Activity of fosfomycin and amikacin against fosfomycin-heteroresistant Escherichia coli strains in a hollow-fiber infection model

Abstract

Objectives:To evaluate human-like intravenous doses of fosfomycin (8g/Q8h) and amikacin (15mg/kg/Q24h) efficacy in monotherapy and in combination against six fosfomycin-heteroresistant Escherichia coli isolates using a hollow-fiber infection model (HFIM).Materials and methods:Six fosfomycin-heteroresistant E. coli isolates (4 with strong mutator phenotype) and the control strain E. coli ATCC 25922 were used. Mutant frequencies for rifampin (100mg/L), fosfomycin (50 and 200mg/L) and amikacin (32mg/L) were determined. Fosfomycin and amikacin MICs were assessed by agar dilution (AD), gradient strip (GSA) and broth microdilution (BMD) assays. Fosfomycin and amikacin synergies were studied by checkerboard and time-kill assays at different concentrations. Fosfomycin (8g/Q8h) and amikacin (15mg/kg/Q24h) efficacy alone and in combination were assessed using a HFIM.Results:Five isolates were resistant to fosfomycin by AD and BMD, but all susceptible by GSA. All isolates were considered susceptible to amikacin. Antibiotic combinations were synergistic in two isolates and no antagonism was detected. In time-kill assays, all isolates survived under fosfomycin at 64mg/L, although, at 307mg/L, only the normomutators and two hypermutators survived. Four isolates survived under 16mg/L amikacin and none at 45mg/L. No growth was detected under combination conditions. In HFIM, fosfomycin and amikacin monotherapies failed to sterilise bacterial cultures, however, fosfomycin and amikacin combination showed a rapid eradication.Conclusions.There may be a risk of treatment failure of fosfomycin-heteroresistant E. coli isolates using either amikacin or fosfomycin in monotherapy. These results support that the combination amikacin-fosfomycin can rapidly decrease bacterial burden and prevent the emergence of resistant subpopulations against fosfomycin-heteroresistant strains.

FIG 1

Drug interaction scores were calculated and are represented as δ scores on heatmaps obtained from the full ZIP model matrix analysis of the checkerboard assays using different fosfomycin and amikacin concentration ranges. The full matrix averages of three replicates are expressed as ZIP score ± the SD. Red and green areas represent synergy and antagonism, respectively. White rectangles show the maximum synergy area. Dose-response curves for fosfomycin and amikacin (mean ± SD) are found below each heatmap.

FIG 2

HFIM of fosfomycin-heteroresistant E. coli isolates and the control strain E. coli ATCC 25922 mimicking a fosfomycin dosage of 8 g/Q8h, an amikacin dosage of 15 mg/kg/Q24h, and dosages of fosfomycin (8 g/Q8h) and amikacin (15 mg/kg/Q24h) in combination. Filled symbols indicate total bacterial population concentrations. The limit of detection (LOD) was set at 2 log₁₀ CFU/ml.