Genomic islands in pathogenic and environmental microorganisms

Key Points Genomic islands (GEIs) are characterized by their large size (>10 kb), their frequent association with tRNA-encoding genes and a different G+C content compared with the rest of the chromosome. Many genomic islands are flanked by repeat structures and carry fragments of other mobile and accessory genetic elements, such as bacteriophages, plasmids and insertion sequence (IS) elements. Some GEIs can excise themselves spontaneously from the chromosome and can be transferred to other suitable recipients. GEIs contribute to bacterial genome plasticity and, together with other mobile and accessory genetic elements, to the 'horizontal gene pool' of a given bacterial population. A hypothetical 'life cycle' of GEIs includes the insertion of mobile genetic elements into the bacterial chromosome. Through rearrangements and consecutive insertion and deletion events, the organization and gene content of the original element becomes modified and can lose the features of mobile elements. Owing to the action of bacteriophage integrases that are encoded on genomic islands, these genetic elements can be deleted from the chromosome and, upon transfer into a suitable host, can be chromosomally inserted by site-specific recombination. GEIs contribute to fitness and adaptation. GEIs typically provide a gain-of-function to the host bacterium. As GEIs promote the transfer of multi-gene families, entire phenotypes can be changed in a single-step gene-transfer event. GEIs are expected to have a role in ecological niches where microbial cell numbers and diversity are high and/or in environments that are constantly changing. The GEIs identified so far are relevant in the context of pathogenicity, symbiosis, antibiotic resistance, xenobiotic degradation, and primary and secondary metabolism. It is expected that the functional diversity of GEIs is even greater than is currently known. As GEIs are widely distributed in pathogenic, non-pathogenic and environmental microorganisms, they represent a paradigm rather than a paradox for microbial evolution, underlining the importance of horizontal gene transfer in this process. Horizontal gene transfer is an important mechanism for the evolution of microbial genomes. Pathogenicity islands — mobile genetic elements that contribute to rapid changes in virulence potential — are known to have contributed to genome evolution by horizontal gene transfer in many bacterial pathogens. Increasing evidence indicates that equivalent elements in