Characterization of Nasal and Blood Culture Isolates of Methicillin-Resistant Staphylococcus aureus from Patients in United States Hospitals

Abstract

ABSTRACTA total of 299 nares and 194 blood isolates of methicillin-resistantStaphylococcus aureus(MRSA), each recovered from a unique patient, were collected from 23 U.S. hospitals from May 2009 to March 2010. All isolates underwentspaand staphylococcal cassette chromosomemecelement (SCCmec) typing and antimicrobial susceptibility testing; a subset of 84 isolates was typed by pulsed-field gel electrophoresis (PFGE) using SmaI. Seventy-sixspatypes were observed among the isolates. Overall, for nasal isolates,spatype t002-SCCmectype II (USA100) was the most common strain type (37% of isolates), while among blood isolates,spatype t008-SCCmectype IV (USA300) was the most common (39%). However, the proportion of all USA100 and USA300 isolates varied by United States census region. Nasal isolates were more resistant to tobramycin and clindamycin than blood isolates (55.9% and 48.8% of isolates versus 36.6% and 39.7%, respectively; for both,P< 0.05). The USA300 isolates were largely resistant to fluoroquinolones. High-level mupirocin resistance was low among allspatypes (<5%). SCCmectypes III and VIII, which are rare in the United States, were observed along with several unusual PFGE types, including CMRSA9, EMRSA15, and the PFGE profile associated with sequence type 239 (ST239) isolates. Typing data from this convenience sample suggest that in U.S. hospitalized patients, USA100 isolates of multiplespatypes, while still common in the nares, have been replaced by USA300 isolates as the predominant MRSA strain type in positive blood cultures.