Loss and gain of aminoglycoside resistance in global clone 2 Acinetobacter baumannii in Australia via modification of genomic resistance islands and acquisition of plasmids.

OBJECTIVES: The objective of this study was to examine the evolution of carbapenem-resistant global clone 2 (GC2) Acinetobacter baumannii in Australia focusing on the complement of aminoglycoside resistance genes and their location in resistance islands and plasmids.

METHODS: Sixty-two carbapenem-resistant GC2 A. baumannii isolates with various aminoglycoside resistance profiles and resistance gene content that were recovered over the period 1999-2010 from hospitals on the east coast of Australia were examined. PCR was used to link relevant contigs retrieved from whole genomes sequenced using Illumina HiSeq and assembled de novo using Velvet. Resistance phenotypes were extended to include additional antibiotics using a disc diffusion assay.

RESULTS: Sixty-one isolates were ST208 (formerly ST92; Oxford scheme) and one was ST425. All isolates included the oxa23 carbapenem resistance gene in Tn2006 located in the same position in AbGRI1-2, along with the ISAba1-sul2-CR2Δ-tetA(B)-tetA(R)-CR2-strB-strA configuration. All isolates harboured either AbGRI2-1 carrying the aacC1 (gentamicin resistance) cassette or a variant derived from it via loss of some of the island content. When aacC1 was lost, aminoglycoside resistance was sometimes regained via acquisition of aadB (gentamicin, kanamycin and tobramycin resistance) in pRAY*-v1 or TnaphA6 (amikacin, kanamycin and neomycin resistance) in a repAci6 plasmid. A small cryptic plasmid or a deletion variant of this plasmid was always present and a large cryptic plasmid was also variably present.

CONCLUSIONS: The extensively antibiotic-resistant GC2 isolates from Sydney, Brisbane and Canberra appear to have arisen from a single import that was introduced into Australia in, or prior to, 1999 that then evolved and spread.