Caractérisation des phages et des mécanismes anti-phages chez Lactococcus lactis

Authors: Labrie, Simon
Advisor: Moineau, Sylvain
Abstract: Lactococcus lactis is a lactic acid bacterium widely used for production of diverse cheeses. Nonetheless, this bacterium is susceptible to bacteriophages that are sporadically causing fermentation problems. The phages of L. lactis are among the most studied. However, many notions remain vague about their evolution and biology, and this is especially notable in response to anti-phage systems. Three different aspects of L. lactis phage biology were studied in this thesis: their genetic diversity, their evolution and the anti-phage mechanisms. The genome of the phage P335 was sequenced and analyzed revealing the heterogeneity of the groups bearing the same name and it was also possible to point out the emergence of sub-groups within this group. A module of moron genes possessing homology with prophages from other taxons was identified in the phage P335 illustrating the genomic plasticity of these phages. The second objective of this project was to demonstrate the genetic plasticity of the group P335 and their evolution when challenged with Abi phage resistance mechanisms. Phage mutants were isolated and characterized. One of the phage mutants had exchanged as much as 79% of its genome to acquire resistance to both AbiK and AbiT. The phage mutants have exchange genetic module from at least two different prophages within their host genome. Finally, AbiT mode of action was studied. Many phage mutants resistant to AbiT were isolated and characterized. Three AbiT targets/activators were identified. Two are early genes of unknown function. The third target is a late gene found in the morphogenesis module and is coding for the major capsid protein. Lactococcus lactis and their bacteriophages are an important model for the understanding of lactic acid bacteria phage/host interaction. This project increases our knowledge on the evolution of P335 phages and their genetic diversity. The amazing genetic plasticity of these phages allows their populations to rapidly evolve when confronted to a selective force such as anti-phage systems. Moreover, the mode of action of AbiT is more complex that initially estimated since it targets three phage components in different functional modules.
Document Type: Thèse de doctorat
Issue Date: 2010
Open Access Date: 16 April 2018
Grantor: Université Laval
Collection:Thèses et mémoires

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