Identification and molecular characterization of pSK41-bearing methicillin-resistant Staphylococcus aureus (MRSA) from ongoing, large-scale surveillance of invasive disease, United States 2005-06

Background: Recent reports have associated large, pSK41-like conjugative plasmids in Staphylococcus aureus and other staphylococci with resistance to multiple antimicrobial classes and possibly with the mobilization of other plasmids. The prevalence of pSK41-like plasmids among invasive methicillin-resistant S. aureus (MRSA) isolates in the United States is unknown. Objective: Identify and characterize invasive MRSA isolates bearing pSK41-like plasmids, determine their association with antimicrobial resistance, and identify molecular features of pSK41-like plasmid-bearing strains. Methods: All sterile-site MRSA isolates received between 2005-2006 from 8 geographically diverse metropolitan areas in the United States (ABCs surveillance sites) were screened for pSK41-like plasmids with both conventional and real-time PCR assays that amplified highly-conserved regions of the traE gene. These molecular data were correlated with isolate pulsed-field gel electrophoresis (PFGE) type, antimicrobial susceptibility, and the epidemiologic context of the corresponding infection. Results: A total of 2,455 isolates were evaluated. These represented 2,045 healthcare-associated infections, 386 community-associated infections and 24 of infections of unknown origin. Overall, 95 (3.9%) isolates were PCR-positive for traE. These were significantly more likely to be healthcare-associated: 95.8% of traE-positive isolates were HA versus 82.8% of traE-negative (p<0.01), representing 4.4% of HA isolates and 0.7% of CA isolates overall. Most traE-positive isolates were either USA100 (n=56; 58.9%) or USA300 (n=26; 27.3%), but the strain type diversity and distribution among traE-positive strains did not differ significantly from the traE-negative background. However, regardless of PFGE type, traE-positive isolates were more likely to be non-susceptible to one or more of: clindamycin, doxycycline, tetracycline, gentamycin, mupirocin, rifampin and trimethoprim-sulfamethoxazole (p<0.005) and levofloxacin (p<0.05). Most were non-susceptible to three or more different antimicrobial classes, although three traE-positive isolates were found to be resistant to oxacillin and penicillin alone. Plasmid carriage rates did not differ significantly by year. Conclusions: Isolates containing pSK41-like plasmids were relatively uncommon (<4.0%) among MRSA isolates from normally sterile sites. Prevalence of these plasmids did not differ by MRSA strain type. Nearly all pSK41-bearing isolates demonstrated broad, multi-class antimicrobrobial resistance. As these plasmids are conjugative and potentially associated with the acquisition and transmission of novel antimicrobial resistance, further monitoring is warranted.