Unusual detection of an Acinetobacter class D carbapenemase gene, blaOXA-23, in a clinical Escherichia coli isolate.

Carbapenem resistance in Acinetobacter baumannii is typically mediated by the acquisition of class D -lactamases belonging to gene clusters of blaOXA-23-like, blaOXA-40-like, and blaOXA-58like (1). In the Enterobacteriaceae, carbapenem-hydrolyzing class D -lactamases are encoded by a dissimilar set of genes, the blaOXA-48-like genes (2). The presence of blaOXA-23 in Enterobacteriaceae is exceptional and has been described once in Proteus mirabilis, where it was chromosomally encoded (3). As a result of national surveillance efforts to characterize noncarbapenem-susceptible Enterobacteriaceae isolates, we detected a case of blaOXA-23 in Escherichia coli, which to the best of our knowledge is the first description of plasmid-borne blaOXA-23 in E. coli. The isolate was recovered from the urine sample of an elderly woman. However, the isolate was not likely to be a cause of infection, as urine microscopy did not show an increased number of white blood cells. The isolate exhibited resistance to -lactams, fluoroquinolones, and chloramphenicol. Sensitivity to tigecycline, colistin, and aminoglycosides was observed (Table 1). Comprehensive PCR screening and sequencing for -lactamase genes was performed (4, 5). The isolate carried blaOXA-23, which had 100% identity with A. baumannii blaOXA-23. It was also positive for TEM-1, OXA-1, and CMY-2 but was negative for other carbapenemases and extended spectrum -lactamases. The isolate had a multilocus sequence type of ST471. (http://www .pasteur.fr/recherche/genopole/PF8/mlst/EColi.html). Two large plasmids of approximately 50 and 100 kb were detected (6), with Southern blot hybridization localizing blaOXA-23 to the 50-kb plasmid. Solid-medium conjugation assays were performed to assess the transferability of blaOXA-23 from the clinical isolate to the azideresistant recipient E. coli J53 and to a spontaneous rifampin-resistant mutant of A. baumannii, ATCC 17978. However, we did not manage to isolate any transconjugants, suggesting that blaOXA-23 was harbored on a non-self-conjugative plasmid. The immediate nucleotide sequences flanking blaOXA-23 were determined by inverse PCR and the primer walking approach (7). E. coli blaOXA-23 is flanked by two copies of insertion sequence IS1 (Fig. 1). In contrast, in A. baumannii, blaOXA-23 is found as part of a transposon (Tn2006, Tn2007, Tn2008) associated with insertion sequence ISAba1 or ISAba4 (8, 9) (Fig. 1). One hundred eighty-three bases upstream, the E. coli blaOXA-23 start codon was completely identical to ISAba1 (GenBank accession number GQ861438.1). Within this region, 35 and 10 ISAba1-associated promoters were identified and likely to be functional (10) (Fig. 1). The 764-bp stretch after the blaOXA-23 stop codon comprised a partially truncated putative AAA ATPase identical to that in A. baumannii Tn2006, Tn2007, and Tn2008 (Fig. 1). We hypothesized that E. coli blaOXA-23 may be carried within a transposon, not unlike those identified in A. baumannii but interrupted by IS1. However, attempts to PCR map the region using both ISAba1 and ISAba4 as reference sequences did not yield amplicons, suggesting that it was in a genetic configuration different from that found in A. baumannii. In summary, we describe the novel detection of an E. coli isolate carrying an Acinetobacter blaOXA-23 gene associated with Enterobacteriaceae IS1 on a 50-kb non-self-conjugative plasmid. The discovery highlights the potential for the tremendous spread of carbapenemases.