Temporal evaluation of antimicrobial resistance development in serial Pseudomonas aeruginosa blood culture isolates

Aims This study aimed to evaluate antibiotic-specific temporal patterns of the first detection of phenotypic resistance in serial Pseudomonas aeruginosa blood culture isolates and within-patient susceptibility transitions over time.Material and methods This single-center retrospective observational cohort study included P. aeruginosa blood culture isolates from hospitalized patients between 2010 and 2025. In patients with repeated blood cultures, antimicrobial susceptibility was evaluated by antibiotic. We analyzed changes in resistance between the first and last isolates using the McNemar test, and assessed time–to–resistance using Kaplan–Meier analysis.Results A total of 1086 blood culture isolates from 523 patients were analyzed. 144 patients had repeated blood cultures and were included in the serial–isolate analyses. McNemar analysis showed more frequent transitions to resistance for imipenem, meropenem, piperacillin/tazobactam, cefepime, and amikacin. Phenotypic resistance was detected earlier for carbapenems (median 118 days for imipenem and 119 days for meropenem), whereas amikacin showed a longer median time to first detection of phenotypic resistance (403 days). Sensitivity analyses under alternative event-time assumptions showed broadly consistent temporal patterns.Conclusions The timing of first detection of phenotypic resistance in P. aeruginosa bacteremia varied across antibiotic classes, indicating that susceptibility profiles may change during serial follow-up.