Carbapenem and colistin resistance patterns, mechanism and molecular epidemiology of Acinetobacter species in Korea

Abstract

Recent increasing carbapenem resistance in Acinetobacter species is very worrisome, resulting in a very limited antimicrobial treatment option. Colistin is the only potent therapeutic options available for treatment of nosocomial infections caused by multi-drug resistant Acinetobacter species. However, colistin-resistant Acinetobacter species of clinical isolates were reported. In present study, it was investigated that resistance patterns and mechanisms to carbapenems and colistin in Acinetobacter species isolated from clinical specimens from 19 hospitals in six provinces in Korea in 2008, and the epidemiological traits of carbapenem-non-susceptible Acinetobacter baumannii (CNSAB), as well as the usefulness of phylogenic grouping based on partial rpoB gene sequencing in defining the epidemiological traits of CNSAB.A total of 547 non-duplicate clinical isolates of Acinetobacter species were collected from 19 hospitals. Reduced susceptibility rates to imipenem or meropenem were 70% (272/388) for A. baumannii, 5% (4/82) for A. nosocomialis, 13% (8/62) for A. pitti, and 0% (0/13) for A. bereziniae. Colistin-resistant clinical isolates were one A. baumannii, one A. bereziniae and two Acinetobacter genomospecies 14 TU. Molecular characterization of the 272 CNSAB isolates using multilocus sequencing typing identified five sequence types (STs): ST92, ST75, ST137, ST138 and ST69. Those STs were clustered into clonal complex 92, sharing alleles at five to six of seven housekeeping gene loci. ST92 was the predominant ST and was associated with European clone II disseminated worldwide. CNSAB of CC92 carried the blaOXA-23 gene (n = 169), the blaOXA-51-like gene preceded by ISAba1 (n = 89) or both (n = 14). Sequence -based analysis of the rpoB gene showed single-nucleotide substitutions in zone 2 of the rpoB gene, resulting in division into five groups (A to E groups). Notably, all CNSAB isolates belonged to subgroup B-0 with a G428T substitution in zone 2. All of the other isolates belonging to group C, D and E were susceptible to carbapenems.In conclusion, carbapenem resistance rates of A. baumannii are very high, over 70%, whereas colistin resistance rate of Acinetobacter species is very low until now. CNSAB isolates of CC92 with the G428T substitution in zone 2 of the rpoB gene are disseminated nationwide in our country. A. baumannii with the G428T substitution in zone 2 may be more likely to acquire carbapenem resistance than are other isolates.