Background. A community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infection has been defined as an MRSA infection in a patient who lacks specific risk factors for healthcare exposure. We sought to determine whether the absence or presence of these risk factors still predicts the phenotypic or genotypic characteristics of MRSA strains. Methods. All clinical MRSA isolates were prospectively collected at the University of Chicago Hospitals from July 2004 through June 2005. Patients were interviewed and/or their medical records were reviewed. Isolates underwent genotyping and susceptibility testing. Data on patients and isolates were stratified in accordance with 8 frequently cited criteria for the identification of CA-MRSA and compared for concordance. Results. Among 616 unique patients from whom MRSA isolates were recovered, 404 (65.6%) had risk factors for healthcare exposure. Of the 404 isolates recovered from these patients, 166 (41.1%) were clindamycin susceptible, 190 (47.0%) carried staphylococcal cassette chromosome mec (SCCmec) type IV, 145 (35.9%) carried the Panton-Valentine leukocidin genes (PVL +), and 162 (40.1%) were identified as sequence type (ST) 8 by multilocus sequence typing (MLST), all of which are characteristics commonly attributed to CA-MRSA strains. Conclusions. Association with the healthcare environment now has little predictive value for distinguishing patients with infection due to multidrug resistant MRSA isolates from those infected by CA-MRSA isolates, that is, isolates that are clindamycin-susceptible, PVL +, ST8, and/or contain SCCmec type IV. Defining CA-MRSA by the absence of risk factors for healthcare exposure greatly underestimates the burden of epidemic CA-MRSA disease.
Bibliographical noteFunding Information:
Financial support: CDC (R01 CCR523379 and R01 CI000373-01 to R.S.D and S.B., R01 CI000373-01 to M.Z.D.), the NIAID (R01 AI40481-01A1 to R.S.D and S.B., 1R01AI067584-01A2 to S.E.C. and M.A.H), and the Grant Healthcare Foundation (to R.S.D and S.B.).
Potential conflicts of interest: R.S.D. is supported by grants from the National Institute of Allergy and Infectious Diseases (NIAID) (1R01AI067584-01A2) and the Centers for Disease Control and Prevention (CDC) (1 U01 CI000384-01), and he has received grant support from Sage Products, Pfizer, Sanofi Pasteur, and Chlorox. He has served on paid advisory boards for Clorox, Sanofi Pasteur, GlaxoSmithKline, Pfizer, and the MRSA National Faculty Meeting (sponsored by Astellas and Theravance), and he has received lecture fees from Nabi Biopharmaceuticals and Pfizer. The other authors report no relevant conflicts.