Emergence of community-associated methicillin-resistant Staphylococcus aureus at a Memphis, Tennessee children's hospital

Background: An epidemiologic investigation was performed because of a perceived increase in infections caused by community-associated methicillin-resistant Staphylococcus aureus (MRSA) among children in the greater Memphis area. Methods: We reviewed medical records of 289 children evaluated from January 2000 to June 2002 at a children's hospital. Clinical criteria were applied to classify MRSA isolates as community-associated (n = 151) or health care-associated (n = 138). The relatedness of 33 archived S. aureus isolates was evaluated using pulsed field gel electrophoresis (PFGE) of Sma1-digested genomic DNA; a common pulsed field type was defined as greater than or equal to80% similarity based on Dice coefficients. PFGE profiles were compared with those in a national database of MRSA isolates. Results: During the first 18 study months, 46 of 122 MRSA isolates (38%) were community-associated; this proportion increased to 106 of 167 isolates (63%) during the last 12 study months (P < 0.0001). Community-associated isolates were recovered from normally sterile sites as frequently as were health care-associated isolates (16% versus 13%). PFGE revealed that 15 of 16 community-associated isolates shared a common pulsed field type (USA300) observed in community-associated MRSA infections elsewhere in the United States and characterized by staphylococcal cassette chromosome mec type IV, clindamycin susceptibility and erythromycin resistance mediated by an msrA-encoded macrolide efflux pump. Conclusions: Community-associated MRSA has emerged as a potentially invasive pathogen among children in the greater Memphis area, and this phenomenon is not explained by spread of nosocomial strains into the community.