Activity of vancomycin combined with linezolid against clinical vancomycin-resistant Enterococcus strains
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Department of Medical Microbiology, Faculty of Medicine, Istanbul University, Istanbul, Turkey
Submission date: 2019-04-16
Final revision date: 2019-07-08
Acceptance date: 2019-07-12
Online publication date: 2020-07-03
Because multi-drug-resistant Gram-positive bacteria have been isolated frequently worldwide and are difficult to treat, alternative treatment choices are required. Combination antibiotherapies have a distinct advantage over monotherapies in terms of their broad spectrum and synergistic effect. In the present study, it was aimed to investigate the in vitro activity of vancomycin combined with linezolid against clinical vancomycin-resistant enterococci (VRE) strains with high-level aminoglycoside resistance.

Material and methods:
A total of 30 randomly selected clinical VRE strains were studied. Susceptibility to agents tested was investigated using broth microdilution assay. The inoculum of strain was adjusted to approximately 5 × 105 CFU/ml in the wells. The results were interpreted in accordance with Clinical and Laboratory Standards Institute guidelines. In vitro activities of anti¬biotics in combination were assessed using the broth microcheckerboard technique. The fractional inhibitory concentration indexes (FICIs) were interpreted as follows: synergism, FICI ≤ 0.5; additive/indifference, FICI ≤ 0.5 – ≤ 4; antagonism, FICI > 4.

All strains were resistant to vancomycin and susceptible to linezolid. The MIC50,90 and MICrange values of antimicrobials were 512, 512, and 512–1024 µg/ml for vancomycin; 2, 2, and 2–4 µg/ml for linezolid. The rate of synergy was found to be 46.6% (14/30) for linezolid combined with vancomycin. No antago-nism was observed.

The results of the study suggest that this combination may contribute to the treatment of VRE infections for their synergistic effect and because no antagonism was observed.