The characterisation of Shewanella algae

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The genus Shewanella comprises an extremely diverse group of facultative anaerobes that are widely distributed in freshwater and marine environments, including intertidal and benthic zones, their sediments and oil field wastes throughout the world. They are Gram-negative bacilli that are 1 - 2 μm in length and 0.4 - 0.7 μm in width which are motile via a single polar flagellum, exhibit un-paralleled respiratory diversity, and have robust sensing and regulatory systems which allow them to survive environments with low temperatures (less than 4°C), high salt concentrations and an extensive range of barometric pressures. These features lend themselves to phenotypic and physiological differences within the genus, but also have elicited interest in their use in biotechnology, including for bioremediation and microbial fuel cells. There are 63 species that comprise the Shewanella genus, and a handful of these are known to cause disease in humans and animals. The main species associated with human infection is Shewanella algae (S. algae), which naturally resides in aquatic environments and has been isolated from marine and freshwater sediments, oil fields, animals, marine life (including fish, sea lions, echinoderms, birds and poultry), and from human clinical material as the causative organism of diseases such as otitis media, cellulitis, septicemia and increasingly gastroenteritis. To date, there have been limited studies investigating the mechanisms of pathogenicity and antibiotic resistance of S. algae. The work presented in this dissertation has sought to address a number of gaps in knowledge regarding the pathogenesis of the emerging human pathogen S. algae using a systems biology approach. S. algae has the ability to cause mono-microbial infections in humans, ranging from infections of the skin and soft tissues, to blood borne and enteric infections. This thesis presents the first genome sequences of S. algae isolated from Sydney, Australia, and the first proteomic investigations which, combined, identify the presence and expression of potential virulence in this emerging human pathogen. This dissertation has linked the S. algae genotype to the phenotype, giving a more holistic understand of the bacterium which is crucial to understanding any roles it has in pathogenesis. We identified a range of genes encoding putative virulence factors in S. algae, including toxins, haemolysins, adhesins, secretion systems, proteases and genes required for biofilm formation and motility/chemotaxis. Furthermore, the investigation into the expression of these proteins, via the differential growth media in the proteome and secretome, have highlighted that many of the genes encoding for these virulence factors require specific conditions for their expression.
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