Acinetobacter baumannii is an important nosocomial pathogen, with isolates often resistant to multiple antibiotics. An increasing number of antibiotic resistance genes found in the mobile gene pool of Acinetobacter species are part of transposons mobilised by ISAba1, which includes a strong outward facing promoter that can drive the expression of adjacent genes. Some antibiotic resistance genes are known to have been mobilised from a bacterial chromosome. For example, the carbapenem resistance gene oxa23 originates from a gene (oxaAr) in the chromosome of Acinetobacter radioresistens that has been mobilised by ISAba1.
Tn6252 includes oxa235, another carbapenem resistance gene, bounded by inversely-oriented copies of ISAba1. The upstream ISAba1 provides a strong promoter that drives expression of the oxa235 gene, conferring moderate levels of resistance to the clinically important carbapenems. Here, we examined the distribution of Tn6252 and the origin of the oxa235 gene.
In the GenBank non-redundant database 21 A. baumannii sequences were found to have at least one copy of Tn6252 (August, 2021), with at least 17 different positions identified (12 in the chromosome and 5 in plasmids). The oxa235 gene was found to most closely match a gene encoding a class D β-lactamase reported to be from the chromosome of an Acinetobacter schindleri isolate (98.92% identity). The central segment of Tn6252 shared 92-96% DNA identity to the corresponding region in most A. schindleri chromosomes with complete or draft sequences available but was more distantly related to Acinetobacter lwoffii sequences (86-88% identity).
Phylogenetic analysis of variants currently assigned to A. lwoffii and A. schindleri chromosomes in RefSeq revealed two groups corresponding to those in A. lwoffii (oxa134 group) and in A. schindleri (oxa276 group). These intrinsic genes are not known to confer resistance to carbapenems. oxa235 clearly grouped with those from A. schindleri. While an intrinsic A. schindleri gene is the most closely related match for oxa235, it is possible that closer matches may be found in the future.