Cefiderocol: EUCAST criteria for disc diffusion and broth microdilution for antimicrobial susceptibility testing

Abstract

Objectives: The reproducibility of cefiderocol MIC determination using broth microdilution (BMD) in iron-depleted CAMHB (ID-CAMHB) was investigated, and the EUCAST disc diffusion (DD) method for cefiderocol susceptibility testing was developed and validated against reference BMD.

Methods: Cefiderocol values were determined for wild-type (WT) and non-WT isolates using BMD plates with ID-CAMHB (Thermo Scientific, Oakwood, USA) per EUCAST guidelines. DD was performed using standard EUCAST methodology on unsupplemented Mueller-Hinton agar with cefiderocol 30 μg discs. Control agents were included in all tests. MICs were correlated with zone diameters (ZD), and ZD breakpoints (BP) best corresponding to the MIC BPs were determined. Areas of technical uncertainty (ATU) were included where appropriate. External laboratory validation of cefiderocol DD was performed per the EUCAST SOP 9.2.

Results: MIC and ZD distributions for cefiderocol against WT isolates were established. Cefiderocol ZD BPs were set at susceptible ≥22 mm, resistant <22 mm for Enterobacterales and Pseudomonas aeruginosa and ATUs were decided. For Acinetobacter baumannii and Stenotrophomonas maltophilia, ZD cut-off values of ≥17 mm and ≥20 mm corresponded to MIC values of ≤2 and ≤0.5 mg/L, respectively. Cefiderocol ZDs for Escherichia coli ATCC 25922 (target 27 mm) and P. aeruginosa ATCC 27853 (target 26 mm) were within ±3 mm of the target values. For DD, there was no problematic variation between discs, media or laboratories.

Conclusions: DD is a robust and easy-to-perform method for cefiderocol susceptibility testing. For isolates with results in the ATU, an MIC test should be performed to confirm the results.

Figure 1.

MIC WT distributions for cefiderocol against E. coli (a), K. pneumoniae (b), P. aeruginosa (c), A. baumannii (d) and S. maltophilia (e), based on the WT distributions from this study [suggested tentative epidemiological cut-off values (TECOFFS) are shown]; white bars correspond to isolates with difficult-to-read MICs.

Figure 2.

Examples of reading cefiderocol endpoints when trailing occurs; examples from the EUCAST Reading Guide. This figure appears in colour in the online version of JAC and in black and white in the print version of JAC.

Figure 3.

MIC–zone diameter (ZD) correlations for cefiderocol for Enterobacterales (a), E. coli (b), K. pneumoniae (c), P. aeruginosa (d), A. baumannii (e), S. maltophilia (f). Each isolate was tested with cefiderocol discs from two manufacturers and on Mueller–Hinton media from two manufacturers, resulting in four correlates per isolate. Green = susceptible/below PK/PD MIC BP; orange/red = resistant/above PK/PD MIC BP; Red dotted line = ZD BP (Enterobacterales and P. aeruginosa) or ZD cut-off (A. baumannii and S. maltophilia).

Figure 4.

Inhibition zone diameter distributions for quality control strains from the external validation study versus test site, disc manufacturer and Mueller–Hinton manufacturer for (a) E. coli ATCC 25922 and (b) P. aeruginosa ATCC 27853. Grey shading indicates EUCAST QC ranges for E. coli ATCC 25922 (24–30 mm) and P. aeruginosa ATCC 27853 (23–29 mm). Com, commercially prepared; IH, prepared in-house. This figure appears in colour in the online version of JAC and in black and white in the print version of JAC.