From these data, you propose a model of how s. aureus changes from a harmless bacterium on the skin to a serious pathogen inside a wound.

Staph, as well as other pathogens, may change genetically by means of plasmids. Plasmids are DNA molecules which carries a rich diversity of genes that benefit the host cell. In this specific case, the host cell is the Staph bacteium. Among other things, plasmids carry antibiotic resistance genes. 

To illustrate how an ordinary Staph aureus bacterium can evolve into a methicillin-resistant bacterium, let's use the patient who doesn't finish an antibiotic prescription of penicillin. The Staph is weakened but not killed. Bacterial plasmids work quickly to produce genes that make the bacteria resistant to the drug. These parent bacteria replicate and pass on the plasmids, along with the DNA they contain, to the "daughter" cells. Voila! Methicillin-resistant Staphylococcus aureus! With the new DNA in place, each subsequent generation of Staph will also be resistant. But do not think for one moment that MRSA is otherwise somehow different from Staphylococcus aureus. MRSA IS Staph. Staph with an added feature. 

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