Sydney Medical School, School of Molecular Bioscience
Abstract
Analysis of multiply antibiotic-resistant Acinetobacter baumannii belonging to Global Clone 1. A. baumannii resistant to multiple antibiotics challenges the treatment of infections caused by this organism. Multiply-antibiotic resistant isolates mainly belong to two major global clones GC1 and GC2. The objective of this study was to analyze the antibiotic resistance in Australian GC1s. Twenty six MAR GC1 isolates were recovered from Australian hospitals and examined to determine the genetic basis of their resistance. The results showed the variety of mobile elements, genomic islands, plasmids, transposons and insertion sequences that incorporate antibiotic resistance genes. AbaR islands were found to carry genes conferring resistance to multiple antibiotics. IS26-mediated deletions and homologous recombination between Tn6018 copies in the AbaR islands were shown to be responsible for generating new variants. AbaR0, the ancestor of AbaRs was found and all of the variants seen so far could be derived from it. Carbapenem resistance was rare but three strains carried the oxa23 carbapenem resistance gene in an AbaR4 island, which includes a backbone related to that of AbaR islands. In one isolate, AbaR4 was found where AbaR-type islands are usually found indicating that transposon backbone of AbaR4 can target the same position as AbaR0/AbaR3. The other two strains carried AbaR4 in a conjugative plasmid that can potentially disseminate the oxa23 gene into strains of different types. The small plasmid pRAY* was responsible for introducing the tobramycin resistance gene aadB into GC1s. Hence, this study shows the significance of plasmids incorporating additional determinants in GC1s. The most unexpected finding was horizontal transfer of DNA segments that contain ISAba1-ampC generating resistance to third generation cephalosporins. Overall, Australian GC1 isolates included a diverse collection. However, two outbreak GC1 strains were identified in a single ward of one of the Sydney hospitals. Outbreak strains persisted for a period of time and were then replaced by a GC2 strain
