Evaluation of cefiderocol activity for Gram-negative bacteria and performance of cefiderocol disk diffusion testing using multiple brands of Mueller Hinton Agar
Authors: Potter Robert , Wallace Meghan , Burnham Carey-Ann 
Published: October 2021

Abstract
Cefidericol is a cephalosporin-siderophore antibiotic for the treatment of multidrug resistant Gram-negative bacteria. Similar to other cephalosporin antibiotics, the lethal mechanism of action is due to inhibition of penicillin binding proteins leading to lysis of the bacteria. However, unlike previously developed antibiotics, the siderophore portion of cefidericol is able to bind iron and then be actively transported into the periplasmic space. To ascertain the feasibility of cefidericol antibiotic susceptibility testing in the Barnes-Jewish Clinical Microbiology Laboratory, we collected a cohort of multidrug Enterobacteriacae (5 Enterobacter cloace, 8 Escherichia coli, 12 Klebsiella pneumoniae), Pseudomonas aeruginosa (n=23), Stenotrophomonas maltophila (n=24), and Acinetobacter baumannii (n=25). We evaluated activity of cefidericol on these strains, and the performance of disk diffusion using three different brands of Mueller-Hinton Agar (BD, Hardy, and Remel). The reference method for comparison was an FDA-cleared broth microdilution panel containing cefidericol (ThermoFisher Scientific). Using CLSI breakpoints, we found that disk diffusion with BD agar had 96% categorical agreement for Enterobacterales, 100% for P. aeruginosa, 92% for A. baumannii, 96% for S. maltophila. We found that Hardy had 96% categorical agreement for Enterobacterales, 92% for P. aeruginosa, 92% for A. baumannii, 96% for S. maltophila. Finally, we found that Hardy had 96% categorical agreement for Enterobacterales, 92% for P. aeruginosa, 92% for A. baumannii, 96% for S. maltophila. Minor errors on any media never exceed 4% and there were no very major errors. Resistance to cefidericol within our cohort of selected antibiotic resistant bacteria was rare, one E. coli isolate and two P. aeruginosa isolates had minimal inhibitory concentrations (MICs) &gt; 32 μg/mL. The highest MICs for one isolate of A. baumannii and one isolate S. maltophila was 8 μg/mL and 4 μg/mL, respectively, both of which were intermediate. There was no difference in the distribution of zone disk diffusion diameter for A. baumannii or Enterobacterales. However, there was a significant difference in the distribution of zone disk diffusion diameters for P. aeruginosa and S. maltophila on BD vs Hardy agar. The median for P. aeruginosa on BD is 25 mm while it is 29 mm on Hardy. The trend for S. maltophila is the opposite as the median for BD was 31.5 mm and 28.5 mm for Hardy. Use of FDA vs CLSI vs EUCAST breakpoints significantly changes outcome of susceptibility testing for broth microdilution and disk diffusion. As one example for broth microdilution of A. baumannii, we had one isolate intermediate using CLSI breakpoints, 4 resistant using EUCAST breakpoints, and 4 resistant and 3 intermediate isolates using FDA breakpoints. Our work demonstrates that cefedericol testing can be performed in a routine format, with certain organismal differences on Mueller-Hinton agar, and that different interpretative criteria significantly change outcomes.