Use of Fosfomycin Etest® to Determine In Vitro Susceptibility of Clinical Isolates of Enterobacterales Other Than Escherichia coli, Non-Fermenting Gram-Negative Bacilli, and Gram-positive Cocci.

Clinical isolates of Enterobacterales other than Escherichia coli (EOTEC), non-fermenting Gram-negative bacilli, and Gram-positive cocci were tested for susceptibility to fosfomycin using Etest® and reference agar dilution. Applying EUCAST (v. 11.0, 2021) intravenous fosfomycin breakpoints, Etest® MICs for EOTEC showed essential agreement (EA), categorical agreement (CA), major error (ME), and very major error (VME) rates of 70.4%, 88.4%, 4.1%, and 32.1%, respectively. No species of EOTEC tested with acceptable rates for all of EA (≥90%), CA (≥90%), ME (≤3%), and VME (≤3%). Etest® MICs for Enterococcus faecalis, interpreted using CLSI oral/urine criteria (M100, 2021), showed EA, CA, minor error, ME, and VME rates of 98.5%, 81.2%, 18.8%, 0%, and 0%. Against Staphylococcus aureus, EA, CA, and ME rates were 84.1%, 98.7%, and 1.3% (EUCAST intravenous criteria). S. aureus isolates with fosfomycin MICs >32 μg/ml (resistant) were not identified by agar dilution. We conclude performing fosfomycin Etest® on isolates of S. aureus will reliably identify fosfomycin-susceptible isolates with low, acceptable rates of MEs and VMEs. Testing of urinary isolates of E. faecalis by Etest® is associated with an unacceptably high rate of minor errors (18.8%) but low, acceptable rates of MEs and VMEs when results are interpreted using CLSI criteria. Isolates of EOTEC tested by Etest® with resulting MICs interpreted by EUCAST criteria were associated with an unacceptably high VME rate (32.1%). In vitro testing of clinical isolates beyond E. coli, E. faecalis, and S. aureus to determine susceptibility to fosfomycin is problematic with current methods and breakpoints.