Medium compositions and culture conditions for the assay of fosfomycin susceptibility by Etest

The Etest (AB Biodisk, Solna, Sweden), a susceptibility testing method, was used for fosfomycin and was evaluated by comparison with the agar dilution method for 73 clinical isolates of Escherichia coli, including two resistant strains, and an optimal Etest method for fosfomycin was determined. Media and culture conditions greatly affected the minimum inhibitory concentrations (MICs) determined by the Etest for fosfomycin, as shown for the agar dilution methods. Our results showed that the most favorable conditions for the Etest for fosfomycin were with Mueller-Hinton agar (Becton-Dickinson Japan, Tokyo, Japan) under aerobic conditions. However, the MICs for the resistant strains were much higher than those determined by agar dilution methods, using Nutrient agar (Becton-Dickinson Japan) under anaerobic conditions. The addition of glucose-6-phosphate did not significantly affect the Etest results.