Prevalence and mechanisms of carbapenem resistance in Acinetobacter spp. isolated from South Korea

Abstract

The persistent increase of carbapenem-resistant Acinetobacter spp. is a global issue. This study was performed to investigate the prevalence of carbapenem-resistant Acinetobacter spp. and to determine resistance mechanisms in Korea.

A total of 418 non-duplicate Acinetobacter clinical isolates were collected from 28 general hospitals in Korea from October to December 2013. Species identification was performed using rpoB gene sequencing. Antimicrobial susceptibilities were determined by disk diffusion assays following CLSI guidelines. Genes encoding carbapenemases were amplified by PCR, and amplified products were directly sequenced. Multilocus sequence typing (MLST) was used for characterizing isolates.

Of 407 collected isolates, 356 (87.5%) were identified as A. baumannii, and non-baumannii Acinetobacter (NBA) isolates were identified as A. nosocomialis (n = 26, 6.4%), A. pitti (n = 22, 5.4%), A. berezinae (n = 1), A. gyllenbergii (n = 1), and A. haemolyticus (n = 1).

While only 12.4% (44/356) and 11.8% (42/356) of A. baumannii clinical isolates were susceptible to imipenem and meropenem, respectively, susceptible rates of NBA clinical isolates to imipenem (43/58, 84.3%) and meropenem (43/58, 84.3%) were higher.

The blaOXA-23 gene was detected in 310 A. baumannii isolates, while the insertion sequence ISAba1 element associated with the blaOXA-51-like gene was detected in only 13 A. baumannii isolates. The blaOXA-58 gene was detected in 2 NBA isolates. The blaOXA-24 and blaOXA-182 genes were not detected in this study. The blaIMP-1 gene was detected in 1 NBA isolate. While A. baumannii acquired carbapenem resistance by producing OXA-23 carbapenemase or overproducing OXA-51 or both of them, A. nosocomialis and A. pitti acquired that by producing IMP-1 or OXA-58.

Of the 356 A. baumannii clinical isolates, 292 isolates were identified as clonal complex 92 (CC92) by MLST, and remaining isolates were identified as CC110 (n = 7), CC397 (n = 3), CC20 (n = 20), and other sequence types (STs, n = 34).

The isolates belonging to CC92 exhibited different antibiogram compared to the isolates not belonged to CC92. Of 292 CC92 isolates, only 1.4% (4 isolates) were susceptible to carbapenems. Of 66 isolates not belonged to CC92, 62.1% (41 isolates) were susceptible to carbapenems.

Regardless of belonging to CC92 or not, the blaOXA-23 gene was detected in 301 A. baumannii isolates (301/356, 84.6%).

OXA-23-producing A. baumannii clinical isolates not belonging to CC92 and A. nosocomialis have emerged in Korea.