Genomics and ecology of Integrative conjugative elements (ICE) in Vibrio cholerae

Abstract

Lateral gene transfer (LGT) is recognized as a major contributor in bacterial evolution. In this study, the importance of LGT was assessed by analyzing a class of mobile elements, Integrative Conjugative Elements (ICE), and two genomic islands, Vibrio Seventh Pandemic islands (VSP-I and VSP-II) in the bacterial species, Vibrio cholerae. The objective was to understand how acquisition of heterologous genetic material by V. cholerae has influenced its differentiation and adaptation to different niches in the aquatic environment, as well as its role as a human pathogen. ICEs are a class of self-transmissible mobile elements that mediate LGT via conjugation. Members of this class of mobile elements have been isolated from several species of Gram-negative and Gram-positive bacteria, and have been observed to serve as vectors of drug resistance, virulence factors and genetic tools modulating ecological adaptation in V. cholerae.

In this study, two new ICEs discovered in seventh pandemic V. cholerae O1 El Tor strains associated with cholera epidemics in Asia and Africa, were analyzed. Overall, comparative analysis of the ICEs revealed an epidemiological relationship between new and old pandemic clones in cholera endemic areas. Furthermore, analysis of a novel ICE found in a clinical isolate of V. cholerae O37 revealed an unusual genetic organization and also association with a major pathogenicity island in V. cholerae, offering evidence of ICEs as progenitors and/or potential dissemination tools for PAIs, likely associated with the pathogenic potential of non-O1/O139 V. cholerae. It is concluded from an extensive survey of the geographical distribution of ICEs that they are present in environmental V. cholerae populations worldwide.

In summary, through analysis of ICEs and VSP-I and II it was possible to demonstrate the extent to which environmental V. cholerae, autochthonous to the aquatic environment worldwide, comprise a reservoir of mobile genetic elements. It is clear that mobile genetic elements have undergone significant molecular rearrangement, with a significant range of profiles, confirming their genetic plasticity. Finally, their variable content most likely contributes significantly to genomic evolution responsible for adaptation of this bacterial host to new ecological niches.