Quorum sensing is the regulation of gene expression in response to fluctuations in cell-population density. Quorum sensing bacteria produce and release chemical signal molecules called autoinducers that increase in concentration as a function of cell density. The detection of a minimal threshold stimulatory concentration of an autoinducer leads to an alteration in gene expression. Gram-positive and Gram-negative bacteria use quorum sensing communication circuits to regulate a diverse array of physiological activities. These processes include symbiosis, virulence, competence, conjugation, antibiotic production, motility, sporulation, and biofilm formation. In general, Gram-negative bacteria use acylated homoserine lactones as autoinducers, and Gram-positive bacteria use processed oligo-peptides to communicate. Recent advances in the field indicate that cell-cell communication via autoinducers occurs both within and between bacterial species. Furthermore, there is mounting data suggesting that bacterial autoinducers elicit specific responses from host organisms. Although the nature of the chemical signals, the signal relay mechanisms, and the target genes controlled by bacterial quorum sensing systems differ, in every case the ability to communicate with one another allows bacteria to coordinate the gene expression, and therefore the behavior, of the entire community. Presumably, this process bestows upon bacteria some of the qualities of higher organisms. The evolution of quorum sensing systems in bacteria could, therefore, have been one of the early steps in the development of multicellularity.

The production of virulence factors including cholera toxin and the toxin-coregulated pilus in the human pathogen Vibrio cholerae is strongly influenced by environmental conditions. The well-characterized ToxR signal transduction cascade is responsible for sensing and integrating the environmental information and controlling the virulence regulon. We show here that, in addition to the known components of the ToxR signaling circuit, quorum-sensing regulators are involved in regulation of V. cholerae virulence. We focused on the regulators LuxO and HapR because homologues of these two proteins control quorum sensing in the closely related luminous marine bacterium Vibrio harveyi. Using an infant mouse model, we found that a luxO mutant is severely defective in colonization of the small intestine. Gene arrays were used to profile transcription in the V. cholerae wild type and the luxO mutant. These studies revealed that the ToxR regulon is repressed in the luxO mutant, and that this effect is mediated by another negative regulator, HapR. We show that LuxO represses hapR expression early in log-phase growth, and constitutive expression of hapR blocks ToxR-regulon expression. Additionally, LuxO and HapR regulate a variety of other cellular processes including motility, protease production, and biofilm formation. Together these data suggest a role for quorum sensing in modulating expression of blocks of virulence genes in a reciprocal fashion in vivo. The Gram-negative bacterium Vibrio cholerae usually inhabits natural aquatic environments, but it is best known as the causative agent of cholera, a severe diarrheal disease (1). Two factors are critical to V. cholerae virulence-cholera enterotoxin (CT) and an intestinal colonization factor known as the toxincoregulated pilus (TCP). Poorly characterized environmental cues influence the expression of CT and TCP in vivo (2). Two sensory proteins, ToxR and TcpP, likely play a role in detection of the environmental signals, and then initiate a signal transduction cascade that promotes the expression of ToxT, which in turn, directly activates the transcription of genes involved in TCP and CT expression (3).Recent work has established that many species of bacteria monitor their cell-population densities through the exchange of chemical signaling molecules (called autoinducers) that accumulate extracellularly and trigger alterations in behavior at high population densities. This phenomenon is referred to as quorum sensing (4, 5). Quorum sensing controls processes that include bioluminescence, virulence, biofilm formation, and sporulation in various bacterial species. In general, quorum sensing regulates processes that are effective only when a population of bacteria acts in a coordinated manner, but not when the bacteria act as individuals. Gram-negative bacteria typically use acylhomoserine lactones as autoinducers (5, 6), whereas Grampositive bacteria usually use modified oligopeptides as the communication signals (5-7). A link between quorum sensing and virulence has been es...

The marine bacterium Vibrio harveyi possesses two quorum sensing systems (System 1 and System 2) that regulate bioluminescence. Although the Vibrio cholerae genome sequence reveals that a V. harveyi-like System 2 exists, it does not predict the existence of a V. harveyi-like System 1 or any obvious quorum sensing-controlled target genes. In this report we identify and characterize the genes encoding an additional V. cholerae autoinducer synthase and its cognate sensor. Analysis of double mutants indicates that a third as yet unidentified sensory circuit exists in V. cholerae. This quorum sensing apparatus is unusually complex, as it is composed of at least three parallel signaling channels. We show that in V. cholerae these communication systems converge to control virulence.

In bacteria, the regulation of gene expression in response to changes in cell density is called quorum sensing. Quorum-sensing bacteria produce, release, and respond to hormone-like molecules (autoinducers) that accumulate in the external environment as the cell population grows. In the marine bacterium Vibrio harveyi two parallel quorum-sensing systems exist, and each is composed of a sensor-autoinducer pair. V. harveyi reporter strains capable of detecting only autoinducer 1 (AI-1) or autoinducer 2 (AI-2) have been constructed and used to show that many species of bacteria, including Escherichia coli MG1655, E. coli O157:H7, Salmonella typhimurium 14028, and S. typhimurium LT2 produce autoinducers similar or identical to the V. harveyi system 2 autoinducer AI-2. However, the domesticated laboratory strain E. coli DH5␣ does not produce this signal molecule. Here we report the identification and analysis of the gene responsible for AI-2 production in V. harveyi, S. typhimurium, and E. coli. The genes, which we have named luxS V.h. , luxS S.t. , and luxS E.c. respectively, are highly homologous to one another but not to any other identified gene. E. coli DH5␣ can be complemented to AI-2 production by the introduction of the luxS gene from V. harveyi or E. coli O157:H7. Analysis of the E. coli DH5␣ luxS E.c. gene shows that it contains a frameshift mutation resulting in premature truncation of the LuxS E.c. protein. Our results indicate that the luxS genes define a new family of autoinducer-production genes.