Identification, genotypic relation, and clinical features of colistin-resistant isolates of Acinetobacter genomic species 13BJ/14TU from bloodstreams of patients in a university hospital

Abstract

Colistin resistance remains rare among clinical isolates of Acinetobacter species. We noted the emergence of colistin-resistant bloodstream isolates of the Acinetobacter genomic species (GS) 13BJ/14TU from patients at a university hospital between 2003 and 2011. We report here, for the first time, the microbiological and molecular characteristics of these isolates, with clinical features of Acinetobacter GS 13BJ/14TU bacteremia. All 11 available patient isolates were correctly identified as Acinetobacter GS 13BJ/14TU using partial rpoB gene sequencing but were misidentified using the phenotypic methods Vitek 2 (mostly as Acinetobacter baumannii), MicroScan (mostly as A. baumannii/Acinetobacter haemolyticus), and the API 20 NE system (all as A. haemolyticus). Most isolates were susceptible to commonly used antibiotics, including carbapenems, but all were resistant to colistin, for which it is unknown whether the resistance is acquired or intrinsic. However, the fact that none of the patients had a history of colistin therapy strongly suggests that Acinetobacter GS 13BJ/14TU is innately resistant to colistin. The phylogenetic tree of multilocus sequence typing (MLST) showed that all 11 isolates formed a separate cluster from other Acinetobacter species and yielded five sequence types. However, pulsed-field gel electrophoresis (PFGE) revealed 11 distinct patterns, suggesting that the bacteremia had occurred sporadically. Four patients showed persistent bacteremia (6 to 17 days), and all 11 patients had excellent outcomes with cleared bacteremia, suggesting that patients with Acinetobacter GS 13BJ/14TU-associated bacteremia show a favorable outcome. These results emphasize the importance of precise species identification, especially regarding colistin resistance in Acinetobacter species. In addition, MLST offers another approach to the identification of Acinetobacter GS 13BJ/14TU, whereas PFGE is useful for genotyping for this species.

FIG 1

Phylogenetic tree of the 11 Acinetobacter GS 13BJ/14TU bloodstream isolates (isolates 1 through 11) from Chonnam National University Hospital and reference strains of Acinetobacter species from the GenBank database, constructed from 450-bp zone 2 sequences of the rpoB gene. This tree was inferred using the neighbor-joining method and a 1,000-replicate bootstrap analysis. The sequences tested in this study are underlined. New rpoB gene sequences (submitted to GenBank) identified in this study include accession no. KC138716 (isolate 1), KC138717 (isolate 6), and KC138718 (isolate 5). The scale bar indicates the number of base substitutions per site.

FIG 2

Dendrogram of the MLST analysis of 11 bloodstream isolates (isolates 1 to 11) and reference strains of Acinetobacter species. A 2,976-bp concatenated sequence consisting of seven housekeeping genes was inferred using the neighbor-joining method and a 1,000-replicate bootstrap analysis. Corresponding MLST profiles present in the Institut Pasteur MLST database (see http://www.pasteur.fr/mlst) are shown in parentheses. The sequences tested in this study are underlined. New STs recovered in this study include ST198, ST199, ST200, and ST201. The scale bar indicates the number of base substitutions per site.

FIG 3

Restriction endonuclease analysis of genomic ApaI-digested DNA using the PFGE method for 11 nonduplicate Acinetobacter GS 13BJ/14TU bloodstream isolates; all 11 isolates showed different band patterns. Details are summarized in Table 1. Lanes 1 and 15, lambda ladder markers; lanes 2 to 12, isolates 1 to 11; lane 13, Acinetobacter GS 13BJ/14TU ATCC 17905; lane 14, A. baumannii ATCC 19606.