Linezolid

Susceptibility to 13 antibiotics was determined by the disc diffusion method and using the ADAGIO™ Automated System (Bio-Rad, Hercules, CA, USA) as described. The antibiotics tested included ampicillin (10 µg), penicillin (6 µg), ampicillin/sulbactam (20 µg), chloramphenicol (30 µg), vancomycin (5 µg), teicoplanin (30 µg), streptomycin (300 µg), gentamicin (120 µg), ciprofloxacin (5 µg), levofloxacin (5 µg), quinupristin-dalfopristin (15 µg), linezolid (30 µg) and tigecycline (15 µg) (Bio-Rad, Hercules, CA, USA)]. Susceptibility to aminoglycosides, glycopeptides, quinolones and β-lactam antibiotics was also determined by an E-test (M.I.C. Evaluator™, OXOID, Basingstoke, UK). The methods and the interpretation of the results followed the CLSI guidelines [32 ]. Enterococcus faecalis ATCC 29212 and Staphylococcus aureus ATCC 25923 were used as control strains.

The confirmed S. aureus isolates were subjected to antibiotic sensitivity test with linezolid (Bio-Rad), methicillin, ampicillin, ampicillin-sulbactum, amoxicillin-clavulanic acid, ticarcillin-clavulanic acid, imipenem-ethylenediaminetetraacetic acid, piperacillin-tazobactam, cefotaxime, ceftizoxime, ceftriaxone, ceftriaxone-tazobactam, enrofloxacin, ciprofloxacin, vancomycin and gentamicin antibiotic discs procured from HiMedia, India following CLSI (Clinical & Laboratory Standards Institute) guidelines [21 ].

The antimicrobial susceptibility was performed by the disk diffusion method according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) [9 ]. Fifteen antimicrobial agents on Mueller-Hinton agar plates (Becton Dickinson, Franklin Lakes, NJ, USA) were tested: oxacillin, cefoxitin, gentamicin, erythromycin, clindamycin, tetracycline, chloramphenicol, ciprofloxacin, trimethoprim/sulfamethoxazole, fusidic acid, linezolid, rifampicin, tigecycline and vancomycin (Bio-Rad, Marnes la Coquette, France) and penicillin G (Oxoid, Basingstoke, England). The phenotype of resistance to macrolide-lincosamide-streptogramin B was tested and interpreted according to the EUCAST. vancomycin susceptibility was determined with E-test method (bioMerieux, Marcy-l’Etoile, France). Multidrug resistance (MDR) was defined as a resistance to three or more classes of antibiotics.