Contribution of mutational resistance mechanisms and acquired β-lactamases to cefiderocol/xeruborbactam susceptibility in <i>Pseudomonas aeruginosa</i>

Cefiderocol/xeruborbactam is a novel β-lactam/β-lactamase inhibitor combination in which the siderophore-cephalosporin cefiderocol is paired with xeruborbactam, a broad-spectrum inhibitor that targets class A to D β-lactamases. We evaluated the contribution of <i>Pseudomonas aeruginosa</i> resistance mechanisms to cefiderocol/xeruborbactam susceptibility. A panel of 61 <i>P</i>. <i>aeruginosa</i> PAO1 derivatives was tested, including 20 knockout mutants representing key chromosomal resistance mechanisms (e.g., <i>ampC</i> overexpression, efflux upregulation, porin loss, iron uptake deficiency) and 41 transformants producing major circulating β-lactamases. Xeruborbactam was assessed in combination with cefiderocol and cefepime at 4 and 8 mg/L and compared with taniborbactam. Additionally, 99 cefiderocol-resistant clinical <i>P. aeruginosa</i> isolates were evaluated. Cefiderocol/xeruborbactam retained activity against most <i>P. aeruginosa</i> mutants with chromosomally encoded resistance mechanisms. However, the <i>P. aeruginosa piuC-</i>defective mutant yielded increased cefiderocol minimum inhibitory concentrations (MIC = 2 mg/L), which could not be restored by xeruborbactam. Xeruborbactam significantly increased the activity of cefiderocol against the majority of <i>P. aeruginosa</i> PAO1 transformants, including those producing PER-1, SHV-12, KPC Ω-loop mutants, or NDM variants. Cefiderocol/xeruborbactam was active against IMP-type MBLs (which only weakly hydrolyze cefiderocol), including xeruborbactam-resistant enzymes. Relative to taniborbactam, xeruborbactam-based combinations showed similar activity against <i>P. aeruginosa</i> PAO1 transformants, but with slightly higher MIC values when tested against metallo-β-lactamase producers. This MIC increase in xeruborbactam-based combinations was partly because of the constitutive MexAB-OprM efflux in the <i>P. aeruginosa</i> background, as confirmed with <i>P. aeruginosa</i> PAO1 efflux mutants. Importantly, xeruborbactam restored susceptibility in 78% of 99 cefiderocol-resistant <i>P. aeruginosa</i> strains, reducing the MIC₉₀ from 64 to 4 mg/L. Cefiderocol/xeruborbactam shows promising activity against <i>P. aeruginosa</i>.