The strain, called USA300 is most common in the city’s Castro District, where it had an incidence rate of 170 cases per 100,000 persons.
It can cause severe illnesses such as necrotising fasciitis, sepsis, endocarditis and pneumonia and can affect healthy people in the community lacking in traditional MRSA risk factors.
USA300 has two versions, or genotypes: one which is multidrug resistant and one which is not.
The multidrug resistant genotype is 100% resistant to oxacillin, erythromycin, clindamycin and mupirocin. It is also partially resistant to ciprofloxacin and tetracycline.
Although it is susceptible to rifampin, trimethoprim-sulfamethoxazole, gentamicin, vancomycin and linezolid, authors warn that treatment with these antibiotics could encourage the emergence of more resistant strains.
Findings were based on a population-based survey of 2,495 patients with MRSA in San Francisco and a study of 183 patients in an HIV clinic in the city.
Annals of Internal Medicine (2008) 148:249-257