Temporal Changes in Linezolid Minimum Inhibitory Concentration Values in Vancomycin-resistant Enterococci and Methicillin-resistant Staphylococcus aureus Strains

Introduction: Vancomycin-resistant enterococci (VRE) and methicillin-resistant Staphylococcus aureus (MRSA) infections are among the most common Gram-positive nosocomial infections. These isolates are resistant to most antibiotics, limiting the antibiotic options that can be used in treatment and causing treatment failure. Linezolid is an important option in the treatment of resistant Gram-positive infections, and came into use in Turkey in 2006. Linezolid-resistant Enterococci and Staphylococcus strains are rarely reported worldwide. The aim of this study was to investigate whether there was an increase in linezolid minimum inhibitory concentration (MIC) values in VRE and MRSA isolates over time. Materials and Methods: Thirteen VRE and 20 MRSA isolates from 2005-2009 (group 1), 18 VRE and 20 MRSA isolates from 2013-2014 (group 2), and seven VRE and 27 MRSA isolates from 2017-2018 (group 3) obtained from various clinical samples at Kocaeli University Medical Faculty Hospital were included in the study. The linezolid MIC values of the isolates were determined by broth microdilution method. The results were interpreted according to the European Committee on Antimicrobial Susceptibility Testing standards. Results: All of the VRE and MRSA isolates were susceptible to linezolid. Linezolid MIC50 and MIC90 values were 2 mg/l in VRE isolates in all three groups. In MRSA isolates, MIC50 was 2 mg/l in group 1, and 4 mg/l in groups 2 and 3, while MIC90 was 4 mg/l in all groups. Conclusion: Global rates of linezolid resistance has been reported to be <1% for S. aureus and VRE. There were no linezolid-resistant isolates in this study. However, we detected a significant increase in MIC50 and MIC90 values compared to most earlier studies performed in Turkey. This increase is expected due to the widespread use of linezolid over the years. The principles of rational antibiotic use should be applied to maintain the low resistance rates to linezolid, which is one of the few remaining options for the treatment of multidrug-resistant Gram-positive infections.