DNA on the move : investigation into two mobile genetic elements in Vibrio species

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Vibrios are a group of Gram negative rod-shaped bacteria that are ubiquitous in marine and estuarine environments. They exist as both free-living organisms and in association with a variety of hosts such as humans, coral, marine animals and plants. Vibrio cholerae is the most notorious of vibrios, being the causative agent of the devastating intestinal disease cholera in humans. Lateral gene transfer (LGT), a process that allows DNA transfer between bacterial cells, has largely driven the rapid evolution in V. cholerae and other Vibrio species. In some strains of Vibrio species at least 20% of genomic content has arisen from LGT events. With respect to V. cholerae, the two most important virulence factors: cholera toxin encoded by the ctxAB genes and intestinal adhesion encoded on the vibrio pathogenicity island (VPI-1) have been acquired via mobile genetic elements transferred by LGT. Thus, these two virulence factors convert toxigenic V. cholerae into a paradigm for the importance of LGT, demonstrating how seemingly avirulent strains of V. cholerae become capable of causing epidemic/pandemic outbreaks (Uma et al., 2003). Mobile genetic elements include but are not exclusive to: transposons, integrons, conjugative elements and genomic islands. Research performed in this thesis is focussed on the study of the integron and a genomic island and how phenotypes they confer contribute to the adaptation of two Vibrio species: V. rotiferianus and V. cholerae. Briefly, integrons are a two-component genetic recombination system present in the chromosome of almost all Vibrio species. The integron incorporates mobile genes termed gene cassettes into a reserved genetic site via site-specific recombination, named the integron/gene cassette system. The integron consists of three basic elements: an integrase gene (intI), an attachment site (attI) and a promoter (Pc). Gene cassettes generally contain a single open reading frame (ORF) and an IntI-identifiable recombination site called attC. Insertion (and excision) of gene cassettes is driven by an integrase-mediated recombination between attI and attC. Multiple insertion events lead to the accumulation of cassettes to form a cassette array. In vibrios, cassette arrays are uniquely large, sometimes containing hundreds of cassettes that make up a 1-3% of the entire genome. There is a consensus that these gene cassettes add to the adaptive potential of vibrios and have likely been an important driver in the evolution of vibrios into their respective niches. How this is achieved has been difficult to understand given that 80% of gene cassettes are novel and consequently of unknown physiological function. Using a number of chemical, proteomic and molecular techniques this thesis has shown that gene cassette(s) present in the chromosome of a model Vibrio organism (V. rotiferianus DAT722) are altering bacterial surface properties. Changes to bacterial surface properties can be important in bacterial-host interactions importantly; biofilm formation, protozoan grazing and pathogen-host association. This thesis also examines how another mobile genetic element; a novel genomic island, aids in the repair of damaged DNA in V. cholerae, giving the organism an advantage in both the environment and in the disease causing state within humans. Our knowledge of how LGT has and continues to drive bacterial adaptation and evolution has only uncovered the tip of the iceberg.
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