Antibiotic Susceptibility of S. aureus

Confirmed S. aureus isolate(s) from each positive nasal swab were tested for susceptibility to the following 12 classes of antibiotics: aminoglycosides, beta-lactams, cephalosporins, fluoroquinolones, lincosamides, macrolides, oxazolidinones, rifamycin, streptogramins, sulfonamide, nitrofuran, and tetracyclines (see Table S1). The Kirby–Bauer disk diffusion method was used to test each isolate’s susceptibility to all antibiotic classes. Interpretation of zones of inhibition was reported as susceptible, resistant, or intermediately resistant (where applicable) according to the Clinical and Laboratory Standards Institute guidelines (CLSI 2015). In erythromycin-resistant isolates, inducible clindamycin resistance was assessed using the D-zone test (Steward et al. 2005). S. aureus isolates that exhibited complete phenotypic resistance to at least three antibiotic classes were classified as MDRSA (Magiorakos et al. 2012). S. aureus isolates that exhibited complete resistance to oxacillin were classified as phenotypic MRSA (“MRSA phenotype”). MRSA isolates meeting the definition of MDRSA were classified as multidrug-resistant MRSA.