Does spatial emergence of predator-prey sub-populations drive T6SS-aided kin killing in Vibrio cholerae communities?

Abstract

Quorum sensing (QS) is the process of cell-to-cell communication that enables bacteria to synchronize their gene expression patterns. Developing colony communities of Vibrio cholerae display a program of spatio-temporally patterned, and regulated cell death (RCD). The RCD program unfolds in two distinct phases, the first phase of which occurs along the colony rim is QS-regulated and is the focus of my work. Rim cell death is, in part, a consequence of kin-killing mediated by a nano weapon called the Type 6 Secretion System (T6SS), which injects toxic effectors into neighboring cells. Bacteria prevent self-poisoning and defend against incoming kin-T6SS attacks through the production of immunity proteins. These proteins are proposed to bind their cognate T6SS effector toxins and render them ineffective, thus preventing killing. This model regarding effector-immunity interactions led me to hypothesize that T6SS-driven kin- killing in V. cholerae colonies occur as a consequence of reduced expression, abundance or activity of immunity proteins in a sub-population of cells in the colony rim, leading to a heterogenous distribution in immunity proteins at the rim. I used a combination of genetic experiments, fluorescence microscopy and flow cytometric analyses to explore this hypothesis. Through this project, I show that immunity proteins exhibit distinct patterns of abundance at the single cell level along both the colony rim and the colony center, suggesting the role of immune independent defense mechanisms in RCD. My results lead me to propose that in V. cholerae colonies, immunity proteins do not act as the first line of defense against kin-killing mediated by coordinated T6SS attacks. This will need further studies. This work deepens our understanding of defense mechanisms employed by bacteria in spatially structured communities.