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Class 1 Integrons Potentially Predating the Association with Tn402-Like Transposition Genes Are Present in a Sediment Microbial Community

Department of Chemistry and Biomolecular Sciences, Macquarie University, Sydney, NSW 2109, Australia,¹ Genome Atlantic, Dalhousie University, 5859 University Avenue, Halifax, Nova Scotia B3H 4H7, Canada,² Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia,³ Department of Microbiology, Institute of Biology, Sølvgade 83H, DK 1307, Copenhagen K, Denmark⁴

Received 20 December 2005/ Accepted 8 June 2006

Integrons are genetic elements that contribute to lateral gene transfer in bacteria as a consequence of possessing a site-specific recombination system. This system facilitates the spread of genes when they are part of mobile cassettes. Most integrons are contained within chromosomes and are confined to specific bacterial lineages. However, this is not the case for class 1 integrons, which were the first to be identified and are one of the single biggest contributors to multidrug-resistant nosocomial infections, carrying resistance to many antibiotics in diverse pathogens on a global scale. The rapid spread of class 1 integrons in the last 60 years is partly a result of their association with a specific suite of transposition functions, which has facilitated their recruitment by plasmids and other transposons. The widespread use of antibiotics has acted as a positive selection pressure for bacteria, especially pathogens, which harbor class 1 integrons and their associated antibiotic resistance genes. Here, we have isolated bacteria from soil and sediment in the absence of antibiotic selection. Class 1 integrons were recovered from four different bacterial species not known to be human pathogens or commensals. All four integrons lacked the transposition genes previously considered to be a characteristic of this class. At least two of these integrons were located on a chromosome, and none of them possessed antibiotic resistance genes. We conclude that novel class 1 integrons are present in a sediment environment in various bacteria of the ß-proteobacterial class. These data suggest that the dispersal of this class may have begun before the "antibiotic era."

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