title: Horizontal gene transfer in a polyclonal outbreak of carbapenem-resistant Acinetobacter baumannii.
abstract: In the last few years, phenotypically carbapenem resistant Acinetobacter strains have been identified throughout the world, including in many of the hospitals and intensive care units (ICUs) of Australia. Genotyping of Australian ICU outbreak-associated isolates by pulsed-field gel electrophoresis of whole genomic DNA indicated that different strains were cocirculating within one hospital. The carbapenem-resistant phenotype of these and other Australian isolates was found to be due to carbapenem-hydrolyzing activity associated with the presence of the blaOXA-23 gene. In all resistant strains examined, the blaOXA-23 gene was adjacent to the insertion sequence ISAba1 in a structure that has been found in Acinetobacter baumannii strains of a similar phenotype from around the world; blaOXA-51-like genes were also found in all A. baumannii strains but were not consistently associated with ISAba1, which is believed to provide the promoter required for expression of linked antibiotic resistance genes. Most isolates were also found to contain additional antibiotic resistance genes within the cassette arrays of class 1 integrons. The same cassette arrays, in addition to the ISAba1-blaOXA-23 structure, were found within unrelated strains, but no common plasmid carrying these accessory genetic elements could be identified. It therefore appears that antibiotic resistance genes are readily exchanged between cocirculating strains in epidemics of phenotypically indistinguishable organisms. Epidemiological investigation of major outbreaks should include whole-genome typing as well as analysis of potentially transmissible resistance genes and their vehicles.
authors.affiliation: Centre for Infectious Diseases and Microbiology, University of Sydney, New South Wales, Australia.
authors.forename: Jubelle K
authors.initials: JK
authors.lastname: Valenzuela
authors.forename: Lee
authors.initials: L
authors.lastname: Thomas
authors.forename: Sally R
authors.initials: SR
authors.lastname: Partridge
authors.forename: Tanny
authors.initials: T
authors.lastname: van der Reijden
authors.forename: Lenie
authors.initials: L
authors.lastname: Dijkshoorn
authors.forename: Jon
authors.initials: J
authors.lastname: Iredell
mesh_terms: D000151:Acinetobacter Infections / Q000453:epidemiology* / Q000382:microbiology; D040981:Acinetobacter baumannii / Q000145:classification / Q000187:drug effects / Q000235:genetics*; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001315:Australia / Q000453:epidemiology; D001426:Bacterial Proteins / Q000235:genetics; D015780:Carbapenems / Q000494:pharmacology*; D004196:Disease Outbreaks; D024881:Drug Resistance, Bacterial / Q000235:genetics; D022761:Gene Transfer, Horizontal; D006801:Humans; D008969:Molecular Sequence Data; D017422:Sequence Analysis, DNA
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D015780:Carbapenems