Activity of fosfomycin alone or combined with cefoxitin in vitro and in vivo in a murine model of urinary tract infection due to Escherichia coli harbouring CTX-M-15-type extended-spectrum β-lactamase

The efficacy of fosfomycin alone or combined with cefoxitin was investigated in vitro and in a murine model of urinary tract infection due to susceptible Escherichia coli CFT073-RR and its transconjugant CFT073-RR Tc (pblaCTX-M-15) harbouring a plasmid carrying the blaCTX-M-15 gene. In vitro, the combination of cefoxitin and fosfomycin was synergistic and bactericidal and prevented the emergence of fosfomycin-resistant mutants of CFT073-RR and CFT073-RR Tc (pblaCTX-M-15) that were selected with fosfomycin alone. In vivo, the combination conferred an advantage in terms of kidney sterilisation of mice infected with either strain compared with fosfomycin monotherapy.