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Quorum Sensing: Bacteria Talk SenseCell communication in bacteria occurs through a vernacular of small diffusible chemical signals that impact gene regulation during times of high cell density. This form of intercellular signaling, known as quorum sensing, optimizes the metabolic and behavioral activities of a community of bacteria for life in close quarters. Quorum sensing is best characterized as a means of communication within a bacterial species, whereas competitive or cooperative signaling can occur between groups of bacteria or between bacteria and the host. These systems are often integrated into complex, multilayered signal transduction networks that control numerous multicellular behaviors, including biofilm formation and other virulence traits. In addition, quorum signals, sensors, and signaling pathways are increasingly recognized as having biological properties that extend beyond cell communication. The deeper understanding of microbial cell communication promises to shed light on the complexities of the host-microbe relationship and may lead to novel therapeutic applications.

Cell communication and signaling are essential for the proper growth and development of all living multicellular organisms. Because of its universal importance, it is not surprising that many fundamental aspects of cell communication have been evolutionarily conserved between plants, animals, and unicellular eukaryotes, even though these kingdoms diverged more than 1 billion years ago .

Cell communication in bacteria and in some eukaryotic microorganisms occurs in a population-density dependent manner and is based on the production of and response to small pheromone-like biochemical molecules called autoinducers. Differential gene regulation in response to intercellular signaling provides microbes with a means to express particular behaviors only while growing in social communities. This process has been termed quorum sensing to reflect the need for a sufficient population of microbes (and concentration of signal) to activate the system .

Types of AutoinducersMicrobially derived signalling molecules act as auto inducers in bacterial quorum sensing. The Gram-negative bacteria use fatty acid derivatives called Homoserine

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Lactones HSLs whose synthesis is dependent on LuxI homolog or LuxR homolog encoding a transcriptional activator protein responsible for detection of the cognate HSL and the resulting gene expression which results in phenotypic changes. More than 30 species of Gram-negative bacteria use HSL derivatives for the control of the cell density and hence the quorum sensing phenomenon.

The Gram-positive bacteria use amino acids and short peptide derivatives for quorum sensing.

(1) Acyl Homoserine Lactone molecules

The AHL signal molecules from different bacteria are related in structure, but differ in the nature of the acyl side chain moieties attached to them.The acyl group can vary from 4 to 14 carbons depending on the auto inducer. It also possesses a hydroxyl group, a carbonyl group, it is either fully saturated or contains a single carbon-carbon double bond. A significant number of microbial acyl HSLs have even number of carbons in their acyl side chains and are synthesized by different bacterial genera. Many bacterial species can produce more than one type of Acyl Homoserine Lactone and the type of acyl HSL produced by a particular species can be strain dependent.

(2) Synthesis of Autoinducers

The Homoserines found in bacteria are intermediates of the methionine-lysine-threonine bio synthetic pathway. S-adenosylmethionine (SAM) is one of the intermediates of methionine/homocysteine pathway.

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Gram-Negative Bacteria

N>-acylhomoserine lactone (AHL) signaling. The lux-type quorum-sensing system is the archetypal and preeminent mechanism for species-specific communication in gram-negative bacteria . First identified in marine Vibrio species, lux-type quorum sensing is based on the production of and responses to AHLs. In general, lux-type systems consist of 2 components, an autoinducer synthase (e.g., LuxI), which synthesizes AHLs from S-adenyosyl methionine, and a transcriptional regulator (e.g., LuxR). Because of its small size and lipophilic character, AHL freely diffuses across cell membranes. As the population density increases, intracellular AHL binds the functionally linked (cognate) LuxR-like receptor at a sufficient concentration within the cytoplasm to induce differential gene expression.

Gram-Positive Bacteria

Intercellular communication is also used by gram-positive pathogens to control virulence. Rather than using AHL or quinolone-based signaling molecules, cell communication in gram-positive bacteria is based on the production and detection of modified oligopeptides called autoinducing peptides (AIPs). The best-studied system is the quorum-sensing system ofS. aureus, which is encoded by the accessory gene regulator (agr) locus. Analogous to the function that quorum sensing plays in P. aeruginosavirulence, the agr system is central to a complex regulatory network that controls the production of a broad array of S. aureus virulence factors . Moreover, agr also has a complex relationship with biofilm formation, which has significant clinical implications.

Genetic Support of Quorum Sensing

Quorum sensing is of ancient origin in many species, although when it first arose in the evolution of bacteria is not clear . The components of quorum-sensing systems are typically encoded by chromosomal genes, which likely were acquired by horizontal gene transfer . The recent discovery of a functional AHL system within a mobile transposon in Serratia marcescens supports this hypothesis . Once transferred to a new bacterial genome, quorum-sensing systems integrate with

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native signal-transduction systems to produce regulatory networks that are often unique to a given species.

Quorum Sensing and Biofilm

Coincident with the elucidation of cell communication systems in bacteria has been the growing appreciation of the importance of biofilms in bacterial physiology and virulence. Most bacteria in the environment reside in biofilms, as do many of those involved in human infection . Interestingly, another small-molecule signaling system, based on the intracellular second messenger bis-(3'-5')-cyclic dimeric guanosine monophosphate (cyclic-di-GMP), has been shown to control the ability of flagellated bacteria, such as P. aeruginosa, to switch from planktonic to biofilm growth . The close proximity of bacteria and limited diffusion of molecules within the biofilm matrix suggest that quorum sensing may be crucial for the development of biofilm-associated infections.

P. aeruginosa. The first evidence that quorum sensing may influence biofilm formation was reported in association with P. aeruginosa, when alasI mutant was found to produce a thinner biofilm that was more susceptible to disruption by detergents . These findings, along with reports of the contribution of quorum sensing to biofilm formation in other bacteria, have led to considerable interest in the development of quorum-sensing inhibitors as a means to prevent or treat biofilm-associated infections. In fact, recent work has shown that compounds that inhibit quorum sensing impede biofilm formation by P. aeruginosa . It should be noted, however, that P. aeruginosa strains that lack functional quorum-sensing systems can, nevertheless, still cause infection .

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Quorum Sensing as a Therapeutic Target

Recently, compounds that inhibit quorum sensing have received considerable attention as a potentially novel class of antimicrobial agents. Pharmacologic inhibition of quorum sensing is a particularly attractive approach for the prevention or treatment of chronic infections with high bacterial cell density or limited diffusion environments, such as chronic lung infections in patients with cystic fibrosis or chronic wound infections . In addition, it has been hypothesized that the development of resistance to quorum sensing inhibitors should be limited, because these agents would attenuate virulence but not impede bacterial growth .

Pharmacologic interference of intercellular signaling can be envisioned at several steps in the quorum-sensing circuitry. Potential strategies include inhibiting receptor synthesis or function, reducing production or release of functional autoinducer, stimulating autoinducer degradation, or inhibiting autoinducer-receptor binding. Examples of each of these mechanisms can be found in nature . For instance, the observation that fronds of the Australian red seaweed Delisea pulchra are rarely fouled with marine biofilms led to the discovery of a class of halogenated furanones with quorum-sensing inhibitory activity. Structurally similar to AHLs, these furanones appear to act as competitive inhibitors of LuxR-type receptors . With use of natural furanones as lead compounds, synthetic halogenated furanones with potent in vitro and in vivo anti-quorum-sensing activity have been developed .