A 19-year longitudinal study to characterize carbapenem-nonsusceptible Acinetobacter isolated from patients with bloodstream infections and the contribution of conjugative plasmids to carbapenem resistance and virulence.

Background: This study aimed to characterize carbapenem-nonsusceptible Acinetobacter (CNSA) isolated from patients with bacteremia from 1997 to 2015.

Methods: A total of 173 CNSA (12.3%) was recovered from 1403 Acinetobacter isolates. The presence of selected β-lactamase genes in CNSA was determined by PCR amplification. The conjugation test was used to determine the transferability of metallo-β-lactamase (MBL)-carrying plasmids. Whole genome sequencing in combination with phenotypic assays was carried out to characterize MBL-plasmids.

Results: In general, a trend of increasing numbers of CNSA was observed. Among the 173 CNSA, A. baumannii (54.9%) was the most common species, followed by A. nosocomialis (23.1%) and A. soli (12.1%). A total of 49 (28.3%) CNSA were extensively drug-resistant, and all were A. baumannii. The most common class D carbapenemase gene in 173 CNSA was bla (32.4%), followed by ISAba1-bla (20.8%), ISAba1-bla (20.2%), and IS1006/IS1008-bla (11.6%). MBL genes, blabla, and bla were detected in 9 (5.2%), 20 (11.6%), and 1 (0.6%) CNSA isolates, respectively. Transfer of MBL genes to AB218 and AN254 recipient cells was successful for 7 and 6 of the 30 MBL-plasmids, respectively. The seven AB218-derived transconjugants carrying MBL-plasmids produced less biofilm but showed higher virulence to larvae than recipient AB218.

Conclusions: Our 19-year longitudinal study revealed a stable increase in CNSA during 2005-2015. bla, ISAba1-bla, and ISAba1-bla were the major determinants of Acinetobacter carbapenem resistance. MBL-carrying plasmids contribute not only to the carbapenem resistance but also to A. baumannii virulence.