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

Methicillin resistance Staphylococcus aureus(MRSA) is another group of Staphylococcus which is resistant to many antibiotics specially methicillin.

So antibiotics will only work on those Staphylococcus aureus which are sensitive for the drug methicillin but will not work on MRSA. As a result, the bacteria which are sensitive to antibiotics will eradicate and MRSA strain will flourish due to having resistance for methicillin.

So there must be some drug-resistant bacteria present with non-resistant bacteria at the starting of the treatment and after six months non- resistant were killed and resistant bacteria flourished because of having antibiotic resistant genes so they are selected naturally.