Risk Factors Associated with Mupirocin Resistance in Methicillin-Resistant Staphylococcus aureus

Implementation of meticillin-resistant Staphylococcus aureus (MRSA) decolonisation programmes has been increasing and the emergence of mupirocin resistance has been reported. However, the patient-level risk factors associated with mupirocin resistance are not clear. In this study, independent predictors of mupirocin resistance in MRSA among Providence Veterans Affairs Medical Center patients with MRSA-positive culture dates between 1 July 2004 and 30 June 2008 were identified using a frequency-matched case–control study. Forty cases (mupirocin-resistant) were matched on culture date quarter and year to 270 controls (mupirocin-susceptible). The adjusted conditional logistic regression model identified three significant independent predictors associated with mupirocin resistance in MRSA: (1) exposure to mupirocin in the year prior to the culture date [odds ratio (OR): 9.84; 95% confidence interval (CI): 2.93–33.09]; (2) Pseudomonas aeruginosa infection in the year before the culture-related admission (4.85; 1.20–19.61); and (3) cefepime use in the year prior to culture (2.80; 1.03–7.58). In sensitivity analyses, previous mupirocin exposure was associated with low-level [minimum inhibitory concentration (MIC) 8–128 mg/L; 23 cases, 202 controls; OR: 6.32; 95% CI: 1.58–25.33] and high-level (MIC ≥256 mg/L; 17 cases, 151 controls; OR: 11.18; 95% CI: 1.89–66.30) mupirocin resistance. To our knowledge, this is the first case–control study to reveal a strong association between previous mupirocin exposure and subsequent mupirocin resistance in MRSA, with demonstrated robustness in low- and high-level mupirocin resistance. Mupirocin susceptibility monitoring is critical for facilities instituting decolonisation with mupirocin as increased use may reduce effectiveness through resistance