Genetic structures at the origin of acquisition and expression of the carbapenem-hydrolyzing oxacillinase gene blaOXA-58 in Acinetobacter baumannii

Abstract

Genetic structures surrounding the carbapenem-hydrolyzing oxacillinase gene blaOXA-58 were characterized in a series of OXA-58-positive Acinetobacter baumannii strains isolated from different countries. We showed that in most of the cases, acquisitions of the blaOXA-58-containing overall structure, including insertion sequence elements, may be likely the results of recombination events. In type strain A. baumannii MAD, the genetic structure surrounding the blaOXA-58 gene was bracketed by two 27-bp repeated sequences. The isolation of a clonally related OXA-58-negative A. baumannii isolate that possessed the same plasmid backbone as A. baumannii MAD but lacked this genetic structure indicated that the mechanism of acquisition could be reversible. Parts of the structure identified in A. baumannii MAD were conserved in other blaOXA-58-positive isolates from various European countries. Primer extension experiments showed that blaOXA-58 expression was related to promoter sequences brought by different insertion sequence elements, such as an ISAba3-like element, ISAba1, ISAba2, and IS18. This work identified novel structures at the origin of acquisition and expression of a carbapenem-hydrolyzing beta-lactamase identified in non-clonally related A. baumannii isolates.

FIG. 1.

Schematic map of bla_OXA-58-positive structures identified in A. baumannii isolates. (1), the structure identified in two bla_OXA-58-negative isolates; (2), the structure from A. baumannii MAD; (3) to (11), structures mapped in other isolates, as indicated in Fig. 1. Genes and their corresponding transcription orientations are indicated by horizontal arrows. The horizontal dotted line indicates the sequence separating the truncated ISAba3 element from the Re27-1 repeat sequence. The two different transcription regulator genes (araC1 and araC2), the threonine efflux protein gene (lysE) and the esterase gene (Est) are indicated. Vertical arrows are for the two Re27 sequences. Positions of the primers are indicated as referred to Table 2. The whole genetic structure which has been acquired or lost by recombination is indicated with the dashed lines. The figure is not to scale.

FIG. 2.

Promoter structures for bla_OXA-58 expression as determined by 5′-RACE experiment. The +1 initiation sites of transcription and the different start ATG codons are in bold. The inverted repeats of the IS elements are shaded in gray. The −35 and −10 motifs of the promoters are boxed. The target site duplications generated by IS transposition, if any, are double underlined. (A) Genetic structure identified from an ISAba2-positive A. baumannii CLA isolate; (B) genetic structure identified from an ISAba1-, ISAba2-, and IS18-negative A. baumannii 10 isolate; (C) genetic structure identified from an ISAba1-positive A. baumannii 13015 isolate; (D) genetic structure identified from an IS18-positive A. baumannii CH29 isolate.