In vitro dynamics and mechanisms of cefiderocol resistance development in wild-type, mutator and XDR Pseudomonas aeruginosa.

Objectives: To analyse the dynamics and mechanisms of stepwise resistance development to cefiderocol in Pseudomonas aeruginosa.

Methods: Cefiderocol resistance evolution was analysed in WT PAO1, PAOMS (mutS mutator derivate) and three XDR clinical isolates belonging to ST111, ST175 and ST235 clones. Strains were incubated in triplicate experiments for 24 h in iron-depleted CAMHB with 0.06-128 mg/L cefiderocol. Tubes from the highest antibiotic concentration showing growth were reinoculated into fresh medium containing concentrations up to 128 mg/L for 7 consecutive days. Two colonies per strain and experiment were characterized by determining the susceptibility profiles and WGS.

Results: Evolution of resistance was significantly enhanced in PAOMS, but was variable for the XDR strains, including levels similar to PAOMS (ST235), similar to PAO1 (ST175) or even below PAO1 (ST111). WGS revealed 2-5 mutations for PAO1 lineages and 35-58 for PAOMS. The number of mutations in the XDR clinical strains ranged from 2 to 4 except for one of the ST235 experiments in which a mutL lineage was selected, thus increasing the number of mutations. The most frequently mutated genes were piuC, fptA and pirR, related to iron uptake. Additionally, an L320P AmpC mutation was selected in multiple lineages and cloning confirmed its major impact on cefiderocol (but not ceftolozane/tazobactam or ceftazidime/avibactam) resistance. Mutations in CpxS and PBP3 were also documented.

Conclusions: This work deciphers the potential resistance mechanisms that may emerge upon the introduction of cefiderocol into clinical practice, and highlights that the risk of resistance development might be strain-specific even for XDR high-risk clones.