Methicillin-resistant Staphylococcus aureus (MRSA) with the staphylococcal cassette chromosome mec (SCCmec) type IV allele is most commonly associated with community-acquired MRSA (CA-MRSA) infection; however, such organisms have also been identified in the healthcare setting. The objective of the present study was to characterize the epidemiology of and clinical outcomes associated with SCCmec-IV MRSA infection acquired in the healthcare setting, compared with infection caused by MRSA of other SCCmec types.
We evaluated a cohort of 100 inpatients with MRSA infection that met the Centers for Disease Control and Prevention definition for healthcare-associated infection and compared the patients' demographic characteristics, the antimicrobial susceptibilities of the MRSA isolates, the infection types, and the associated clinical and microbiological outcomes. For each MRSA isolate, the SCCmec type and the presence of Panton-Valentine leukocidin (PVL) were determined by polymerase chain reaction methods.
SCCmec-IV MRSA isolates were isolated from 53 patients (42% of these isolates were positive for PVL), and SCCmec-II or SCCmec-III MRSA was isolated from 47 patients (3% of these isolates were positive for PVL). No differences were noted between the patients in the SCCmec-II/III group and the patients in the SCCmec-IV group with respect to age (median, 55 vs 50 years); sex (77% vs 64% of patients were male); medical service (surgical service, 60% in both groups; ICU admission, 55% vs 53%), Acute Physiology and Chronic Health Evaluation II score (median, 8 vs. 7); infection type; or underlying comorbidities, except for presence of a burn wound (13% vs 2%; P < .04). Patients in the SCCmec-II/III group were more likely to have multiple sites of infection (P = .006) and a longer length of stay (LOS) prior to detection of MRSA than were patients in the SCCmec-IV group (median, 4 vs 1 days; P < .001). Total LOS was significantly greater for patients in the SCCmec-II/III, compared with those in the SCCmec-IV group (P = .006). Multiple logistic regression identified liver disease and longer LOS prior to detection of MRSA as predictors of infection with SCCmec-II/III MRSA. Rates of susceptibility to clindamycin, gentamicin, ciprofloxacin, levofloxacin, and tetracycline was significantly greater among SCCmec-IV MRSA isolates, compared with type II/III isolates (P < or = .05). Compared with SCCmec-IV isolates acquired in the community, the susceptibility rates among healthcare-associated SCCmec-IV isolates was significantly less for clindamycin, gentamicin, and levofloxacin, indicating that these organisms may quickly acquire resistance to non- beta -lactam antibiotics, as do SCCmec-II/III strains.
SCCmec-IV MRSA appears to have become established in hospitals. The onset of infection caused by SCCmec-IV strains is earlier than the onset of infection with SCCmec-II/III strains; however, associated types of infection are similar. Infection with SCCmec-II/III MRSA is currently associated with an adverse impact on outcome, compared with infection with SCCmec-IV MRSA. Further research is warranted to determine the impact of SCCmec type IV strains in hospital settings.