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.

B

Explanation:

The rate of mutations on the S. Aureus population increased when exposed to antibiotics over time. This gave rise to a few mutants of the bacteria that could  survive the methicillin antibiotic. This is because the mutation altered a protein target of methicilin (beta-lactam binding proteins) on which the antibiotic acted upon. These populations increased as the others were eliminated by the antibiotic – the way natural selection works. Over time, these mutations were transferred to other populations of bacteria by these MRSA through ‘sexual’ horizontal gene transfer, causing whole bacteria populations to be resistant against methicilin.

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