Characterization of the Type VI Secretion System in the Bacterial Pathogen Acinetobacter baumannii

• Author / Creator

  Weber, Brent S

• Antibiotic resistant bacteria that cause hospital-acquired infections are a mounting concern for healthcare systems globally. Multidrug resistant (MDR) Acinetobacter baumannii has emerged as a common cause of nosocomial infections, and some isolates are now pandrug resistant. These infections can be polymicrobial, with one or several other different bacteria being co-isolated with A. baumannii, suggesting interactions with other microorganisms is common for this pathogen. The type VI secretion system (T6SS) has recently been described as an offensive molecular weapon used by many Gram-negative bacteria to kill competing bacteria. This multi-component apparatus facilitates the contact-dependent injection of toxic effector proteins into nearby prey cells and can allow the bacterium expressing the T6SS to dominate a particular environment. Production of cognate immunity proteins prevents self-inflicted intoxication by binding to and inactivating the toxins, which provides a means of discriminating self from non-self. The T6SS is dynamic and energetically costly, and therefore appears to be exquisitely regulated in most bacteria. Apart from increased antibiotic resistance, little is known about the factors that have contributed to the rapid rise of A. baumannii as a nosocomial pathogen. In this thesis, we combined bioinformatic and genetic analyses to experimentally characterize the T6SS in A. baumannii. We found that many species of Acinetobacter secrete a conserved T6SS protein called Hcp, indicating a functional secretory system. Through phenotypic screens and genome sequencing of clinical isolates, we identified a novel regulatory system that controls expression of the T6SS, which confers an anti-bacterial phenotype. This regulatory mechanism resulted in the loss of antibiotic resistance in those cells which activated their T6SS, indicating an incompatibility between these two phenotypes. We found that this regulation was controlled by a conjugative MDR plasmid, with proteins expressed by the plasmid repressing T6SS. We present the hypothesis that A. baumannii differentiate into antibiotic resistant cells or bacterial killers, with the two phenotypes being mutually exclusive. The biogenesis of the T6SS has been intensely studied during the past decade, but has not been investigated in detail for A. baumannii. We screened mutants in the model organism A. baylyi to uncover the genetic requirements for elaborating a functional T6SS. These experiments revealed several essential genes that were required for T6SS function, but had not been characterized in other bacteria. This led to the biochemical characterization of the novel T6SS component TagX, which hydrolyzes peptidoglycan, that we propose facilitates the transit of the T6SS through the producing organism’s peptidoglycan layer. Furthermore, we describe the first T6SS-dependent anti-bacterial effectors of A. baumannii, and elucidate the requirement of VgrG proteins for their effector activity. Overall, the studies presented in this thesis provide a comprehensive understanding of the T6SS in this emerging pathogen and inform future studies on the role of this secretory system in the pathobiology of A. baumannii.

• Subjects / Keywords

  • T6SS
  • Type VI secretion system
  • Acinetobacter
  • Secretion
  • Anti-bacterial
  • Acinetobacter baumannii

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3QV3CC0M

• 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.