Title: Impact of phage-host interactions on the behavior of Salmonella Typhimurium
Other Titles: Invloed van faag-gastheer interacties op het gedrag van Salmonella Typhimurium
Authors: Cenens, William
Issue Date: 3-Dec-2014
Abstract: Salmonella enterica strains significantly contribute to the infectious disease burden in both developed and developing countries, with a majority of their infections being food- or waterborne. Since the evolution and physiology of these important pathogens are partly shaped by the predatory, parasitic and symbiotic interactions with the bacterial viruses (i.e. bacteriophages or phages) that tend to infect them, this dissertation has focused on dissecting the impact of phage P22 infection on S. Typhimurium behavior. While population-level approaches have traditionally described phage–host interactions of temperate phages such as P22, as being either lytic or lysogenic, it was decided to more closely examine P22–S. Typhimurium infection dynamics at the single cell level, by using a fluorescent DNA labelling approach that enabled specific tracking of the whereabouts of the P22 chromosome during all stages of infection using time-lapse fluorescent microscopy (chapter 3). Aside from revealing interesting dynamics of phage and host DNA, this analysis also provided molecular evidence of the existence of the phage carrier state in which an episomal cluster of P22 genomes remains located at the cell pole of the host cell and becomes segregated asymmetrically between siblings. It seems that this state invariably precedes either lytic or lysogenic proliferation. Moreover, during establishment of lysogeny, phage free cells were able to segregate from the phage carrier cell before actual phage chromosome integration occurred, and experiments further revealed that these phage free siblings remained resistant to superinfection by P22 during several generations, suggesting the cytoplasmic inheritance of a resistance factor. This transient resistance is hypothesized to increase the window of stable coexistence between phages and bacteria (chapter 3). The P22 pid (P22 encoded instigator of dgo expression) gene was previously discovered in our research group and found to specifically derepress the dgoRKDAT operon (encoding the necessary proteins for galactonate metabolism) in S. Typhimurium. Further investigation of the pid gene and Pid protein in this study now revealed that its expression was tightly linked to the phage carrier state of phage P22 (chapter 4). Although protein-protein interaction studies so far have failed to identify interaction partners of the host, they could demonstrate that Pid interacts with itself. In fact, the first crystal structure model of Pid revealed it to form a trimeric structure, due to a coiled-coil formation from the alfa-helices of three Pid molecules. The exact molecular mechanism by which Pid mediates dgoRKDAT derepression, as well as the physiological impact of the Pid/dgoRKDAT interaction for S. Typhimurium remain subject to further study. In essence, using dynamic (single) cell biology this dissertation has uncovered novel phage–host interactions between S. Typhimurium and P22 that surpass the classical bifurcation of lytic and lysogenic phage development. More specifically, by closely examining the phage carrier state, it was shown that this state is able to specifically affect gene expression of the host cell, and that it allows for the emergence of phage free siblings that are transiently resistant to P22 infection.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Food and Microbial Technology
Division of Gene Technology (-)

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