Evaluation of the Susceptibility of Staphylococcus aureus Strains to Various Antibiotics

Abstract

Objective: Staphylococcus aureus continues to be a major health problem due to widespread methicillin resistance. The aim of this study was to determine the susceptibility to various antibiotics of 100 methicillin-resistant S. aureus (MRSA) and 100 methicillin-susceptible S. aureus (MSSA) strains isolated from various clinical samples between January 2016 and November 2019. Methods: Gradient test strips were used to test the susceptibilities of isolates to vancomycin, teicoplanin, and linezolid. The susceptibilities to other antibiotics were determined by automated system and evaluated based on EUCAST recommendations. Results: Seventy five percent of MRSA strains and 66% of MSSA strains included in the study were isolated from inpatients and 40% of MRSA and 21% of MSSA strains were isolated from intensive care patients. Isolates were obtained from tissue-abscess, blood, sterile body fluid, deep tracheal aspirate and sputum, urine, and catheter tip cultures, respectively. We did not detect S. aureus resistance to vancomycin, teicoplanin and linezolid in our study. Vancomycin, teicoplanin and linezolid MIC50/MIC90 values for MSSA strains were found to be 1/1.5 µg/ml, 0.75/1.5 µg/mL, 0.75/1 µg/mL and 1.5/2 µg/mL, 1.5/2 µg/mL, 0.75/1.5 µg/mL for MRSA strains, respectively. Based on MIC90 values, linezolid was 1.3 times more effective than vancomycin and 2 times more effective than teicoplanin in MRSA strains; linezolid was 1.5 times more effective than vancomycin and teicoplanin in MSSA strains. MRSA strains showed high resistance rates to all antibiotics, except daptomycin, tigecycline, cotrimoxazole and fusidic acid. All antibiotics were found to be highly effective in MSSA strains. Conclusions: MIC values of glycopeptides in MSSA isolates were found to be close to MIC values of MRSA isolates. Regarding the MIC90 values, linezolid was found to be more effective than glycopeptides. All other antibiotics were highly effective in MSSA strains