Synergistic Combination of Linezolid and Fosfomycin Closing Each Other’s Mutant Selection Window to Prevent Enterococcal Resistance

Abstract

Enterococci, the main pathogens associated with nosocomial infections, are resistant to many common antibacterial drugs including β-lactams, aminoglycosides, etc. Combination therapy is considered an effective way to prevent bacterial resistance. Preliminary studies in our group have shown that linezolid combined with fosfomycin has synergistic or additive antibacterial activity against enterococci, while the ability of the combination to prevent resistance remains unknown. In this study, we determined mutant prevention concentration (MPC) and mutant selection window (MSW) of linezolid, fosfomycin alone and in combination including different proportions for five clinical isolates of Enterococcus and characterized the resistance mechanism for resistant mutants. The results indicated that different proportions of linezolid combined with fosfomycin had presented different MPCs and MSWs. Compared with linezolid or fosfomycin alone, the combination can restrict the enrichment of resistant mutants at a lower concentration. A rough positive correlation between the selection index (SI) of the two agents in combination and the fractional inhibitory concentration index (FICI) of the combination displayed that the smaller FICI of linezolid and fosfomycin, the more probable their MSWs were to close each other. Mutations in ribosomal proteins (L3 and L4) were the mechanisms for linezolid resistant mutants. Among the fosfomycin-resistant mutants, only two strains have detected the MurA gene mutation related to fosfomycin resistance. In conclusion, the synergistic combination of linezolid and fosfomycin closing each other’s MSW could effectively suppress the selection of enterococcus resistant mutants, suggesting that the combination may be an alternative for preventing enterococcal resistance. In this study, the resistance mechanism of fosfomycin remains to be further studied.

Dose Optimization of Combined Linezolid and Fosfomycin against Enterococcus by Using an In Vitro Pharmacokinetic/Pharmacodynamic Model

“…According to the in vitro dynamic time-kill curve, the maximum bactericidal rate of coadministration was not significantly increased relative to fosfomycin at 4 to 6 h, but the bactericidal effect became progressively stronger after 6 h. This might occur because linezolid limits the rapid bactericidal effect of fosfomycin during early bacterial reproduction, but in later stages, when bacterial populations reach a stable period, the combination of the two drugs produces a more potent bactericidal effect. Moreover, in a previous study, we found that the combination of linezolid and fosfomycin against Enterococcus could effectively close the resistance mutation-selection windows of the other, which is a potential mechanism to explain the ability of coadministration to inhibit bacterial regeneration at a later stage ( 8 ). Additionally, the production of enterococcal biofilms in an in vitro model protected a subpopulation of bacterial inoculum from fosfomycin, thereby regenerating a subpopulation when fosfomycin concentration was below the MIC of the isolates ( 11 ).…”

“…Hence, combination therapy has been proposed to alleviate the development of drug resistance and increase efficacy. Previous studies have confirmed that linezolid combined with fosfomycin (FOF) can effectively inhibit vancomycin-resistant and -sensitive Enterococcus strains ( 7 , 8 ). However, these studies have been limited to the qualitative determination of the synergistic effects of the two drugs without a quantitative perspective on the mechanisms by which the drugs act on bacteria.…”

Development and Validation of an HPLC-UV Method for Quantitation of Linezolid: Application to Resistance Study Using in vitro PK/PD Model

“… 30 However, this value was twofold higher than that in clinical trial. Combination is another solution, Jiang et al 36 observed that linezolid combined with fosfomycin closing each other’s mutant selection window through Mueller–Hinton Agar dilution method.…”

Factorial design and post-antibiotic sub-MIC effects of linezolid combined with fosfomycin against vancomycin-resistant enterococci

“…Qi et al (14) suggested linezolid in combination with fosfomycin as a potential treatment for VRE infection. In a study of the synergistic combination of linezolid and fosfomycin, Jiang et al (15) showed that the combination effectively inhibits the selection of enterococcal-resistant mutants by closing each other's mutant selection window. Given the limited treatment options available, new combination therapies of the two above-mentioned drugs represent a potential treatment for VRE.…”