Methicillin-Resistant Staphylococcus aureus USA300 Clone in Long-Term Care Facility

We performed a longitudinal analysis of 661 methicillin-resistant Staphylococcus aureus (MRSA) isolates obtained from patients in a long-term care facility. USA300 clone increased from 11.3% of all MRSA isolates in 2002 to 64.0% in 2006 (p<0.0001) and was mostly recovered from skin or skin structures (64.3% vs. 27.0% for non-USA300 MRSA; p<0.0001).

Isolates were tested for oxacillin resistance by the salt agar method, and the presence of the mecA gene was confirmed by PCR. Susceptibility to other antimicrobial drugs was determined by using microbroth dilution with the MicroScan WalkAway 96 instrument (Dade Behring, Deerfield, IL, USA). Inducible clindamycin-resistance testing (D-zone test) was performed by using the agar diskdiffusion method for isolates obtained during 2005-2006. Results were interpreted in accordance with guidelines (M7-A5) of the Clinical and Laboratory Standards Institute (Wayne, PA, USA; www.clsi.org).
Nonduplicated MRSA isolates were genotyped by pulsed-field gel electrophoresis (PFGE) after digestion of chromosomal DNA with SmaI (6), spa typing (7), and multilocus sequence typing (MLST) (8). USA300 was defined by the presence of Panton-Valentine leukocidin (PVL) genes (lukF-PV and lukS-PV) and the arginine catabolic mobile element (ACME), detected by PCR. Staphylococcal cassette chromosome mec (SCCmec) type was identified by using a PCR-based protocol (9). All strains underwent spa typing and were tested for PVL and ACME genes. MLST was conducted for 12 strains that could not be characterized otherwise.
Chi-square tests were used for bivariate analysis, and χ 2 tests for trend were used to evaluate secular trends. All statistical analysis was conducted by using Stata version 9.1 (Stata Corp., College Station, TX, USA).

Conclusions
Increasing incidence of MRSA infections in this LTCF during 1997-2006 is attributable to 2 clonal groups; USA100 predominated until 2003, and USA300 predominated during 2004 2006. Emergence of a new MRSA clone in healthcare facilities may be followed by a decrease in incidence of other MRSA clones (10). Unfortunately, this decrease was not observed in this LTCF, where the incidence of the previously predominant clone USA100 remained unabated. As a consequence, emergence of USA300 led to a 2-fold increase in MRSA incidence and an increase to 73% in the rate of methicillin resistance among S. aureus. Moreover, because USA300 has a tropism for skin and skin structure infections (3), its emergence caused a shift in MRSA specimen sources.
Although primarily encountered in urinary and respiratory specimens until 2001 in this study, as in previous studies performed in LTCFs, MRSA isolates have mostly originated from skin or skin structure since 2002. This finding has important implications for MRSA transmission and should be taken into account when designing infection control policies in LTCFs. For example, nasal decolonization as a means to prevent MRSA infection implies that MRSA reservoirs reside in endogenous sources. However, skinskin and skin-fomite contact may represent common alternative routes of transmission for USA300 (11).
In contrast to reports characterizing USA300 strains as typically not multidrug resistant (1), we found that up to 30.9% of USA300 isolates were multidrug resistant. This finding suggests that USA300 isolates were acquired under antimicrobial drug pressure in the LTCF or during a stay in another hospital rather than while in contact with the community. Results from this study and others illustrate the fitness trait of USA300 clonal lineage (12). To prevent further spread of multidrug-resistant USA300 in LTCFs would require enhanced infection control policies, which may include isolation or cohorting of infected patients, antimicrobial drug stewardship, and systematic use of alcohol-based handwashing products. However, given that these policies have proven difficult to implement in tertiary care hospitals, it may be even more challenging in LTCFs, which have limited staff and infection control resources (5,(13)(14)(15).
This study had some limitations. Increased incidence of MRSA-positive cultures could be related to changes in sampling policies in this institution (e.g., more frequent sampling, surveillance cultures). However, the increased incidence rate for MRSA is not likely related to these changes because the annual number of cultures positive for any pathogen gradually decreased over the study period,  Although our study was limited by its reliance on retrospective data collection, it illustrates the need for further investigations in LTCFs regarding risk factors and appropriate interventions to minimize further transmission of MRSA. LTCFs, long thought to be reservoirs of nosocomial MRSA clones, now emerge as an important reservoir for USA300 and could play a role in the emergence of multidrug-resistant USA300.