Amikacin resistance plasmids in extensively antibiotic-resistant GC2 Acinetobacter baumannii from two Australian hospitals

Sir, Previously, we studied a collection of extensively antibiotic-resistant Acinetobacter baumannii belonging to global clone 2 (GC2) from Australian hospitals in Sydney, Brisbane and Canberra.1 These 61 closely related carbapenem-resistant isolates shared a common set of resistance genes, but were subdivided into five different groups based on their aminoglycoside resistance phenotypes. One group harboured aacC1 (gentamicin resistance), aphA1 in Tn6020 (kanamycin and neomycin resistance) and aphA6 in TnaphA6 (amikacin, kanamycin and neomycin resistance). We subsequently showed that aacC1 and aphA1 were located in a chromosomal resistance island, AbGRI2-1, in two representatives of this collection that did not carry TnaphA6.2 The aphA6 gene was originally recovered from an A. baumannii isolate on a 63 kb plasmid that was transferrable to other Acinetobacter spp.3 However, this plasmid was not sequenced. Recently, TnaphA6 was found on a completely sequenced 70 kb conjugative plasmid, pAb-G7-2 (GenBank accession number KF669606),4 which was in a GC1 isolate from Melbourne. pAb-G7-2 belongs to the repAci6 plasmid family.4 The archetype of the repAci6 family,5 pACICU2, was originally reported to be cryptic.6 However, it was recently shown to carry TnaphA6 in the same position as pAb-G7-2 and to transfer amikacin resistance.7