Sastre-Femenia MÀ; Fernández-Muñoz A; Gomis-Font MA; Taltavull B; López-Causapé C; Arca-Suárez J; Martínez-Martínez L; Cantón R; Larrosa N; Oteo-Iglesias J; Oliver A

Abstract: Cefiderocol is a new siderophore-cephalosporin showing potent activity against Pseudomonas aeruginosa, including isolates showing extensively drug-resistant (XDR) or difficult-to-treat resistant (DTR) phenotypes. However, there is still a limited understanding of the potential resistance mechanisms. The objective of this study was to analyse the activity of cefiderocol in a nationwide Spanish survey, determine its stability against most relevant resistance mechanisms, and analyse potential drivers of resistance through whole-genome sequencing. Cefiderocol MICs were determined by broth microdilution in cation-adjusted iron-depleted Müller-Hinton broth for 1735 isolates from 66 Spanish hospitals and compared with those of a panel of 13 antipseudomonal agents. All XDR/DTR strains and those resistant to ceftolozane/tazobactam, ceftazidime/avibactam, imipenem/relebactam or cefiderocol were subjected to whole-genome sequencing (NovaSeq, Illumina, San Diego, California, US) and bioinformatic analysis, including specific pipelines developed for the assessment of acquired resistance determinants, the mutational resistome and iron-uptake genes. Cefiderocol showed the highest percentage of susceptibility (99.7%) and conserved activity (<5-10% resistance="resistance">2025 doi: 10.1016/j.ijantimicag.2025.107563 https://pubmed.ncbi.nlm.nih.gov/40609707