Impact of media brand on cefiderocol disk diffusion results

Cefiderocol is a siderophore cephalosporin that utilizes iron transport systems to cross cell membranes. This unique strategy complicates antimicrobial susceptibility testing (AST) due to variable iron content in media. While guidance for using iron-depleted media exists for broth microdilution (BMD), disk diffusion (DD) with commercial media is a common AST method in the clinical laboratory. We investigated cefiderocol DD result variability using multiple Mueller-Hinton agar (MHA) brands. DD results using Remel (Thermo Fisher Scientific, San Diego, CA), Hardy (Hardy Diagnostics, Springboro, OH), and BBL (Becton Dickinson, East Rutherford, NJ) MHA were compared to those of BMD using iron-depleted cation-adjusted Mueller-Hinton broth. BMD reproducibility, BMD trailing endpoints, and DD intra- and inter-brand variability in zones of inhibition were investigated. Forty-seven multidrug-resistant clinical isolates and three Antibiotic Resistance Bank isolates composed of Pseudomonas aeruginosa, carbapenemase (CP-) and non-carbapenemase-producing (non-CP-) carbapenem-resistant Enterobacterales (CRE), Acinetobacter baumannii complex, Stenotrophomonas maltophilia, and Burkholderia cepacia complex were tested. Categorical agreement (CA) ≥ 90% was only demonstrated using CLSI breakpoints with BBL agar. Intra- and inter-brand variability in DD were highest for P. aeruginosa and CRE, with 25% (5/20) and 16.7% (3/18) exhibiting discrepant AST interpretations, respectively. Isolates not susceptible to cefiderocol via BMD were commonly associated with AST interpretation errors and lower CA. Using commercial MHA for DD resulted in frequent AST interpretation discrepancies, particularly for isolates that were not susceptible to cefiderocol by BMD. Methods and quality control may need to be revisited to ensure the reliability of DD for cefiderocol AST.

The novel mechanism of action of cefiderocol overcomes a variety of resistance mechanisms associated with gram-negative bacteria and positions the agent as an attractive option for treating infections involving multidrug-resistant pathogens. The availability of accurate, timely antimicrobial susceptibility testing methods for cefiderocol in clinical microbiology laboratories is critical as cefiderocol-resistant isolates have been described and may contribute to treatment failure. Iron-depleted broth microdilution testing may not be feasible for use in many clinical laboratories. While disk diffusion is an appealing, practical method to implement, our data demonstrate reproducibility issues across agar brands, most notably for organisms that do not test susceptible to cefiderocol when using broth microdilution. Discrepancy errors and misclassifications of resistant isolates as susceptible, and susceptible isolates as resistant, may mislead clinicians and compromise the treatment efficacy. More work is needed to standardize practical yet reproducible methods for cefiderocol antimicrobial susceptibility testing.