Multicenter Evaluation of the Novel ETEST Fosfomycin for Antimicrobial Susceptibility Testing of Enterobacterales, Enterococcus faecalis, and Staphylococcus Species

Fosfomycin is a phosphonic acid derivative active against a wide spectrum of Gram-positive and Gram-negative pathogens. It is used for the treatment of uncomplicated urinary tract infections (uUTI) or severe infections by oral or intravenous (i.v.) administration. In order to improve its performance and robustness, the fosfomycin strip, an antibiotic gradient diffusion strip, was redeveloped and evaluated in the multicenter study summarized in this paper. ETEST fosfomycin (ETEST FO) clinical performance was evaluated by three study sites on 152 Enterococcus faecalis, 100 Staphylococcus spp. and 330 Enterobacterales in comparison with the CLSI and EUCAST agar dilution reference method. Referring to FDA performance criteria, the ETEST FO achieved 91.0% of essential (EA) and 99.0% of categorical agreement (CA) for Escherichia coli. In addition, 98.0% EA and 93.4% CA were achieved for E. faecalis, with no very major errors (VME) or major errors (ME). According to EUCAST breakpoints for intravenous fosfomycin use, Enterobacterales and Staphylococcus spp. also met ISO acceptance criteria for EA and CA (EA 91.5%, 94.0%, respectively, and CA 98.0% for both). A VME rate of 8.8% was observed for Enterobacterales but the MICs were within EA. A trend to predict lower MICs for Citrobacter spp., E. coli and Salmonella enterica and to predict higher MICs for Klebsiella pneumoniae MICs was observed, while ETEST FO should not be used for Enterobacter cloacae, because of low EA and a high VME rate. The study results support the efficiency of the novel ETEST FO, making it an easy-to-handle tool as a substitute to the classical agar dilution method.