Antimicrobial Susceptibilities of Commonly Encountered Bacterial Isolates to Fosfomycin Determined by Agar Dilution and Disk Diffusion Methods

Abstract

ABSTRACTWe studied the antimicrobial activity of fosfomycin against 960 strains of commonly encountered bacteria associated with urinary tract infection using standard agar dilution and disk diffusion methods. Species studied included 3 common species ofEnterobacteriaceae,Pseudomonas aeruginosa,Acinetobacter baumannii, andStenotrophomonas maltophilia; methicillin-susceptible and -resistantStaphylococcus aureus; and vancomycin-susceptible and resistantEnterococcus faecalisandE. faecium. MICs and inhibition zone diameters were interpreted in accordance with both the currently recommended Clinical and Laboratory Standards Institute (CLSI) criteria for urinary tract isolates ofEscherichia coliandEnterococcus faecalisand the European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria forEnterobacteriaceae. Tentative zone diameter interpretive criteria were developed for species not currently published by CLSI or EUCAST.Escherichia coliwas uniformly susceptible to fosfomycin, as were most strains ofKlebsiella pneumoniaeandEnterobacter cloacae. A. baumanniiwas resistant to fosfomycin, while the prevalence of resistance inP. aeruginosaandS. maltophiliawas greatly affected by the choice of MIC breakpoint. New tentative zone diameter criteria forK. pneumoniae,E. cloacae,S. aureus, andE. faeciumwere able to be set, providing some interim laboratory guidance for disk diffusion until further breakpoint evaluations are undertaken by CLSI and EUCAST.