In vitro and intracellular activities of fosfomycin against clinical strains of Listeria monocytogenes

This study was designed to evaluate the potential role of fosfomycin as a therapeutic agent in human listeriosis. The in vitro activity of fosfomycin against 154 Listeria monocytogenes clinical isolates under conditions that mimic the induction of prfA expression was determined and was correlated with fosfomycin intracellular antimicrobial activity. In vitro, partial induction of prfA expression is achieved through bacterial growth in brain–heart infusion agar supplemented with activated charcoal (BHIC). A fosfomycin pharmacokinetic/pharmacodynamic breakpoint of ≤64 mg/L was estimated using a Monte Carlo simulation to assess the success of an intravenous fosfomycin dose of 300 mg/kg/day over 5000 individuals. Eighty strains (51.9%) were susceptible to fosfomycin in BHIC, with minimum inhibitory concentrations (MICs) of ≤64 mg/L; 13 strains (8.4%) had the epidemic clone (EC) marker. In addition, 27 strains (17.5%) had a three doubling dilutions reduction in the MIC from ≥1024 mg/L to 128 mg/L (96–128 mg/L by Etest). The fosfomycin modal MIC is lower under prfA expression. However, this effect is smaller in terms of clinical categorisation of isolates and can be influenced by the serotype and clonal type. In A549 cells, the reductions in bacterial inocula of the two susceptible isolates studied after 1 h and 24 h of incubation with fosfomycin at 0.5× the human maximum serum concentration (C_max) were 45.8% and 46.6%, and 93.8% and 99.1%, respectively. Slightly higher reductions were found with fosfomycin at 1× C_max. The resistant strain tested showed significantly lower reductions in all assays.