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acquired resistance is conferred by changes in the bacterial genome of one strain, including mutations in housekeeping genes or acquisition of mobile genetic elements carrying resistance genes (Schwarz and Chaslus-Dancla, 2001; Guardabassi and Courvalin, 2006). Bacteria contain extremely efficient genetic transfer systems capable of exchanging and accumulating antimicrobial resistance genes. Resistance genes can move between chromosomal and extra-chromosomal DNA elements, and they may move between bacteria of the same or different species or to bacteria of different genera by horizontal gene transfer. The most important vehicles for transfer of resistance genes in bacteria are mobile genetic elements, such as plasmids, transposons, integrons and gene cassettes, and genomic islands (Bennett, 1995; Schwarz and Chaslus-Dancla, 2001). Plasmids are extrachromosomal, replicable circular DNA molecules that vary in size between less than two and several hundred kb. They replicate independently of bacterial chromosomal DNA. Plasmids have been identified in most bacterial species and may have the capacity to be transferred (conjugative plasmids) or co-transferred (non-conjugative plasmids) from one bacterium to another, thus resulting in widespread dissemination of plasmid-encoded characteristics within a bacterial population. In addition to manifold traits, such as metabolic or virulence properties, plasmids may code for resistance to antimicrobial agents. These resistance plasmids carry one or more resistance genes, thus a single plasmid may code for resistance to up to 10 different antimicrobial agents simultaneously. Plasmids are known to be vectors for transposons and integrons/gene cassettes (Carattoli, 2003). Transposons (jumping genes) are short sequences of DNA that can move between plasmids, between a plasmid and the bacterial chromosome, or between a plasmid and a bacteriophage. Unlike plasmids, transposons are not able to replicate independently; their stable maintenance necessitates integration into the chromosomal or plasmid DNA. Sizes and structures of transposons vary, but usually they harbor a transposase gene encoding their integration and excision, and in most cases one or more resistance genes. Transposons can either be conjugative or non-conjugative, and they are easily acquired by plasmids and then incorporated into bacterial DNA. Often several transposons are clustered on the same plasmid, resulting in the transfer of multiple resistance determinants with a single conjugation event (Liebert et al., 1999). Integrons are naturally occurring gene expression elements. Among the known classes of integrons, class 1 and class 2 are most frequently found. They represent intact or defective transposons and are usually composed of a 50 - and a 30 -conserved region and an interposed variable region, which