Carbapenemase and efflux pump genes in Acinetobacter calcoaceticus Acinetobacter baumannii complex strains from Singapore

Sir, We recently published a paper describing the distribution of OXA and IMP b-lactamases in Acinetobacter spp. from Singapore. Since that manuscript was submitted, there has been accumulating evidence suggesting that OXA-51-type b-lactamases are ubiquitous in Acinetobacter baumannii sensu stricto. To resolve the identity of isolates from our original study that were identified using biochemical tests as A. baumannii, but were negative for blaOXA-51-type genes, we used PCR to re-test all 12 strains using a new set of universal blaOXA-51-type primers. We also sequenced the 16S rDNA gene (first 500 bp) using primer 8FPL described by Relman and in-house primer 515R (50-TTA CCG CGG CAG CTG GCA C-30), and the 16– 23S rRNA gene intergenic spacer using the primers described by Chang et al. In addition, we tested for the presence of the efflux genes adeB, adeE and adeY using the primers described by Chu et al., since it has been shown that the AdeABC multidrug efflux pumps are intrinsic to A. baumannii, whereas the AdeDE and AdeXY pumps are found predominantly in Acinetobacter genomospecies 3. The final definition of strains to species level was based primarily on the 16–23S rRNA gene intergenic spacer sequences but also took into consideration the results of all the other tests (Table 1). Five strains were positive for blaOXA-51-type genes with the new primer set and remain identified as A. baumannii. Another five strains were re-identified as either Acinetobacter genomospecies 3 or Acinetobacter genomospecies 13TU. Conflicting results did not allow us to discriminate within the Acinetobacter calcoaceticus–Acinetobacter baumannii complex for two isolates. The segregation of carbapenemase genes between the different Acinetobacter genomospecies is striking (Table 1). Of particular note, the IMP-4 metallo-b-lactamase is found in either Acinetobacter genomospecies 3 or Acinetobacter genomospecies 13TU but not in A. baumannii. Our data do support the hypothesis that blaOXA-51-type genes are intrinsic to A. baumannii. The distribution of efflux genes is also consistent with that reported by Chu et al. The A. calcoaceticus–A. baumannii complex is composed of four closely related species (A. calcoaceticus, A. baumannii, Acinetobacter genomospecies 3 and Acinetobacter genomospecies 13TU), which are difficult to differentiate phenotypically using traditional laboratory biochemical tests and commercial kits. In routine clinical practice, this distinction between the