Distribution of the blaOXA-23-containing transposons Tn2006 and Tn2008 in Australian carbapenem-resistant Acinetobacter baumannii isolates.

Sir, Worldwide, resistance to carbapenems, imipenem and meropenem is widespread among Acinetobacter causing infections and most resistant isolates are also resistant to many other antibiotic classes. Several class D b-lactamases can hydrolyse carbapenems, conferring resistance. The blaOXA-23 gene was the first to be found and was originally identified in a conjugative plasmid from a clinical Acinetobacter baumannii isolate recovered in the UK in 1985 and the plasmid was shown to be transferable to other Acinetobacter species, but not to Escherichia coli. The blaOXA-23 gene is now predominant among A. baumannii isolates in many countries. – 4 The blaOXA-23 gene originated in Acinetobacter radioresistens and has been mobilized into other species by ISs. However, resistance is only observed if expression is elevated by the presence upstream of a strong promoter, typically an appropriately oriented copy of ISAba1. To date, several transposons or transposon-like structures carrying blaOXA-23 have been identified. However, only Tn2006 (Figure 1a) has been shown to move and it has been found in several locations surrounded by a 9 bp duplication of the target sequence. Less is known about Tn2008, which consists of a single copy of ISAba1 adjacent to a continuous 1612 bp segment from an A. radioresistens isolate and, in the original sequence (GenBank accession number EU594641), Tn2008 is flanked by a 9 bp duplication of the target sequence (TTAATGTTT). The blaOXA-23-containing segment in both Tn2006 and Tn2008 is A. radioresistens-derived. However, it may have been picked up twice independently as the distance between the ISAba1 and the initiation codon of blaOXA-23 is 34 bp in Tn2006 and 27 bp in Tn2008 and there are single nucleotide differences in both the ISAba1 and the blaOXA-23-containing segment (Figure 1a). We recently identified a segment related to Tn2008 in a conjugative repAci6 plasmid pABUH1 (GenBank accession number AYOH01000010), but for comparison used the incorrect shorter sequence (GenBank accession number GQ861438) of Mugnier et al. rather than the original sequence (GenBank accession number EU594641). In fact, pABUH1 carries Tn2008 in the same location and flanked by the same 9 bp duplication as in the original report. To determine whether Tn2008 is present in Australian carbapenem-resistant A. baumannii isolates, the genome sequences of .100 isolates were examined for the presence of Tn2008. All of the global clone 2 (GC2) isolates described previously carried Tn2006 in AbGRI1-2 (Tn6167), as seen for A91. Among additional isolates, three 2008 GC2 isolates from Flinders Medical Centre (08325850, 08317005 and 08315000) carried Tn2008. In addition to the resistances conferred by blaOXA-23, these isolates were resistant to third-generation cephalosporins, tetracycline, spectinomycin, streptomycin, sulfamethoxazole, tetracycline and various aminoglycosides. Tn2008 was in a chromosomal gene encoding a short-chain dehydrogenase and surrounded by a different 9 bp duplication (AAGCGACTC) (Figure 1b). The sequence has been deposited in GenBank under accession number KP780408. BLAST searches were used to determine whether Tn2008 is a discrete entity that is found in further locations. A third location in the draft genomes of GC1 isolates AB5075 (GenBank accession number JHUI01000012), IS-58 (GenBank accession number AMGH01000100) and ANC 4097 (GenBank accession number APRF01000011) was identified. In this case, the interrupted gene is in the chromosome and annotated as a ‘phage-related’ membrane protein. The duplicated bases are CAATTCAAC (Figure 1b). After this work was completed, Tn2008 was found in a fourth location inside ISAba125, again surrounded by a 9 bp duplication. A further location for Tn2008 was in plasmid pNB09A30 from Acinetobacter baylyi (GenBank accession number JF731029), but in this case an additional two bases have been carried with it (Figure 1c). Tn2008 in this location interrupted by a copy of ISAba29 is found in the draft genome of the GC1 isolate Naval-83 (GenBank accession number AMFK00000000). The sequence reported as Tn2008 by Mugnier et al. appears to represent an incompletely sequenced version of Tn2008::ISAba29 (Figure 1c). The fact that Tn2008 has been found in four (potentially five) different locations, each of which is uninterrupted in other available sequences, is consistent with repeated transpositional movement of the DNA segment originally named as Tn2008, even though it includes only one ISAba1. Examination of the outer end of the blaOXA-23-containing segment revealed a match with 10 of 11 bases at the inner end of the 16 bp inverted repeats (IRs) of ISAba1, with the right-hand end of Tn2008 located 5 bp away (Figure 1d). The fortuitous presence of this potential transposase binding site in the A. radioresistens-derived segment appears to allow Tn2008 to move.