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Phage-host interactions in Streptococcus thermophilus
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Date
2021-05-28
Authors
Lavelle, Katherine
Journal Title
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Publisher
University College Cork
Published Version
Abstract
Streptococcus thermophilus is a Gram-positive, ovococcoid bacterium which belongs to the biotechnologically important Lactic Acid Bacteria. S. thermophilus is subject to heavy industrial, and indeed, artisanal use for the production of fermented milk products including yogurt and various cheeses. Such extensive application exposes this economically important species to (bacterio)phage predation which may lead to compromised product quality, delayed production schedules and, consequently, financial losses. Phages of S. thermophilus belong to one of five known groups – cos (now termed Moineauvirus), pac (now termed Brussowvirus), 987, 5093 and P738. While the Moineauvirus and Brussowvirus phage species have retained dominance in industrial production settings, recent biodiversity studies have confirmed increasing incidences of 987 and 5093 species isolation, highlighting the rapid circulation of S. thermophilus phages, once established on a proficient host. Further to this, the 987, 5093 and P738 species have emerged through recombination events with phages capable of infecting other ovococcal species (such as Lactococcus lactis and pathogenic streptococci), highlighting the rapid evolutionary ability of S. thermophilus siphophages. To protect against this persistent and ever evolving risk of phage proliferation, it is necessary to elucidate the molecular players – both host and phage encoded - which underpin the phage-host interaction process of S. thermophilus.
In this thesis, a detailed scientific investigation into molecular aspects of the phage- host interaction process between S. thermophilus, and its phages was undertaken. This investigation commenced with an extensive phage screen to assess S. thermophilus phage diversity and was followed by a detailed bioinformatic analysis of the proteins that form part of the host recognition device of selected phages, prompting a re-classification of the overall baseplate structure of S. thermophilus Moineauvirus and Brussowvirus phages.
The scientific efforts described above were followed by the generation of non- CRISPR mediated bacteriophage insensitive mutants, which were subjected to genomic analysis. This allowed the identification of cell wall polysaccharide- associated genes which are critical to facilitate phage infection. Subsequently, the genomic locus which encodes the rhamnose containing cell wall polysaccharides of various S. thermophilus strains was assessed for its diversity and gene content, with assigned putative functions allowing for the establishment of a biosynthesis pathway for this polymeric cell wall-associated glycan, designated here as Rgp, in this species. Finally, the biochemical structure of the Rgp isolated from representative strains of S. thermophilus was elucidated allowing for a genome to structure relationship to be established, while also enhancing our understanding of phage-host interactions in this species, thereby allowing parallels to be drawn concerning phage-host interactions that occur among members of ovococcoid species.
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Keywords
Bacteriophage , Host interactions , Cell wall polysaccharide , Dairy fermentation
Citation
Lavelle, K. 2021. Phage-host interactions in Streptococcus thermophilus. PhD Thesis, University College Cork.