Inhibition of fosfomycin resistance protein FosA by phosphonoformate (foscarnet) in multidrug-resistant gram-negative pathogens

FosA proteins confer fosfomycin resistance to Gram-negative pathogens via glutathione-mediated modification of the antibiotic. In this study, we assessed whether inhibition of FosA by sodium phosphonoformate (PPF) (foscarnet), a clinically approved antiviral agent, would reverse fosfomycin resistance in representative Gram-negative pathogens. The inhibitory activity of PPF against purified recombinant FosA from Escherichia coli (FosA3), Klebsiella pneumoniae (FosA^KP), Enterobacter cloacae (FosA^EC), and Pseudomonas aeruginosa (FosA^PA) was determined by steady-state kinetic measurements. The antibacterial activity of PPF against FosA in clinical strains of these species was evaluated by susceptibility testing and time-kill assays. PPF increased the Michaelis constant (K_m) for fosfomycin in a dose-dependent manner, without affecting the maximum rate (V_max) of the reaction, for all four FosA enzymes tested, indicating a competitive mechanism of inhibition. Inhibitory constant (K_i) values were 22.6, 35.8, 24.4, and 56.3 M for FosA^KP, FosA^EC, FosA^PA, and FosA3, respectively. Addition of clinically achievable concentrations of PPF (667 M) reduced the fosfomycin MICs by 4-fold among 52% of the K. pneumoniae, E. cloacae, and P. aeruginosa clinical strains tested and led to a bacteriostatic or bactericidal effect in time-kill assays among representative strains. PPF inhibits FosA activity across Gram-negative species and can potentiate fosfomycin activity against the majority of strains with chromosomally encoded fosA. These data suggest that PPF may be re-purposed as an adjuvant for fosfomycin to treat infections caused by some FosA-producing, multidrug-resistant, Gram-negative pathogens.