Carbapenem heteroresistance in Acinetobacter baumannii and Pseudomonas aeruginosa blood isolates from a Turkish tertiary hospital: a comprehensive study.

AIMS: To investigate the prevalence of carbapenem resistance and carbapenem heteroresistance (CAHR) in Acinetobacter baumannii and Pseudomonas aeruginosa blood isolates.

METHODS

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 method. CAHR was investigated by population analysis profiling (PAP) and diffusion-based tests, including disk diffusion and antibiotic gradient tests. Carbapenemase genes in carbapenem-resistant and carbapenem-heteroresistant isolates were identified by polymerase chain reaction.

RESULTS

Based on the Clinical Laboratory Standards Institute suceptibility 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 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.

CONCLUSIONS

Upon comparison of the results obtained with diffusion-based tests and PAP, diffusion-based tests were found to be unreliable for detecting CAHR.