Carbapenem Heteroresistance in Acinetobacter baumannii and Pseudomonas aeruginosa Blood Isolates from a Turkish Tertiary Hospital: A Comprehensive Study.

To investigate the prevalence of carbapenem resistance and carbapenem heteroresistance (CAHR) in Acinetobacter baumannii and Pseudomonas aeruginosa. The susceptibility of 408 bloodstream isolates (256 A. baumannii and 152 P. aeruginosa) to imipenem (IMI) and meropenem (MER) was determined using the broth microdilution (BMD) method. CAHR was investigated using population analysis profiling (PAP) and diffusion-based tests, including the Kirby-Bauer disk diffusion and gradient tests. Carbapenemase genes in resistant and heteroresistant isolates were identified by polymerase chain reaction. Based on the Clinical Laboratory Standards Institute breakpoints, the resistance rates for A. baumannii were 88.3% for IMI and 87.5% for MER, while P. aeruginosa showed resistance rates of 43.4% for IMI and 46.7% for MER. Among the carbapenem-resistant A. baumannii strains, blaOXA-51 (100%) and blaOXA-23 (86.3%) were the most prevalent. Among carbapenem-resistant P. aeruginosa strains, blaOXA-10 (26.8%) was the most frequent. The PAP method revealed that the IMI- and MER-HR rates were 24.1% and 30%, respectively, in A. baumannii isolates, while in P. aeruginosa isolates, the rates were 57.9% and 13.8%, respectively. blaOXA-51 was detected in all carbapenem-resistant A. baumannii subpopulations, whereas blaOXA-23 was detected in 75% (n=12) of isolates. No carbapenemase genes were identified in any of the carbapenem-resistant P. aeruginosa subpopulations. Upon comparison of the results obtained with diffusion-based tests and PAP, diffusion-based tests were found to be unreliable for detecting CAHR.