Aminoglycoside resistance in multiply antibiotic-resistant Acinetobacter baumannii belonging to global clone 2 from Australian hospitals

Objectives: To examine the distribution and context of aminoglycoside resistance genes in multiply antibiotic-resistant Acinetobacter baumannii isolates from Australia that are members of the global clone 2 and carry the bla(OXA-23) gene conferring resistance to carbapenems. Methods: Sixty-one multiply antibiotic-resistant A. baumannii strains isolated between 2000 and 2010 at six Australian hospitals that belonged to global clone 2 and carried the blaOXA-23 gene were studied. Various molecular techniques were used to determine their relatedness and to detect antibiotic resistance genes and insertion sequences. Structures surrounding the aminoglycoside resistance genes were sequenced. Results: The isolates all shared several antibiotic resistance genes, including the sul2 sulphonamide resistance gene, but varied in their pattern of resistance to aminoglycosides. The aminoglycoside resistance profiles of isolates were accounted for by four resistance genes-aadB, aacC1, aphA1b and aphA6-in various combinations. The aadB gene cassette was located at a secondary site on a 6 kb plasmid similar to pRAY. The aphA6 gene was in a transposon, TnaphA6, bounded by directly oriented copies of ISAba125. The aacC1 gene cassette in a class 1 integron and Tn6020 carrying aphA1b were always present together, but were not linked. Conclusions: Imipenem-resistant global clone 2 A. baumannii isolates containing blaOXA-23 have been present in Australian hospitals for at least 10 years. Variation in this global clone 2 type has occurred with the introduction of various aminoglycoside resistance genes carried on a small plasmid or within transposons.