In partial fulfillment of the Requirements for the Degree of

Doctor of Philosophy in Biology

Eryn Bernardy

Will defend her thesis

"Natural competence and Type VI secretion in Vibrio cholerae"

Tuesday, July 12^th

1:00pm

Petite Institute for Bioengineering and Biosciences (IBB), room 1128.

Thesis Advisor:
Dr. Brian Hammer (School of Biology)

Committee Members:
Dr. Frank Stewart  (School of Biology)

Dr. Thomas DiChristina (School of Biology)

Dr. William Ratcliff (School of Biology)

Dr. Cheryl Tarr (Centers for Disease Control and Prevention)

Abstract:

The waterborne bacterium Vibrio cholerae, responsible for epidemics of cholera diarrhea, associates with the human gut and with chitinous surfaces in aquatic reservoirs. Prior studies of two clinical V. cholerae isolates revealed that natural competence for genetic transformation, a horizontal gene transfer mechanism, requires the chitin-induced TfoX regulator, and quorum sensing transcription factor HapR made at high cell density. To further understand this regulation, I helped identify, in a genetic screen, CytR, a new positive regulator required for competence gene expression and natural transformation. Recently, this complex regulatory network in V. cholerae was shown to also control a type VI secretion system (T6SS) that allows contact-dependent killing of other bacteria by injecting toxic proteins.  I characterized a diverse set of sequenced V. cholerae isolates, revealing that transformation was rare in all isolates, while constitutive type VI killing was common among environmental but not clinical isolates. These latter results were consistent with a “pathoadaptive” model that tight regulation is beneficial in a host, while constitutive killing is advantageous in the environment. We hypothesized that two sequenced V. cholerae isolates with distinct T6SSs could generate structured populations from initially well-mixed conditions by killing competitors, but not kin. Indeed, when both isolates were rendered T6SS^-, a well-mixed population was observed via fluorescence microscopy. In contrast, mutual killing generated clonal patches with each isolate segregating into distinct groups. Structural dynamics were recapitulated with three mathematical models and a cooperation model developed supports that this assortment promotes cooperation among kin. My work in V. cholerae has helped elucidate a complex regulatory network controlling multiple important phenotypes, diversity of these phenotypes among species members, and ecological consequences of antagonistic microbial interactions in the environment.