In Vitro Activities of Mupirocin, Tigecycline, Ceftaroline, Vancomycin, Linezolid and Daptomycin in Clinical Isolates of Methicillin-Resistant Staphylococcus aureus by E-Test Methodology

Introduction: In 2013, the Center for Disease Control (CDC) designated methicillin-resistant Staphylococcus aureus (MRSA) as a serious threat. In addition to its intrinsic virulence, MRSA has become resistant to numerous antibacterial agents. In many instances, mupirocin is used empirically to decolonize patients harboring MRSA to decrease the possibility of progression to disease. In vitro susceptibility information is critical to identify patients who would benefit from use of mupirocin for decolonization and treatment of infections caused by MRSA. Methods: One-hundred and sixty-three recent MRSA single patient clinical isolates were collected from the Clinical Microbiology Laboratory. In-vitro susceptibility testing was performed using E-test methodology for tigecycline, ceftaroline, daptomycin, vancomycin, linezolid, and mupirocin. Results: Of the 163 MRSA isolates tested, >99% demonstrated susceptibility to tigecycline, ceftaroline, daptomycin, vancomycin, and linezolid. Seventy (43%) had vancomycin MICs ≥ 1.5 μg/ml, twenty-four isolates (15%) were resistant to mupirocin, and three appeared to express mupirocin hetero-resistance. Conclusion: While antibiotic susceptibility to mupirocin is not routinely performed in clinical microbiology laboratories, the level of resistance to mupirocin identified in this surveillance study suggests that susceptibility testing should be added to routine MRSA panels.