To assess the susceptibility of ceftolozane/tazobactam (C/T) and comparators to Pseudomonas aeruginosa isolates recovered from respiratory-tract-infections (RTI) between 2016–2022 in the French SMART study.

Antibiotic susceptibility testing and minimum inhibitory concentrations (MICs) of 717 P.aeruginosa isolates collected in five French hospitals were determined and interpreted according to the EUCAST-2022 guidelines. P. aeruginosa isolates resistant (R) to imipenem and/or C/T were screened by PCR for extended-spectrum-β-lactamases (ESBLs), AmpC and carbapenemase genes. The identified genes were sequenced and the variants determined.

All in all, 96.5 % of P. aeruginosa isolates were susceptible to C/T, comparable to the susceptibilities of meropenem-vaborbactam (MVB = 96.5 %), imipenem/relebactam (IMI/REL = 96.9 %) and ceftazidime-avibactam (C/A = 97.0 %). MIC₅₀ and MIC₉₀ for C/T were 0.5 and 2 mg/L respectively against the 717 isolates. Among the 242 isolates (33.7 %) resistant to at least one anti-Pseudomonas β-lactam, close to 90 % were susceptible to C/T, C/A, MVB and IMI/REL. Among the 80 isolates resistant to piperacillin-tazobactam, cefepime and ceftazidime, 76.3 % were susceptible to C/T and only IMI/REL and amikacin reached susceptibility exceeding 80 %. Among the 32 isolates resistant to imipenem and meropenem, susceptibility exceeding 60 % was observed only for IMI/REL, C/T, and C/A. For these strains, the MIC₅₀ of C/T was 2 mg/L, while that of C/A was at the resistance threshold (8 mg/L). IMI/REL had the strongest activity (72 %) against the 25 isolates resistant to C/T. Lastly, 53 imipenem and/or C/T-R isolates harbored a class C β-lactam (blaPDC) variant, and one of them also carried the bla_PER-1 gene and another, the bla_VIM-2 gene.

C/T is a reliable treatment option in RTI caused by P. aeruginosa.