Type VI Secretion System-Mediated Microbial Competition of Vibrio cholerae

• Author / Creator

  Unterweger, Daniel

• Vibrio cholerae is a Gram-negative bacterial species that consists of over 200 serogroups with differing pathogenic potential. Only O1 serogroup strains have been associated with the pandemic spread of cholera, a disease characterized by watery diarrhoea that can lead to severe dehydration and death. The current 7th cholera pandemic is still ongoing and follows six pandemics recorded during the 18th and 19th centuries. O1 serogroup strains make use of the main virulence factors – toxin coregulated pilus and cholera toxin – to colonize the small intestine of the human host and induce watery diarrhoea. Although strains not belonging to the O1 or O139 serogroups lack or do not effectively make use of these virulence factors, strains like the O37 serogroup strain V52 or the O39 serogroup strain AM-19226 can cause local outbreaks and cholera-like symptoms characterized by a mild form of diarrhoea. All V. cholerae strains sequenced to date harbour genes for the type VI secretion system (T6SS), a molecular puncturing device with homology to the tail-spike complex of the T4 bacteriophage that allows translocation of effectors into neighbouring eukaryotic and prokaryotic cells. Translocated effectors are toxic unless the target cell produces an immunity protein against the cognate effector. Proteins of the T6SS are encoded in three gene clusters on the large and small chromosome of V. cholerae. The T6SS of pandemic strains is activated in vivo during infection. V. cholerae employs its T6SS to kill other prokaryotic cells using effectors with peptidoglycan-degrading, pore-forming, and lipase activities. The T6SS also confers virulence toward eukaryotic phagocytic cells such as amoebae and macrophages. An effector with an actin-crosslinking domain is toxic to eukaryotic cells. The T6SS is differentially regulated among V. cholerae strains. The O37 serogroup strain V52 has an active T6SS under laboratory conditions. The O1 serogroup strain C6706 represses its T6SS under laboratory conditions and depends on inducing signals like mucin to activate the secretion system. Immunity protein-encoding genes are differentially regulated than the remaining genes of the T6SS gene clusters and are expressed under laboratory conditions in the O1 serogroup strain C6706. As a result, pandemic V. cholerae O1 serogroup strain C6706 is immune to a T6SS-mediated attack by the O37 serogroup strain V52. Three immunity proteins TsiV1, TsiV2 and TsiV3 protect from the cognate T6SS effectors TseL, VasX and VgrG-3, respectively. Characterization of environmental V. cholerae strains isolated from the Rio Grande Delta revealed intraspecific competition of V. cholerae such that V. cholerae strains kill each other in a T6SS-dependent and -independent manner. Bioinformatics analyses of 37 V. cholerae strains indicated that effector and immunity protein-encoding genes encoded within otherwise conserved gene clusters differ widely among V. cholerae strains. Effector modules were defined that contain effector and immunity protein-encoding genes which are likely exchanged between V. cholerae strains. The set of effector modules determines the compatibility group of a V. cholerae strain. Strains of the same compatibility group are able to coexist in direct contact and do not kill each other in a T6SS-dependent manner, because immunity proteins protect from cognate effectors. Strains of different compatibility groups kill each other in a T6SS-dependent manner because of the inability of the immunity proteins to protect from the effectors. All analyzed pandemic strains can be assigned to a single compatibility group and are thus expected to coexist among each other but not with nonpandemic strains. Further analysis of pandemic strains revealed that strains responsible for the sixth and likely also for preceding pandemics are weak microbial competitors. The bioinformatics analysis of V. cholerae strains also revealed diverse T6SS effectors. A chimeric adaptor protein was identified and characterized that mediates the translocation of diverse effectors of the T6SS encoded in modules. After T6SS-mediated effector export is completed, Tap-1 is retained in the bacterial cell to load other T6SS machines. The presented work advances our understanding of the diversity of T6SS effector and immunity proteins among V. cholerae strains, and proposes a model for the translocation of diverse effectors via adaptor proteins. The T6SS effector module sets could be targeted as a biomarker for pandemic strain identification for epidemic risk assessment. Classification of the V. cholerae strains based on effector module sets may affect future outbreak management, diagnostics, and therapeutic approaches.

• Subjects / Keywords

  • Type VI Secretion System
  • Vibrio cholerae
  • Microbial competition

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3PG1J326

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.