Structure and context of Acinetobacter transposons carrying the oxa23 carbapenemase gene

The oxa23 gene encoding the OXA-23 carbapenemase (and several minor variants of it) is widespread in Acinetobacter baumannii clinical isolates and compromises treatment with carbapenem antibiotics. The gene is derived from the chromosome of Acinetobacter radioresistens where it is an intrinsic gene, here designated oxaAr. In A. baumannii and other Acinetobacter species, oxa23 is usually preceded by an IS, ISAba1, which supplies the strong promoter required for the gene to confer clinically relevant levels of resistance. The oxaAr gene appears to have been mobilized twice creating Tn2008 and Tn2008B, both of which consist of a single ISAba1 and an A. radioresistens-derived fragment. Tn2006 and Tn2009 are clearly derived from Tn2008B and are each made up of Tn2008B with an additional segment of unknown origin and an additional ISAba1, creating a compound transposon. Tn2006, Tn2008 and possibly Tn2008B are globally disseminated, while Tn2009 has as yet only been found in China. Of the four ISAba1-associated transposons, Tn2006 has been most frequently observed worldwide and Tn2006 in Tn6022, known as AbaR4, appears to contribute significantly to the dissemination of oxa23. Moreover, AbaR4, Tn2006, Tn2008 and Tn2009 have each been found in conjugative plasmids, further facilitating their spread.