Cefiderocol Susceptibility Among Clinical Isolates of Ceftazidime/Avibactam-Resistant Pseudomonas aeruginosa: Exploring the Contribution of Efflux Pumps

Background/Objectives: This study investigated the emergence of cefiderocol (FDC) non-susceptibility among clinical isolates of ceftazidime–avibactam-resistant Pseudomonas aeruginosa in Chile, a setting where FDC has not yet been introduced into clinical practice. Methods: Seventy-seven ceftazidime–avibactam-resistant isolates were analyzed through antimicrobial susceptibility testing, efflux pump inhibition assays, and whole-genome sequencing. Results: Of the 77 isolates, 21 carried blaVIM, 33 carried blaPER, and 23 did not carry any extended-spectrum β-lactamase nor carbapenemases (NP isolates). Among FDC-non-susceptible isolates, seven of them were intermediate (MIC = 8 μg/mL) and eight were resistant (MIC ≥ 16 μg/mL). PER was significantly more prevalent in non-susceptible than susceptible isolates (p = 0.010). MIC90 were markedly higher for NP and PER isolates, being 32 μg/mL and 16 μg/mL, respectively, compared to 0.5 μg/mL for VIM isolates. Phenotypic analyses demonstrated that inhibition of efflux pumps significantly reduced FDC MIC, particularly among non-susceptible isolates, indicating that active efflux contributes to FDC resistance. Whole-genome sequencing revealed predominance of the high-risk clone ST309 among PER-producing isolates. Comparative genomic analyses showed that FDC non-susceptibility was associated with the cumulative burden of mutations affecting iron-uptake pathways, AmpC/PBPs, and efflux pumps/permeability. Accessory genome analysis identified an ISPa38-like element carrying mercury-resistance genes that was significantly more common among FDC-susceptible isolates. Conclusions: FDC non-susceptibility in P. aeruginosa is a complex phenotype driven by the interaction of acquired β-lactam resistance determinants, chromosomal adaptations, and active efflux mechanisms. The detection of this phenotype in an FDC-naïve setting suggests that resistance may emerge indirectly through collateral selection, raising concerns about the future effectiveness of FDC against high-risk multidrug-resistant lineages like PER-producing ST309.