UBC Theses and Dissertations
Comparative analysis of Salmonella-infecting bacteriophages and characterization of bacteriophage-host interactions Fong, Karen
Numerous outbreaks in North America have been attributed to non-typhoidal Salmonella enterica. Bacteriophages (phages), viral bacterial predators, represent agents that could be used for controlling Salmonella; yet, relatively little is known about phages and their host interactions. Therefore, the purpose of this thesis was to characterize phages of Salmonella on their phenotypic and genomic determinants and phage-host interactions. Salmonella phages (n=97) were isolated from sites within British Columbia, Canada. Host range analysis revealed diverse patterns of lysis, with several broad host range phages identified. Initial screening demonstrated that phage SI1 exhibited remarkable stability at a variety of pH and temperature values. Artificially- contaminated sprouting alfalfa seeds treated with SI1 resulted in a significant (p<0.05) reduction of 38.3±3.0% of viable S. Enteritidis following two h of phage treatment, indicating the potential for usage of phage SI1 in food.Comparative genomic analyses of 45 of the newly isolated phages revealed an abundance of sequence diversity. Genome alignment grouped the phages into 12 clusters with three singletons. Phages within clusters exhibited high genome homology (>98% nucleotide identity), yet between clusters, genomes exhibited a span of diversity (<50% nucleotide identity). Alignment of the major capsid protein also supported the clustering pattern observed with whole genome alignment. We further observed associations between genomic relatedness and the site of isolation, as well as genetic elements related to DNA metabolism and host virulence. Five mutants of tetracycline-resistant S. Agona, resistant to phage SI1 infection, displayed attenuated virulence and antimicrobial resistance. Using a differentiated Caco-2 cell line, ∆95 and ∆96 displayed significantly (p<0.05) attenuated invasion compared to the wild- type strain, with ∆96 also exhibiting increased susceptibility to tetracycline. Concordantly, insertions were observed in rfaL involved in lipopolysaccharide biosynthesis, indicating the site of phage attachment. Mutations were observed in a vgrG gene involved in type VI secretion. Mutations in the tetracycline resistance cassette were not revealed; it may be hypothesized that altered transcriptional activity could account for the observed tetracycline susceptibility. Collectively, these findings support the knowledge framework for phage diversity and phage-host interactions that are required for developing phage-based applications for various sectors, including biocontrol, detection and typing.
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