Abstract
Quorum sensing regulates cell density-dependent phenotypes and involves the synthesis, excretion and detection of so-called autoinducers. Vibrio harveyi strain ATCC BAA-1116 (recently reclassified as Vibrio campbellii), one of the best-characterized model organisms for the study of quorum sensing, produces and responds to three autoinducers. HAI-1, AI-2 and CAI-1 are recognized by different receptors, but all information is channeled into the same signaling cascade, which controls a specific set of genes. Here we examine temporal variations of availability and concentration of the three autoinducers in V. harveyi, and monitor the phenotypes they regulate, from the early exponential to the stationary growth phase in liquid culture. Specifically, the exponential growth phase is characterized by an increase in AI-2 and the induction of bioluminescence, while HAI-1 and CAI-1 are undetectable prior to the late exponential growth phase. CAI-1 activity reaches its maximum upon entry into stationary phase, while molar concentrations of AI-2 and HAI-1 become approximately equal. Similarly, autoinducer-dependent exoproteolytic activity increases at the transition into stationary phase. These findings are reflected in temporal alterations in expression of the luxR gene that encodes the master regulator LuxR, and of four autoinducer-regulated genes during growth. Moreover, in vitro phosphorylation assays reveal a tight correlation between the HAI-1/AI-2 ratio as input and levels of receptor-mediated phosphorylation of LuxU as output. Our study supports a model in which the combinations of autoinducers available, rather than cell density per se, determine the timing of various processes in V. harveyi populations.
Highlights
The term ‘‘quorum sensing’’, introduced by Peter Greenberg in 1994 [1], refers to a concept according to which bacteria constantly produce and excrete low-molecular-weight signaling molecules, called autoinducers (AIs), into the medium
Patterns of accumulation of the three autoinducers change during growth of V. harveyi The extracellular concentrations of the three AIs were determined in a wild type population of V. harveyi grown in liquid autoinducer bioassay (AB) medium at regular intervals (Fig. 2)
During the early and mid-exponential growth phases only AI-2 is present in detectable amounts, in the late exponential growth phase AI-2 predominates over HAI-1, and the stationary phase is characterized by essentially equal molar concentrations of HAI-1 and AI-2
Summary
The term ‘‘quorum sensing’’, introduced by Peter Greenberg in 1994 [1], refers to a concept according to which bacteria constantly produce and excrete low-molecular-weight signaling molecules, called autoinducers (AIs), into the medium. Vibrio harveyi strain ATCC BAA-1116 (recently reclassified as Vibrio campbelli [2,3]), one of the best studied model organisms for quorum sensing, produces and responds to three different classes of AIs: the species-specific HAI-1 [N-(3-hydroxybutyryl)-homoserine lactone], AI-2 (furanosyl borate diester), which is synthesized by many bacterial species, and the genus-specific CAI-1 [(Z)-3aminoundec-2-en-4-one (Ea-C8-CAI-1)] [4,5,6,7] These three AIs are recognized by the three membrane-bound hybrid sensor kinases LuxN, LuxQ (in cooperation with the periplasmic AI-2binding protein LuxP) and CqsS respectively (Fig. 1) [5,6,8,9,10]. At high AI concentrations, genes involved in bioluminescence [16], biofilm formation [17] and extracellular proteolysis [18] are induced, while genes for type III secretion [19] and siderophore production [20] are repressed
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