Functional characterization and clinical implication of a novel epidemic carbapenem-resistant Acinetobacter baumannii genetic marker.

Objectives: The aim of the study was to evaluate the distribution and function of contact-dependent growth inhibition (CDI) systems associated with carbapenem-resistant Acinetobacter baumannii (CRAB) isolates.

Methods: Isolates were examined by multilocus sequence typing (MLST) and polymerase chain reaction (PCR) for the presence of CDI genes in CRAB and carbapenem-susceptible A. baumannii (CSAB) from patients with invasive disease in a medical center in Taiwan. Inter-bacterial competition assays were performed to characterize the in vitro function of the CDI system.

Results: A total of 89 (61.0%) CSAB and 57 (39.0%) CRAB isolates were collected and examined. ST787 (20/57; 35.1%) was the predominant sequence type among CRAB, followed by ST455 (10/57; 17.5%). More than half the CRAB (56.1%, 32/57) belonged to CC455 and more than one third (38.6%, 22/57) to CC92. A novel CDI system, cdi, was found in 87.7% (50/57) of the CRAB but in only 1.1% (1/89) of the CSAB isolates (P<0.00001). The cdi was also identified in 94.4% (17/18) of previously genome-sequenced CRAB isolates and only one CSAB isolate from Taiwan. Two other previously reported CDI (cdi and cdi) were not found in these isolates, except both were found in one CSAB. All six CRAB without cdi showed growth inhibition by a CSAB carrying cdi in vitro. All clinical CRAB isolates belonging to the predominant CC455 carried the newly identified cdi CONCLUSIONS: This CDI system was widespread in CRAB clinical isolates and appeared to be an epidemic genetic marker for CRAB in Taiwan. The cdi was functional in vitro in bacterial competition assay.