Community-adapted methicillin-resistant Staphylococcus aureus (MRSA): population dynamics of an expanding community reservoir of MRSA.

To define methicillin-resistant Staphylococcus aureus (MRSA) reservoirs in the community and their population dynamics, we studied the molecular epidemiology of a random sample (n=490) from a collection of 2154 inpatient and outpatient MRSA isolates during a 7-year period in San Francisco. We noted a progressive replacement of type II staphylococcal chromosomal cassette (SCC)mec-bearing isolates with type IV SCCmec-bearing isolates, which coincided with >4-fold increase in methicillin resistance between 1998 and 2002. Type IV SCCmec-bearing isolates involved in the increase in methicillin resistance belonged to 4 molecular genotypes. These 4 genotypes were associated predominantly with community-onset disease, rather than hospital- or long-term-care facility-onset disease (76.9% vs. 19.4% vs. 3.7%; P=.0005), suggesting that they are not feral descendants of hospital isolates. The longitudinal results linked the dramatic increase in MRSA infections to an expanding community reservoir of MRSA genotypes with intrinsic community survival advantage.