A-042. Genetic Basis of Multi-Resistance in Nosocomial Strains of Acinetobacter baumannii

J. K. Mak1, M-J. Kim1, J. Pham2, J. Tapsall2, P. A. White1;
1Univ. of New South Wales, Sydney, AUSTRALIA, 2Prince of Wales Hosp., Sydney, AUSTRALIA.

Multi-resistant Acinetobacter baumannii are a major concern in many hospitals world-wide. Between May 2005 and March 2007, a collection of 37 A. baumannii was obtained from four large hospitals in Sydney, Australia. Eighteen isolates were responsible for two nosocomial outbreaks, while the remaining 19 isolates from the other two hospitals were collected sporadically over eight months. All 37 isolates were multiresistant, as they were resistant to between six and 21 of the 23 antibiotics tested. The aim of this study was to characterise the mechanisms of antibiotic resistance by identifying the gene/s responsible for each resistance phenotype. Resistance genes were detected using polymerase chain reaction (PCR) and repetitive extragenic palindromic (REP)-PCR was used to assess clonality. Thirteen isolates carried class 1 integrons, amongst these four different gene cassette arrays were found, encoding resistance to aminoglycosides, sulphonamides and two open reading frames with no known function. Twenty nine isolates contained the intrinsic gene ampC, with ISAba1. This insertion sequence upregulates transcription by providing an alternative promoter, leading to higher levels of cephalosporin resistance. In the same isolates, ISAba1 was found upstream of the oxacillinase gene blaOXA-23, and accounted for carbapenem resistance. These 29 isolates were also positive for tetB, which conferred tetracycline resistance. blaTEM-1 was found in 31 isolates which contributed to beta-lactam resistance. Fluoroquinolone resistance in 36 isolates was conferred by a Ser-83 to Leu mutation in GyrA. The streptomycin resistance gene strB was found in 30 isolates. A total of nine resistance genes and the point mutation in gyrA were identified which accounted for resistance to 19 of the 23 antibiotics. REP-PCR identified eight clonal types, with one dominant clonal type seen in outbreak and sporadic cases. Previous studies have shown differences in phenotype and genotype between outbreak and sporadic strains, but this was not observed in this study. The genetic basis of nearly every resistance phenotype in outbreak and sporadic strains of A. baumannii was elucidated.