Acinetobacter baumannii is an opportunistic pathogen, especially in intensive care units, and multidrugresistant isolates have increasingly been reported during the last decade. Despite recent progress in knowledge of antibiotic resistance mechanisms in A. baumannii, little is known about the genetic factors driving isolates toward multidrug resistance. In the present study, the A. baumannii plasmids were investigated through the analysis and classification of plasmid replication systems and the identification of A. baumannii-specific mobilization and addiction systems. Twenty-two replicons were identified by in silico analysis, and five other replicons were identified and cloned from previously uncharacterized A. baumannii resistance plasmids carrying the OXA-58 carbapenem-hydrolyzing oxacillinase. Replicons were classified into homology groups on the basis of their nucleotide homology. A novel PCR-based replicon typing scheme (the A. baumannii PCR-based replicon typing [AB-PBRT] method) was devised to categorize the A. baumannii plasmids into homogeneous groups on the basis of the nucleotide homology of their respective replicase genes. The AB-PBRT technique was applied to a collection of multidrug-resistant A. baumannii clinical isolates carrying the bla OXA-58 or bla OXA-23 carbapenemase gene. A putative complete conjugative apparatus was identified on one plasmid whose selfconjugative ability was demonstrated in vitro. We showed that this conjugative plasmid type was widely diffused in our collection, likely representing the most important vehicle promoting the horizontal transmission of A. baumannii resistance plasmids.The foundation of plasmid biology was largely built on the genetic analysis of plasmid strategies for broad-host-range replication in Gram-negative bacteria. Mechanisms which guarantee the autonomous replication, addiction systems based on toxin-antitoxin factors, partitioning systems ensuring stable inheritance during cell division, and other virulence and antimicrobial resistance determinants have been described for plasmids circulating in the Enterobacteriaceae family and Pseudomonas spp. (17). Enterobacterial plasmids have also been classified into homogeneous groups on the basis of their replication controls by conjugation (plasmid incompatibility) and molecular methods (Southern blot hybridization with replicon probes and PCR-based replicon typing) (5,8,10,11). Currently, 27 incompatibility groups are recognized in the Enterobacteriaceae by the Plasmid Section of the National Collection of Type Cultures (Colindale, London, United Kingdom). In contrast, limited information is available on the plasmids circulating in Acinetobacter spp., even though Acinetobacter baumannii is an important pathogen in intensive care units (13,28). Moreover, despite recent progress in the study of antibiotic resistance mechanisms in A. baumannii, little is known about the genetic factors that have driven the recent evolution of A. baumannii toward multidrug resistance. A. baumannii may develop resistance to...
ISAba1 is an insertion sequence that is widely distributed in Acinetobacter baumannii. We demonstrated here that ISAba1 and the composite transposon Tn2006 are capable of transposition, generating 9-bp target site duplications. The expression of the ISAba1 transposase-encoding gene was downregulated by translational frameshifting.
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