Source: http://www.asmscience.org/content/concept/Entity/ASM/Microbiology/Bacteria_and_Archaea/Bacteria/Bacterial_Cellular_Processes/Bacterial_Cell_to_Cell_Communications/Quorum_Sensing/Signal_Transduction/Signal_Molecules/Autoinducers/Furanosyl_Borate_Diester
Timestamp: 2019-04-21 08:37:11+00:00

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One of the environmental signals measured by Vibrio cholerae is its own cell density, which it achieves by a quorum-sensing mechanism. During inhabitation of aquatic environments, V. cholerae lives in association with various species of phytoplankton and zooplankton, often in the form of biofilms. Once V. cholerae has entered the host and traversed the hostile stomach environment, it must penetrate the mucous layer and adhere to and colonize the epithelial cells of the small intestine. To achieve this, V. cholerae produces a number of virulence factors, including the cholerae toxin (CT) and the toxin coregulated pilus (TCP). TCP is a type IV pilus encoded by the Vibrio pathogenicity island (VPI) whose probable function is to mediate adherence to the intestinal mucosal cells. When the quorum-sensing pathways of V. cholerae were being dissected at the molecular level, it was noted that the simultaneous mutation of both the CAI-1 and AI-2 systems did not abolish density-dependent light induction from the lux operon. The mechanisms of quorum-sensing control of biofilm formation in V. cholerae is further complicated by a recent finding that the concentration of the autoinducer CAI-1 is higher in biofilms than in planktonic cultures. To assess the significance of quorum sensing, it is important to carry out experiments under conditions that mimic as closely as possible the natural habitat of V. cholerae.
This chapter reviews the role of quorum sensing in Vibrio fischeri, focusing on recent developments in one&apos;s understanding of the genetics and physiology of cell-cell signaling by populations of these bacteria, both in culture and in their light-organ symbioses. Particular emphasis is placed on outlining the regulatory factors and pathways by which quorum sensing coordinates the biological activities of this bioluminescent microbe. Early work showed that V. fischeri strains native to Euprymna scolopes were especially well adapted to this host and that nonnative strains such as MJ1 did not colonize juvenile squid well; thus, MJ1 was not an appropriate strain for studying the symbiosis between V. fischeri and E. scolopes. Production of N-octanoyl-homoserine lactone (C8-HSL) initiates stimulation of the lux operon at moderate cell densities; if ES114 had relatively low expression of AinS and low C8-HSL output, this deficiency could potentially explain why ES114 is so much dimmer than MJ1. Most likely, then, the large difference in the levels of N-3-oxo-hexanoyl-homoserine lactone (3-O-C6-HSL) production and luminescence seen between cultures of ES114 and MJ1 is due to external regulatory influences on the autoinducer synthase genes, and such regulation may be multifactorial. Biochemical and genetic studies of Acyl-homoserine lactone (AHL) signaling in Vibrio harveyi have shown that, in the presence of an inducing concentration of the LuxM-synthesized AHL, the receptor, LuxN,participates in a phosphorelay cascade by stimulating the relative dephosphorylation of LuxU.
This chapter reviews the current understanding of acylated homoserine lactone (AHL) signaling in marine bacterial systems outside of the well-described Vibrio fischeriand Vibrio harveyi models. AHL production has far only been documented in proteobacterial groups. Representatives of these taxa, however, constitute one of the most numerous and functionally diverse classes of microorganisms, and they are particularly abundant in marine environments. Direct chemical analyses, such as those employed by Wagner- Dobler et al., do not have intrinsic biases but are currently at least 10-fold less sensitive than the best biosensors. AHL synthesis is often strongly regulated by other environmental conditions, and these activating conditions may not be recapitulated in standard laboratory culture. The surveys discussed in the chapter should be considered as conservative estimates of AHL production capacity among cultivatable marine bacteria and should be integrated with emerging genomic information on marine bacteria. The Roseobacteria are one of the dominant microbial groups in the ocean, and several subgroups are also the most common AHL signal producers. Ulva zoospores preferentially settled on top of bacteria, suggesting a direct interaction between the bacteria and zoospores and providing evidence that attachment is not a random process. This work led to the discovery that bacterial quorum-sensing molecules, specifically AHLs, are involved in zoospore settlement. The marine environment is a source of abundant materials and resources across the globe. The oceans are sources of diverse and complex quorum-sensing signal molecules produced by many of their endogenous proteobacteria.
Escherichia colicauses three types of illnesses in humans: diarrhea, urinary tract infections, and meningitis in newborns. The acquisition of virulence-associated genes and the ability to properly regulate these, often horizontally transferred, loci distinguishes pathogens from the normally harmless commensal E. coli found within the human intestine. This review addresses our current understanding of virulence gene regulation in several important diarrhea-causing pathotypes, including enteropathogenic, enterohemorrhagic,enterotoxigenic, and enteroaggregativeE. coli—EPEC, EHEC, ETEC and EAEC, respectively. The intensely studied regulatory circuitry controlling virulence of uropathogenicE. coli, or UPEC, is also reviewed, as is that of MNEC, a common cause of meningitis in neonates. Specific topics covered include the regulation of initial attachment events necessary for infection, environmental cues affecting virulence gene expression, control of attaching and effacing lesionformation, and control of effector molecule expression and secretion via the type III secretion systems by EPEC and EHEC. How phage control virulence and the expression of the Stx toxins of EHEC, phase variation, quorum sensing, and posttranscriptional regulation of virulence determinants are also addressed. A number of important virulence regulators are described, including the AraC-like molecules PerA of EPEC, CfaR and Rns of ETEC, and AggR of EAEC;the Ler protein of EPEC and EHEC;RfaH of UPEC;and the H-NS molecule that acts to silence gene expression. The regulatory circuitry controlling virulence of these greatly varied E. colipathotypes is complex, but common themes offerinsight into the signals and regulators necessary forE. coli disease progression.
This chapter emphasizes recurring themes in bacterial signaling. Bacterial metabolites include N-acyl-L-homoserine lactones (AHL) first discovered in Vibrio fischeri and produced by various gram-negative bacteria, furanosyl borate diester (AI-2) first discovered in Vibrio harveyi and produced by a diverse range of bacteria, signaling peptides produced by many gram-positive bacteria, butyrolactones produced by Streptomyces species and 3-hydroxypalmitic acid methyl ester (PAME) produced by Ralstonia solanacearum. The cell density sensing or "quorum sensing" hypothesis proposes that the concentration of an intercellular signal, produced by a growing population and accumulating in the surrounding extracellular environment, is proportional to the cell density of that population. The rationale is that the plant immune system, which responds to tissue damage, is not capable of resolving Erwinia carotovora infections at this cell density. The chapter discusses the implications of intercellular signaling mechanism (ISM)-regulated coordinated behaviors on three levels. First, it considers how intercellular signaling within species can extend the range of phenotypes and niches available to bacteria at the cost of adaptive flexibility. Second, it appraises the evidence for intercellular signaling between species in mixed communities and considers its potential impact on microbial community dynamics. Third, it considers how bacterial ISMs may be exploited by higher organisms in interdomain interactions.
This chapter focuses on quorum sensing in gram-negative bacteria with a special emphasis on the well-studied intercellular communication network found in Pseudomonas aeruginosa. At least 22 gram-negative species have been shown to utilize an acyl-homoserine lactone based quorum sensing system to control various genes, and more than 50 different species have been shown to produce an acyl-homoserine lactone type of cell-to-cell signal. One of the more well-studied pathogens in this group is P. aeruginosa, which contains two separate quorum sensing systems. The study of quorum sensing in P. aeruginosa began when it was discovered that the production of the virulence factor elastase was controlled by LasR, a homolog of LuxR. The genetic organization of the rhl quorum sensing system is also similar to the las quorum sensing system. Evidence for the importance of quorum sensing in infections was also found by randomly mutagenizing the P. aeruginosa wild-type strain PA14 in search of virulence factors. A popular theory is that delaying the production of certain virulence factors may allow P. aeruginosa to face a lesser immune response while its population builds. Biofilm formation is believed to be a critical step in the disease produced when P. aeruginosa chronically infects the lungs of patients with cystic fibrosis. The authors hope the understanding of quorum sensing will provide the background for the development of new and effective antimicrobial therapies that will provide much needed options for the treatment of bacterial infections.

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