Pseudomonas Remains Highly Susceptible to Newer Combinations

A 10-year US surveillance report in Open Forum Infectious Diseases found high in vitro activity for three newer beta-lactam/beta-lactamase inhibitor combinations against Pseudomonas aeruginosa. Across all isolates, ceftazidime-avibactam, ceftolozane-tazobactam, and imipenem-relebactam were active against 97.0% to 97.9%.

Investigators analyzed 14,304 patient-unique isolates collected consecutively from 56 US medical centers during 2015-2024 through the International Network for Optimal Resistance Monitoring program. The centers, mostly tertiary hospitals, represented 33 states and all 9 US Census Divisions. Local criteria had to identify isolates as a probable cause of infection. Susceptibility testing used broth microdilution at a central monitoring laboratory.

Overall susceptibility was 97.0% for ceftazidime-avibactam, 97.6% for ceftolozane-tazobactam, and 97.9% for imipenem-relebactam. Imipenem-relebactam testing covered 2020-2024. Ceftazidime-avibactam nonsusceptibility rose from 1.8% to 3.2% between 2015 and 2024. Ceftolozane-tazobactam nonsusceptibility rose from 1.4% to 2.5%. Imipenem-relebactam nonsusceptibility declined from 2.8% in 2020 to 1.3% in 2024.

Piperacillin-tazobactam was active against 80.2% of isolates overall. Its nonsusceptibility rate increased from 17.0% to 22.6%. Ceftazidime nonsusceptibility increased from 13.4% to 17.8%. Levofloxacin nonsusceptibility declined from 34.9% to 25.4%. Overall, 17.2% of isolates met the study definition for multidrug resistance, and 4.0% met the definition for difficult-to-treat resistance. Susceptibility among multidrug-resistant isolates was 83.0%, 85.9%, and 86.9% for the three newer combinations, respectively; among difficult-to-treat isolates, it was 65.3%, 71.9%, and 64.8%.

The authors said ceftazidime-avibactam and ceftolozane-tazobactam remained highly active despite more than 10 years of clinical use. They reported similar antipseudomonal activity for imipenem-relebactam. The absence of cefiderocol testing was identified as an important limitation.

Source: Sader HS, Mendes RE, Lock J, Papp-Wallace KM, Castanheira M. A 10-year trend of antimicrobial resistance in Pseudomonas aeruginosa from United States medical centers: results from the INFORM Surveillance Program (2015-2024). Open Forum Infect Dis. 2026. doi:10.1093/ofid/ofag504