In 1959, doctors began using the powerful antibiotic methicillin to treat infections of Staphylococcus aureus... ...but within two years, methicillin-resistant strains of S. aureus (MRSA) appeared. How did the resistant strains of S. aureus emerge? a.)In response to treatment of Staphylococcus aureus infections with methicillin, bacterial populations gradually began to synthesize cell walls using a protein that was not affected by methicillin. b.)In response to treatment of Staphylococcus aureus infections with methicillin, some bacteria began to synthesize cell walls using a protein that was not affected by methicillin. These bacteria survived the methicillin treatments and reproduced at higher rates than did other individuals. Over time, these resistant individuals became increasingly common. c.)Staphylococcus aureus bacteria that were able to synthesize cell walls using a protein that was not affected by methicillin survived the methicillin treatments and reproduced at higher rates than did other individuals. Over time, these resistant individuals became increasingly common. I'd also like to know the reason why behind the correct answer.

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The answer is C. Staphylococcus aureus that had a mutation allowed them to synthesize cells walls using a protein that was not affected by methicillin and these bacteria survived the treatment of that antibiotic and were able to reproduce at higher rates. This was a pre-adaptation, something in the DNA of some individuals that was due to a variation or mutation in their DNA. Once this antibiotic was introduced into their environment, those with the mutation had a selective advantage and survived. The selecting agent wasn't "nature" rather, it was the antibiotic. Those with the favorable mutation survived and because they were fittest, reproduced and passed along this variation to offspring. Also, because bacteria are able to conjugate, they exchanged their DNA with other individuals, causing the resistant gene to spread through the bacteria population.

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