Quorum Sensing Regulates Type III Secretion inVibrio harveyiandVibrio parahaemolyticus

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Abstract
In a process known as quorum sensing, bacteria communicate with one another by producing, releasing, detecting, and responding to signal molecules called autoinducers.Vibrio harveyi, a marine pathogen, uses two parallel quorum-sensing circuits, each consisting of an autoinducer-sensor pair, to control the expression of genes required for bioluminescence and a number of other target genes. Genetic screens designed to discover autoinducer-regulated targets inV. harveyihave revealed genes encoding components of a putative type III secretion (TTS) system. Using transcriptional reporter fusions and TTS protein localization studies, we show that the TTS system is indeed functional inV. harveyiand that expression of the genes encoding the secretion machinery requires an intact quorum-sensing signal transduction cascade. The newly completed genome of the closely related marine bacteriumVibrio parahaemolyticus, which is a human pathogen, shows that it possesses the genes encoding both of theV. harveyi-like quorum-sensing signaling circuits and that it also has a TTS system similar to that ofV. harveyi. We show that quorum sensing regulates TTS inV. parahaemolyticus. Previous reports connecting quorum sensing to TTS in enterohemorrhagic and enteropathogenicEscherichia colishow that quorum sensing activates TTS at high cell density. Surprisingly, we find that at high cell density (in the presence of autoinducers), quorum sensing represses TTS inV. harveyiandV. parahaemolyticus.