using methicillin to treat S. aureus infections in a community, all new S. aureus infections were caused by a resistant strain 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.

Explanation

The rapid increase in MRSA indicates that pre-existing resistant strains, which were likely rare, had a significant survival advantage in the presence of methicillin. Natural selection then rapidly increased their frequency in the population as non-resistant strains were killed off.