Antibiotic resistance in global clone 2 Acinetobacter baumannii from Singapore

Abstract

Extensively antibiotic resistant Acinetobacter baumannii belonging to the global clone GC2 are a major challenge in the treatment of infections. This study aimed to analyse acquired antibiotic resistance in a selection of 20 XAR GC2 isolates from Singapore from 1996-2011. The relationships between these isolates were determined by examining the structures of the chromosomal resistance islands AbGRI1, AbGRI2 and AbGRI3, along with ISAba1 locations and MLST profiles. Plasmid diversity was also studied. Most isolates were ST208 or a single locus variant. All had ISAba1 upstream of ampC and a form of AbGRI1 and AbGRI2. AbGRI3 including the armA gene, conferring resistance to all clinical aminoglycosides, was only found in 15 isolates. Shared features allowed evolutionary lineages to be inferred. The 1996 ST208 isolate shared many features with most Australian isolates including AbGRI2-1, and 11 of 12 conserved ISAba1 positions. Four other isolates had AbGRI2-1 but ISAba1 in only 8 conserved positions. The other 15 isolates had AbGRI3 and an IS26-derived variant of AbGRI2-1. They have only 3 of the conserved ISAba1s and 2 more positions that are specific to the AbGRI3 lineage. One of the 6 AbGRI3 variants detected was associated with 6 extra ISAba1s. RepAci6 plasmids of 2 known and 2 novel backbone types were found in the collection; most were transfer proficient. Of the 14 plasmids, 2 were cryptic, 4 carried aphA6 (amikacin resistance) in TnaphA6, 10 had oxa23 in Tn2006 in AbaR4 and one had oxa23 in Tn2008. A RepAci1 plasmid with Tn2006 was mobilised in trans by a RepAci6 plasmid enabling a shared putative oriT to be found. The tet39 tetracycline resistance determinant and the msrE-mphE macrolide resistance genes were found in dif modules, bounded by inversely-oriented XerC-XerD binding sites, in a novel plasmid in one isolate. C and D dif modules were defined and found to usually alternate.