In Vitro Susceptibility to Glycopeptides, Linezolid, Tigecycline, and Ceftaroline against Methicillin Resistant Staphylococcusaureus Isolates in a Tunisian University Hospital

Background: Vancomycin remains the drug of choice for treatment of serious methicillin-resistant Staphylococcus aureus (MRSA) infections. However, Strains with reduced susceptibility to vancomycin have been reported in several countries around the world.Therefore, new antibiotics are introduced into the treatment.

Objectives: The present study aimed to determine the vancomycin, teicoplanin, linezolid, tigecycline and ceftaroline susceptibility patternand to investigate the presence and the frequency of heterogeneous vancomycin intermediate S. aureus (hVISA) among clinical isolates of MRSA in Tunisia.

Methods: A total of 162 non duplicate MRSA strains isolated between 2017 and 2018 were investigated. Vancomycin, teicoplanin, tigecycline, linezolid and ceftaroline minimum inhibitory concentrations (MIC) values were detected by broth microdilution method and interpreted according to the European Committee on Antimicrobial Susceptibility testing criteria. Etest GRD, Etestmacromethod, Mueller-Hinton screen agar, and population analysis profile-area under the curve (PAP-AUC) methods were used to detect hVISA.

Results: The MIC50, MIC90, and MIC ranges were respectively 1, 1, and 0.5-2 µg/ml for vancomycin; 1, 2, and 0.125-4 µg/ml for teicoplanin; 2, 2, and 0.5-4 µg/ml for linezolid; and 0.25, 0.5, and 0.064-0.5 µg/ml for tigecycline. Twelve strains were suspected as hVISA by EtestGRD, 28 by screen agar, and one by Etestmacromethod, but only one strain was confirmed hVISA by PAP-AUC. No vancomycin, tigecycline and linezolid resistance was found among MRSA isolates. Four strains were teicoplanin resistant, and four were intermediate to ceftaroline.

Conclusion: The prevalence of hVISA, teicoplanin and ceftaroline resistance are low. Linezolid and tigecycline were found to be highly active against MRSA isolates. Therefore, they could be considered as alternative agents for the treatment of serious infections. Continuous and regular monitoring of MICs at local and regional level is necessary to guide clinician in their empiric antibiotic selection.

Keywords: Methicillin resistant Staphylococcus aureus, glycopeptide, linezolid, tigecycline, ceftaroline, MIC.