Competitive Traits of El Tor V. cholerae

  • Author / Creator
    Benjamin Kostiuk
  • Vibrio cholerae is a diverse bacterial species that causes a wide range of disease,ranging from the diarrheal disease cholera by toxigenic strains to mild gastroenteritiscaused by non-toxigenic strains. Toxigenic strains are able to cause more severe diseasedue to the secretion of cholera toxin. During its lifecycle, V. cholerae inhabits chitinoussurfaces and the human host. This diversity within the species and environmentalconditions results in opportunities for competition among V. cholerae strains and with othermicroorganisms.V. cholerae uses the type VI secretion system (T6SS) to mediate competition withother bacteria and eukaryotes. The T6SS is a molecular syringe-like apparatus thatcontracts in the host bacterium, sending a spike decorated with effector proteins intoneighbouring bacterial or eukaryotic cells. Effector translocation is lethal unless the targetcell produces cognate immunity proteins specific to the incoming effectors. Despite theT6SS being ubiquitous amongst all sequenced V. cholerae strains, different strains encodedifferent effectors and immunity genes. Therefore, each strain carries three effector genesbut also three immunity genes to neutralize these effectors. If bacteria encode the same effectors, they are able to coexist, while encoding different effectors causes competition. The T6SS effectors appear to be horizontally shared based on strong bioinformatics evidence and early experimental data.All sequenced toxigenic strains encode the same three effectors – termed the AAAmodule set. Initial experiments have demonstrated that toxigenic strains are all able tocoexist while being able to outcompete non-toxigenic strains in a T6SS dependent manner.My Ph.D. work focused on better understanding aspects of the T6SS that are unique totoxigenic strains. Specifically, my thesis characterizes unique aspects of the toxigenicT6SS, including the acquisition of the AAA effector modules, the evolution of the T6SSover time, host regulation of the system, and both the advantages of the system of intra andinterspecies competition.First, I tested a potential mechanism by which T6SS effectors can be exchangedand the consequences of effector exchange. Briefly, using next generation sequencing, Ifound that T6SS effectors modules can be exchanged with no change to the structuralgenes. Additionally, that pressure to exchange T6SS genes also results in the exchange ofgenes elsewhere on the chromosome. Lastly, through competition assays I studied theconsequences of acquiring the AAA module set, and the ability to outcompete non-AAAstrains as well as coexist with other toxigenic strains.Second, I looked at the differences between modern toxigenic strains belonging tothe 7th cholera pandemic, and ancient toxigenic strains belonging to the 2nd and 6thpandemics. By mutating modern V. cholerae strains to resemble the classical strains fromthe 2nd and 6th pandemic, my data suggest that while the ancient toxigenic strains encodethe genes for the T6SS and the AAA module set, mutations were accumulated making thesystem inactive. In general these findings suggest that the T6SS has changed from beinginactive to active throughout pandemics.Third, I investigated host regulation of the T6SS. While most non-toxigenic strainshave constitutively active T6SSs, toxigenic strains tightly regulate the T6SS. I found thatmucin, the proteinaceous component of mucus, is able to active the T6SS. I also found thatbile salts are able to modulate the magnitude of that activity. Activating the T6SS in thehost is advantageous as it allows V. cholerae to participate in intraspecies competition andexclude nontoxigenic strains during infection.Lastly, in partnership with David Fast (from Dr. Foley’s laboratory) within ourdepartment, I was able to investigate the role of the T6SS in interspecies competition withinthe Drosophila melanogaster model of infection. We found that a T6SS-positive toxigenicstrain was more pathogenic in this model, as determined by increased diarrheal symptomsand survivability. The increased virulence was dependent on both the presence of the T6SSand a T6SS prey, Acetobacter pasteurianus. Furthermore, this interaction between the T6SS and A. pasteruianus also evoked the host immune system. Altogether this suggests that the T6SS of V. cholerae, the host immune system and A. pasteurianus all interact tocontribute to host death.My thesis demonstrates that acquiring the AAA compatibility group in an inducible7th pandemic V. cholerae strain results in superior competition both within and outside theV. cholerae species in a host environment.

  • Subjects / Keywords
  • Graduation date
    Spring 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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