Abstract
Acinetobacter baumannii is a successful opportunistic pathogen that can compete for iron under iron-limiting conditions. Here, large novel transposons that carry genes for synthesis and transport of the fimsbactin siderophores present in some A. baumannii strains were examined. Tn6171, originally found in the A. baumannii global clone 1 (GC1) lineage 2 isolate D36, includes tns genes encoding proteins related to the TnsA, TnsB, TnsC transposition proteins (50–59 % identity), TnsD targeting protein (43 % identity) and TnsE (31 % identity) of Tn7, and is found in the chromosome downstream of the glmS gene, the preferred location for Tn7, flanked by a 5 bp target site duplication. Tn6171 is bounded by 29 bp inverted repeats and, like Tn7, includes additional TnsB binding sites at each end. Tn6171 or minor variants were detected in the equivalent location in complete or draft genomes of several further A. baumannii isolates belonging to GC1 [sequence type (ST) 1, ST81, ST94, ST328, ST623, ST717], GC2 (ST2) and ST10. However, in some of these isolates the surrounding glmS region was clearly derived from a different A. baumannii lineage, indicating that the transposon may have been acquired by replacement of a segment of the chromosome. A recombination-free phylogeny revealed that there were several transposon acquisition events in GC1. The GC1 isolates were mainly lineage 2, but a potential third lineage was also detected. A related transposon, designated Tn6552, was detected in ATCC 17978 (ST437) and other ST437 isolates. However, the Tn6552 tnsD targeting gene was interrupted by an ISAba12, and Tn6552 is not downstream of glmS.
Highlights
The Gram-negative opportunistic pathogen Acinetobacter baumannii is mainly associated with nosocomial infections, including pneumonia, blood stream, urinary tract and wound infections [1]
A third gene cluster, known to be responsible for the synthesis and transport of fimsbactins [13], was found only in ATCC 17978 [8, 9, 13, 14]. This cluster includes 15 genes involved in siderophore biosynthesis, 3 genes involved in recognition and uptake of the ferric siderophores, and 2 genes encoding putative efflux pumps, a putative MFS efflux pump and the second efflux pump, which is a member of the multidrug and toxic compound extrusion (MATE) family [8, 9, 13]
Tn6171 contains 36 genes ranging in size from ~600 bp to 2.1 kbp, and four proteins encoded by genes at the left end share 50, 56, 59, 43 and 31% identity with the Tn7 transposition proteins TnsA, TnsB, TnsC, TnsD and TnsE, respectively
Summary
The Gram-negative opportunistic pathogen Acinetobacter baumannii is mainly associated with nosocomial infections, including pneumonia, blood stream, urinary tract and wound infections [1]. Infections caused by two globally distributed clones of A. baumannii, namely global clone 1 and 2 (GC1 and GC2), have been responsible for the majority of treatment difficulties due to high levels of resistance to multiple antibiotics [1,2,3,4]. Beyond drug resistance, various virulence mechanisms, such as biofilm formation or production of surface polysaccharides that protect this organism from the immune system or enable it to compete with its host for micronutrients, have been identified [5, 6]. Amongst the virulence factors identified to date, siderophore- mediated iron-acquisition systems are known to be of major. This cluster includes 15 genes involved in siderophore biosynthesis, 3 genes involved in recognition and uptake of the ferric siderophores, and 2 genes encoding putative efflux pumps, a putative MFS efflux pump and the second efflux pump, which is a member of the multidrug and toxic compound extrusion (MATE) family [8, 9, 13]
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