Gold standard susceptibility testing of Fosfomycin in Staphylococcus aureus and Enterobacterales using a new agar dilution panel

Abstract Objectives Many clinical laboratories have difficulty in routinely performing in vitro fosfomycin susceptibility testing using the agar dilution (AD) method, considered the gold standard method. The objective of our work was to evaluate a rapid commercial fosfomycin agar dilution panel against clinical Staphylococcus aureus and Enterobacterales strains, in two different centres located in Italy and in the UK. Methods A total of 99 Enterobacterales (mostly Escherichia coli and Klebsiella pneumoniae) and 80 S. aureus clinical isolates were used to evaluate the commercial device, a 12-well panel containing fosfomycin incorporated into CA-MH agar supplemented with 25 mg/L of glucose-6-phosphate (Liofilchem S.r.l., Roseto degli Abruzzi, Italy). Testing was performed in two centres (Italy and UK) and kit results were compared against the gold standard in-house AD MIC method. Results According to the EUCAST breakpoints, fosfomycin inhibited 61% of the S. aureus strains, and 76% of the Enterobacterales isolates tested by the AD reference method. There was a Categorical Agreement of 100% and an Essential Agreement of 91.25% for S. aureus; while, the Enterobacterales strains showed a CA of 94% and an EA of 97%. No evaluation errors among S. aureus were observed, while 5% ME and 1% VME for the Enterobacterales were observed. Conclusions Our results confirmed the feasibility to provide fosfomycin susceptibility by the AD commercial panel as a routine substitution of the AD test. The few differences observed were only in strains with MIC around the breakpoint used.