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Efficacy of Vancomycin with Fosfomycin against Vancomycin-Resistant Enterococcus Strains Using Agar Dilution Method by Gulseren Aktas

Background: Enterococcus faecalis and Enterococcus faecium can cause community-acquired and nosocomial infections. Combination antibiotic therapies have a distinct advantage over monotherapies in terms of their synergistic effect. In the study, it was aimed to investigate in vitro activity of vancomycin combined with fosfomycin by agar dilution method against VRE strains.
Methods: A total of 30 clinical VRE strains were included in the study. Bacterial identifications of the strains were undertaken using conventional routine methods. The resistance to vancomycin was investigated using the broth microdilution method compared to fosfomycin by agar dilution method, and the results were interpreted in accordance with the CLSI guidelines. All experiments contained 25 mg/mL glucose-6-phosphate for fosfomycin. The fractional inhibitory concentration (FIC) indexes (FICI) were interpreted as synergism, FICI ≤ 0.5. Additionally, two strains in 30 VRE were studied to determine the time-kill curves to verify the synergistic results. Both antibiotics were studied at 1 x MIC in the tests. Viable counts were determined at 0, 4, 8, 12, and 24-hour intervals. Time-kill curves were constructed by plotting mean colony forming unit counts versus time.
Results: Susceptibility rate to fosfomycin was found at 16.6% (5/30). The MIC50,90 and MICrange values of antimi-crobials were 512, 512, and 512 - 1,024 mg/L for vancomycin, and 128, 128, and 64 - 160 mg/L for fosfomycin. The rate of synergism was found as 100%.
Conclusions: The result shows that the combination of vancomycin with fosfomycin gives hope that it may be an option in the treatment of infections caused by VRE.

DOI: 10.7754/Clin.Lab.2020.200437