Tracking Methicillin-Resistant Staphylococcus aureus among Community Clinical Isolates: A Five-Year Study

Background: Methicillin-resistant Staphylococcus aureus (MRSA) continues to be a significant public health threat worldwide. We sought to track MRSA strains among community isolates over the past 5 years, and analyse their resistance to commonly reported oral staphylococcal antimicrobial agents over the same period, in comparison to methicillin-susceptible S. aureus (MSSA). Methods: From January 2008 to December 2012, all eligible clinical specimens (n = 252,290), including urinary (U; n = 237,551); skin and soft tissue (SST; n = 6,760), respiratory (Rsp; n = 3,985), eye/ear/nasal (EEN; n = 3,223), and sterile site specimens (S; n = 771) were cultured on appropriate media for clinically relevant pathogens. S. aureus was identified as MSSA or MRSA by conventional methods and processed for antimicrobial susceptibility testing (AST) where warranted, as per CLSI guidelines. EEN isolates were excluded from AST. Resistance to nitrofurantoin (FM) and trimethoprim/sulfamethoxazole (SXT) was determined for U isolates; resistance to SXT, clindamycin (CC), and erythromycin (E) was determined for SST, Rsp, and S isolates, in accordance with CLSI guidelines. Only non-duplicate isolates were included in the analysis. Results: MRSA was isolated from 0.07%, 0.09%, 0.07%, 0.09%, and 0.09% of eligible specimens, accounting for 7.2%, 10.3%, 7.9%, 11.4%, and 10.1% of S. aureus in Year 1 (Y1), Y2, Y3, Y4, and Y5, respectively. MRSA and MSSA (n = 209; 2,019) were isolated from SST (n = 169; 1,294), U (n = 22; 288), EEN (n = 12; 376), Rsp (n = 6; 51), and S (n = 0; 10), respectively. MRSA vs MSSA resistance rates for SXT, FM, CC, and E were 1.5 vs 1.7%, 4.5 vs 3.1%, 30.9 vs 17.9% (P <0.001), and 79.4 vs 22.6% (P <0.001), respectively. There was a trend among MRSA vs MSSA isolates for decreasing resistance rates against E observed from Y1 (MRSA, 93.3% vs MSSA, 26.2%) through Y5 (71.1% vs 19.3%), respectively. Conclusions: Rates of MRSA isolation from clinical specimens remained relatively stable over the past five years among community isolates. There was a trend for declining resistance rates against E over the same period. MRSA strains are more likely to be resistant than MSSA strains against CC and E, while SXT continues to have good activity in vitro against community isolates of both MSSA and MRSA.