Unravelling novel mechanisms of horizontal transfer of class I integrons in bacteria. Implications for the dissemination of antibiotic resistance

Abstract

Horizontal gene transfer (HGT) contributes to the genetic diversity and evolutionary trajectories of bacterial populations. In particular, HGT of mobile genetic elements (MGEs) is a major contributor to the emergence, recombination and dissemination of multidrug resistance among bacterial pathogens (Nakamura et al., 2004; Thomas and Nielsen, 2005). MGEs are often shared between bacterial species and separately evolving lineages due to their capacity to physically move within genomes, and also between host genomes and cytoplasms. A variety of MGEs have been described so far (Stokes and Gillings, 2011). Integrons are genetic elements that contain a site-specific recombination system able to capture, express and exchange specific DNA elements, called gene cassettes (Hall and Collis, 1995). The complete integron is not considered to be a mobile element as such as it lacks functions for self-mobility. In contrast, the gene cassettes present in integrons are considered mobile within genomes, although the frequencies and modes of exchange of cassettes are rarely observed experimentally (Guerin et al., 2009; Baharoglu et al., 2010). Nevertheless, sequence similar integrons appear to be widespread among bacterial species and genetic backgrounds, suggesting that they are frequently exposed to mechanisms that allow them to disseminate horizontally through bacterial populations (Stokes and Hall, 1989).