AbstractBackgroundCefiderocol, a novel siderophore cephalosporin, is a promising therapeutic option for infections caused by multidrug-resistant Pseudomonas aeruginosa. We evaluated the activity of cefiderocol against carbapenem-resistant P. aeruginosa (Cr-Pa) isolates and investigated the potential mechanisms involved in resistance.Methods108 CR-Pa isolates collected from patients without prior exposure to the substance were studied. MICs of cefiderocol were determined by broth microdilution using iron-depleted cation-adjusted Mueller-Hinton broth. Whole genome sequencing was performed to investigate the potential resistance mechanisms by comparing resistant and susceptible P. aeruginosa isolates and identifying unique mutations in the resistant group.ResultsOf the 108 isolates, nine were resistant to cefiderocol with MIC values ranging from 4 to 32 mg/L. The genetic analysis revealed a broad spectrum of mutations in the resistant isolates associated with iron uptake systems, efflux pumps, AmpC β-lactamase and penicillin-binding proteins. The most frequently observed mutations among the resistant isolates were located in fptA, fpvB and chtA. Notably, the presence of carbapenemases did not correlate with cefiderocol resistance.ConclusionsOur findings show the low prevalence of cefiderocol resistance among CR-Pa isolates, showing its potential as an effective treatment option. However, the complex genetic landscape of resistance mechanisms, particularly mutations affecting iron transport and other TonB-dependent receptors, requires continuous monitoring and functional analyses to identify and manage potential resistance mechanisms. This study provides a foundation for future research to improve antimicrobial resistance prediction and develop targeted therapies against CR-Pa.