2.3. Antimicrobial Susceptibility Testing

The AMR profile of all S. aureus isolates was determined using the Kirby–Bauer disk diffusion method as described by the Clinical and Laboratory Standards Institute (CLSI) [28] with a panel of 38 antimicrobials representing 14 different antimicrobial classes. The antimicrobials commonly used for antimicrobial susceptibility testing include the fluoroquinolones (nalidixic acid (NA, 30 µg), ciprofloxacin (CIP, 5 µg), levofloxacin (LEV, 5 µg), norfloxacin (NX, 10 µg), ofloxacin (OFX, 5 µg), gatifloxacin (GAT, 5 µg), pefloxacin (PEF, 5 µg)), nonextended spectrum cephalosporins (first-generation cephalosporins: cephalexin (CL, 30 µg), cephradine (CE, 30 µg); second-generation cephalosporins: cefuroxime (CXM, 30 µg), cefaclor (CEC, 30 µg)), extended-spectrum cephalosporins (third-generation cephalosporins: cefotaxime (CTX, 30 µg), ceftriaxone (CRO, 30 µg), ceftazidime (CAZ, 30 µg), cefixime (CFM, 5 µg); fourth-generation cephalosporins: cefepime (FEP, 30 µg)), cephamycins (cefoxitin (FOX, 30 µg)), carbapenems (imipenem (IPM, 10 µg), meropenem (MEM, 10 µg)), tetracyclines (tetracycline (TE, 30 µg), oxytetracycline (OT, 30 µg), doxycycline (DO, 10 µg)), penicillins (ampicillin (AM, 10 µg), amoxycillin (AX, 10 µg), penicillin-G (P, 10 units), methicillin (MET, 5 µg), oxacillin (OX, 1 µg), cloxacillin (CX, 5 µg)), penicillins + β-lactamase inhibitors (amoxicillin-clavulanic acid (AMC, 30 µg)), aminoglycosides (gentamicin (CN, 10 µg), neomycin (N, 30 µg), tobramycin (TOB, 10 µg)), folate pathway inhibitors (trimethoprim-sulfamethoxazole (SXT, 25 µg)), lincosamide (clindamycin (DA, 2 µg)), glycopeptides and lipoglycopeptides (vancomycin (VA, 30 µg)), macrolides (azithromycin (AZM, 15 µg), erythromycin (E, 15 µg)), and polymyxins (colistin (CT)). For colistin, minimum inhibitory concentrations (MICs) were determined by broth microdilution method, according to the CLSI guidelines [28]. The interpretive category (susceptible, intermediate, and resistant) of each isolate was determined according to the CLSI guidelines [28], and in some cases, of European Committee on Antimicrobial Susceptibility Testing (EUCAST) [29]. Isolates resistant to at least one agent in three or more antimicrobial classes were defined as multidrug resistant (MDR) while isolates resistant to at least one agent in all but two or fewer antimicrobial classes were defined as possible extensively drug resistant (pXDR) [30].