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14.1 Introduction
Quorum sensing (QS) comprises of a bacterial communication system wherein the
production and secretion of small signaling molecules known as autoinducers
occurs and these molecules get accumulated in the extracellular environment and
increase in concentration as a function of cell density (Romero et al. 2011). This
process assists bacteria in communication, regulating gene expression, and synchronizing phenotypic expression of biofilm development, motility, bioluminescence,
and virulence factor production such as phospholipase, hemolysin, protease, etc.
(Tang and Zhang 2014). Quorum sensing system was first mentioned in bioluminescent Vibrio fischeri, and the term “quorum sensing” was introduced by E.P. Greenburg
(Nealson and Hastings 1979; Davis 2004). As quorum sensing plays a vital role in
the pathogenesis of human, animal, and plant, the identification of mechanisms that
can cause disruption of quorum sensing molecules in such pathogenic bacteria has
become an interesting area of research in microbiology and many compounds have
been found to be able to inhibit this signaling pathway in various pathogenic organisms and the trials are ongoing (Defoirdt et al. 2013). Identification of signaling
molecules, their receptor and target site, and the mechanism of signaling are the
important aspects of cell-cell communication in bacteria.
Quorum sensing bacteria access the cell population density using autoinducers
(Henke and Bassler 2004). This enables the bacterial group to express specific genes
only at particular population densities. The discovery of species specific as well as
cosmopolitan intracellular signaling molecules unfolds that the bacteria interact
with one another using astonishingly sophisticated mechanisms of communication.
In recent past, bacteria were considered to be living a rather solitary life. But, new
research has revealed that in fact bacteria have a comprehensive chemical signaling
system that enables them to communicate intraspecifically as well as interspecifically. When the population of bacteria increases, the individual organisms produce
and secrete the autoinducers into the extracellular environment. Quorum Sensing is
shown by both Gram-positive and Gram-negative bacteria. Examples of wellstudied quorum sensing molecules include acylhomoserine lactones, autoinducer 2,
and peptide signals, but many other signals, such as indole, cholera autoinducer also
exist (Nazzaro et al. 2013). In addition to signals, signal synthases, signal receptors,
signal response regulators, and regulated genes (Quorum Sensing regulon) are key
components of any quorum sensing system.
Based on signaling molecules and sensing mechanism, there are three major
classes of QS systems:
1. Gram-negative LuxI/LuxR-like quorum sensing system that uses AHLs as signaling molecules (Fuqua et al. 1994).
2. Gram-negative V. harveyi–like two-component signaling circuits that recognize
three different signaling molecules, AHLs, FBD, and an uncharacterized Cal −1
molecule (Bassler et al. 1993, 1994; Henke and Bassler 2004).
3. Gram-positive two-component signaling systems that use modified oligopeptides as autoinducers (Lazazzera and Grossman 1998).
G. Seghal Kiran et al.
14.1 Introduction
Quorum sensing (QS) comprises of a bacterial communication system wherein the
production and secretion of small signaling molecules known as autoinducers
occurs and these molecules get accumulated in the extracellular environment and
increase in concentration as a function of cell density (Romero et al. 2011). This
process assists bacteria in communication, regulating gene expression, and synchronizing phenotypic expression of biofilm development, motility, bioluminescence,
and virulence factor production such as phospholipase, hemolysin, protease, etc.
(Tang and Zhang 2014). Quorum sensing system was first mentioned in bioluminescent Vibrio fischeri, and the term “quorum sensing” was introduced by E.P. Greenburg
(Nealson and Hastings 1979; Davis 2004). As quorum sensing plays a vital role in
the pathogenesis of human, animal, and plant, the identification of mechanisms that
can cause disruption of quorum sensing molecules in such pathogenic bacteria has
become an interesting area of research in microbiology and many compounds have
been found to be able to inhibit this signaling pathway in various pathogenic organisms and the trials are ongoing (Defoirdt et al. 2013). Identification of signaling
molecules, their receptor and target site, and the mechanism of signaling are the
important aspects of cell-cell communication in bacteria.
Quorum sensing bacteria access the cell population density using autoinducers
(Henke and Bassler 2004). This enables the bacterial group to express specific genes
only at particular population densities. The discovery of species specific as well as
cosmopolitan intracellular signaling molecules unfolds that the bacteria interact
with one another using astonishingly sophisticated mechanisms of communication.
In recent past, bacteria were considered to be living a rather solitary life. But, new
research has revealed that in fact bacteria have a comprehensive chemical signaling
system that enables them to communicate intraspecifically as well as interspecifically. When the population of bacteria increases, the individual organisms produce
and secrete the autoinducers into the extracellular environment. Quorum Sensing is
shown by both Gram-positive and Gram-negative bacteria. Examples of wellstudied quorum sensing molecules include acylhomoserine lactones, autoinducer 2,
and peptide signals, but many other signals, such as indole, cholera autoinducer also
exist (Nazzaro et al. 2013). In addition to signals, signal synthases, signal receptors,
signal response regulators, and regulated genes (Quorum Sensing regulon) are key
components of any quorum sensing system.
Based on signaling molecules and sensing mechanism, there are three major
classes of QS systems:
1. Gram-negative LuxI/LuxR-like quorum sensing system that uses AHLs as signaling molecules (Fuqua et al. 1994).
2. Gram-negative V. harveyi–like two-component signaling circuits that recognize
three different signaling molecules, AHLs, FBD, and an uncharacterized Cal −1
molecule (Bassler et al. 1993, 1994; Henke and Bassler 2004).
3. Gram-positive two-component signaling systems that use modified oligopeptides as autoinducers (Lazazzera and Grossman 1998).
G. Seghal Kiran et al.