Doctor of Philosophy in Biology

In the

School of Biological Sciences

Cristian Voicu Crisan

Will defend his dissertation

Antimicrobial Dynamics of the Vibrio cholerae Type VI Secretion System

Wednesday, March 24^th, 2021

11:00 AM

https://bluejeans.com/953791122

 Thesis Advisor:

Dr. Brian Hammer, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Committee Members:

Dr. Raquel Lieberman, Ph.D.

School of Chemistry and Biochemistry

Georgia Institute of Technology

Dr. Matthew Torres, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Dr. Thomas DiChristina, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Dr. Loren Williams, Ph.D.

School of Chemistry and Biochemistry

Georgia Institute of Technology

ABSTRACT: Vibrio cholerae is a common bacterial inhabitant of marine ecosystems and some pathogenic strains cause life-threatening cholera disease when ingested by humans. Like approximately 25% of all Gram-negative bacteria, V. cholerae uses the Type VI Secretion System (T6SS) to translocate cytotoxic proteins into adjacent target cells. After sequencing a diverse set of V. cholerae isolates, I characterized two novel T6SS toxins, which I named TleV1 and TpeV. TleV1 is a predicted phospholipase that is lethal when expressed in the periplasm of Escherichia coli cells and kills target bacteria when delivered in a T6SS-dependent manner. Unlike other V. cholerae T6SS effectors, TpeV does not share homology to known proteins and does not contain motifs or domains indicative of function. E. coli cells expressing periplasmic TpeV have a disrupted membrane potential and are permeabilized. I determined that V. cholerae can use TpeV to permeabilize and eliminate target cells in a T6SS-dependent manner. Human, animal and plant pathogens encode tpeV homologs adjacent to known T6SS genes, indicating that the toxin is a representative member of a large protein family. While many studies have investigated the regulation and toxins of the T6SS, few have examined defensive responses elicited by target cells. I demonstrated that multiple human commensal E. coli strains, which are susceptible to killing via the T6SS, display a significant survival improvement when co-cultured with V. cholerae in the presence of glucose. I identified that the E. coli glucose-responsive gene regulator CRP (cyclic adenosine monophosphate receptor protein) controls resistance against T6SS attacks. E. coli cells with a crp gene disruption are protected against V. cholerae T6SS attacks even in the absence of glucose. In conclusion, I have shown that V. cholerae isolates employ a diverse repertoire of T6SS toxins and described a molecule that confers resistance to T6SS attacks in E. coli cells. My work has expanded our understanding of secreted antimicrobial proteins and the complex dynamics of interbacterial competitions.