Community-associated strains of methicillin-resistant Staphylococccus aureus as the cause of healthcare-associated infection

OBJECTIVE. Methicillin-resistant Staphylococcus aureus ( MRSA) isolates from patients with community-associated infection have been described as strains genetically distinct from the strains isolated from patients with healthcare-associated infection. This study examines the hypothesis that community-associated MRSA ( CA-MRSA) strains now cause serious infections in hospitalized patients. METHODS. Thirty-seven clinical MRSA isolates were randomly selected from blood isolates obtained from July 2003 through June 2004. Strains were tested for staphylococcal chromosomal cassette mec ( SCCmec) type, pulsed-field gel electrophoresis ( PFGE) type, and presence of Panton-Valentine leukocidin ( PVL) genes. Medical records review and epidemiologic classification was performed by an investigator blinded to the results of the bacterial strain analysis. Episodes of bloodstream infection were independently classified as either community-associated or healthcare-associated infections, and bacterial isolates were independently classified as either CA-MRSA strains or healthcare-associated MRSA ( HA-MRSA) strains, according to established definitions. SETTING. A tertiary care Veterans Affairs Medical Center. RESULTS. Twenty-four ( 65%) of 37 MRSA isolates were SCCmec type IV, a genetic type characteristic of CA-MRSA strains; 22 of these 24 isolates belonged to the CA-MRSA clone USA300 and carried PVL genes. Thirteen ( 35%) of the 37 strains were SCCmec type II, of which 12 were USA100-ST5 and 12 lacked PVL genes. Thirty patients ( 81%) had healthcare-associated infections; 18 ( 60%) of these 30 were infected with isolates carrying markers of CA-MRSA strains. Of 7 patients with CA- MRSA infections, 6 were infected with isolates belonging to the USA300 clone. Patients with healthcare-associated bloodstream infections were as likely to be infected with a CA-MRSA strain as patients with a community-associated infection (P=.38). CONCLUSIONS. MRSA strains with molecular characteristics of CA- MRSA strains have emerged as an important cause of serious healthcare-associated infection in our hospital.