A transcriptional regulatory mechanism finely tunes the firing of Type VI Secretion System in response to bacterial enemies

LAZZARO, M.; FELDMAN, M.F.; GARCÍA VÉSCOVI, E.

mBio

ASM Press

Año: 2017 vol. 8

The ability to detect and measure danger from an environmental signalis paramount for bacteria to respond accordingly, deploying strategies that halt orcounteract potential cellular injury and maximize survival chances. Type VI secretionsystems (T6SSs) are complex bacterial contractile nanomachines able to target toxiceffectors into neighboring bacteria competing for the same colonization niche. Previousstudies support the concept that either T6SSs are constitutively active or theyfire effectors in response to various stimuli, such as high bacterial density, cell-cellcontact, nutrient depletion, or components from dead sibling cells. For Serratiamarcescens, it has been proposed that its T6SS is stochastically expressed, with nodistinction between harmless or aggressive competitors. In contrast, we demonstratethat the Rcs regulatory system is responsible for finely tuning Serratia T6SS expressionlevels, behaving as a transcriptional rheostat. When confronted with harmlessbacteria, basal T6SS expression levels suffice for Serratia to eliminate the competitor.A moderate T6SS upregulation is triggered when, according to the aggressor-preyratio, an unbalanced interplay between homologous and heterologous effectors andimmunity proteins takes place. Higher T6SS expression levels are achieved when Serratiais challenged by a contender like Acinetobacter, which indiscriminately fires heterologouseffectors able to exert lethal cellular harm, threatening the survival of theSerratia population. We also demonstrate that Serratia?s RcsB-dependent T6SS regulatorymechanism responds not to general stress signals but to the action of specificeffectors from competitors, displaying an exquisite strategy to weigh risks and keepthe balance between energy expenditure and fitness costs.