Antimicrobial Susceptibility of Pseudomonas aeruginosa from Elderly Patients in Intensive Care Units of United States Medical Centers (2021–2025)

Objectives: The primary objective was to evaluate the antimicrobial susceptibility of Pseudomonas aeruginosa causing infection in elderly (≥65 years old) patients hospitalized in intensive care units (ICUs) of United States medical centers. Susceptibility results from isolates of elderly patients in ICUs were compared to isolates from elderly patients not in ICUs (elderly non-ICU) and adult ICU patients (18 to 64 years old; adult ICU). Methods: P. aeruginosa isolates were consecutively collected from 74 US medical centers in 2021–2025 and susceptibility tested by reference broth microdilution in the monitoring laboratory (Element Iowa City [JMI Laboratories]). The organism collection included 999 isolates from elderly ICU, 2027 isolates from elderly non-ICU, and 1022 isolates from adult ICU patients. Results: The most active agents against P. aeruginosa from all three patient groups were ceftazidime-avibactam (95.8% to 97.3% susceptible), ceftolozane-tazobactam (96.0% to 98.3% susceptible), imipenem-relebactam (97.6% to 98.7% susceptible), and tobramycin (91.4% to 94.7% susceptible). Susceptibility to piperacillin-tazobactam, ceftazidime, cefepime, meropenem, and imipenem were markedly lower among isolates from elderly and adult ICU patients compared to elderly non-ICU patients. Susceptibility to levofloxacin and tobramycin were lower among isolates from adult ICU patients compared to elderly ICU and non-ICU patients. Moreover, the frequency of multidrug-resistant (MDR) isolates was markedly higher among elderly (18.4%) and adult (22.4%) ICU patients compared to elderly non-ICU (11.0%) patients. An annual analysis of susceptibility to selected β-lactams showed a slight variation in susceptibility rates without a clear trend. Conclusions: Ceftazidime-avibactam, ceftolozane-tazobactam, and imipenem-relebactam were highly active and exhibited similar coverage against a large contemporary collection of P. aeruginosa isolates from ICU elderly, non-ICU elderly, and ICU adult patients. Cross-resistance among these β-lactamase inhibitor combinations (BLICs) varied markedly, indicating that all three should be tested in the clinical laboratory and available for clinical use.