“The bacteria use chemicals to transmit several information as speaking to each other. Every family of bacteria using specific language which the other member of this family can understand it perfectly which called Quorum sensing & Quenching system of bacteria.”
Senior Marketing & Technical Support Specialist of Poultry Nutrition – Egypt
WHAT IS QUORUM SENSING (QS)?
Bacteria “talk” to each other using chemicals as their words, a signaling system now known as quorum sensing.
Numerous bacterial species make small-molecule signals, called auto inducers (AI), that they release into their immediate environment to track changes in cell numbers. As the bacterial population grows, increasing in density, AI concentrations also accumulate. When AI levels reach threshold, a series of events are triggered inside bacteria that ultimately result in changing the bacterial population’s behavior and enable it to act as a large multicellular organism.
Important behaviors governed by QS are:
1. Antibiotic resistance
2. Virulence expression
Rather than individual bacterial cells expressing virulence factors, QS enables bacterial populations to achieve a population large enough for virulence factor expression, such as the production of extracellular toxins that damage the intestinal wall, reduce productivity and cause clinical disease.
Most QS systems promote communication between bacteria of the same species (intraspecies communication), with gram-positive and gram-negative bacteria using different systems to talk among themselves.
QS are species specific:
1. Gram-positive Bacteria: Gram-positive bacteria use auto inducing peptide (AIP) as their auto inducers. When gram-positive bacteria detect high concentration of AIP in the environment, AIP bind to a receptor to activate kinase. The kinase phosphorylates a transcription factor, which regulated gene transcription. This is called a two-component system.
2. Gram-negative Bacteria: Gram-negative bacteria produce N-acyl homoserine lactones (AHL) as their signaling molecule. Usually AHLs do not need additional processing, and bind directly to transcription factors to regulate gene expression. But some gram-negative bacteria may use the two-component system as well.
There are some exceptions as E. coli and Salmonella enterica do not produce AHL signals commonly found in other Gram-negative bacteria. However, they have a receptor that detects AHLs from other bacteria and change their gene expression in accordance with the presence of other “quorate” populations of Gram-negative bacteria.
Escherichia coli:
Cell division may be partially regulated by AI-2-mediated quorum sensing. As when grown normally, AI-2 presence is transient.
This species uses AI-2, which is produced and processed by the lsr operon. Part of it encodes an ABC transporter, which imports AI-2 into the cells during the early stationary (latent) phase of growth. AI-2 is then phosphorylated by the LsrK kinase, and the newly produced phospho-AI-2 can be either internalized or used to suppress LsrR, a repressor of the lsr operon (thereby activating the operon). Transcription of the lsr operon is also thought to be inhibited by dihydroxyacetone phosphate (DHAP) through its competitive binding to LsrR.
Salmonella enteric:
Salmonella encodes a LuxR homolog, SdiA, as Salmonella appears to use SdiA to detect the AHL production of other pathogens rather than the normal gut flora but does not encode an AHL synthase.
SdiA detects AHLs produced by other species of bacteria including Aeromonas hydrophila, Hafnia alvei, and Yersinia enterocolitica. When AHL is detected, SdiA regulates the rck operon on the Salmonella virulence plasmid (pefI-srgD-srgA-srgB-rck srgC) and a single gene horizontal acquisition in the chromosome srgE.
Salmonella does not detect AHL when passing through the gastrointestinal tracts of several animal species, suggesting that the normal microbiota does not produce AHLs.
AIM
Cell-to-cell communication at high population density is often termed quorum sensing. The principle involves the production of a signal molecule by a bacterium, which is released into the environment. As cell numbers increase so does the extracellular level of signal, until a threshold is reached. Gene activation, or in some cases de-repression or repression, may then occur via the activity of response regulator systems if other co-regulatory factors are satisfied.
The successful modulation of phenotype is essential for the colonization and proliferation of bacteria within the complex ecosystem of the gastrointestinal tract. To accomplish this, bacteria obtain and respond to information from the environment. One important parameter is the other bacteria present. The ability to correctly sense self, and also others, must therefore be advantageous for the control of mass action processes by the bacterial population. Within the gut ecosystem that may include processes involved in colonization including those determining bio. Information, pathogenicity, dispersal and DNA transfer. The ability to sense other bacteria may have important consequences for competitive and nutritional strategies controlling for example, entry into stationary phase, dispersal and the production of antimicrobial compounds. The ability to interfere with the signalling of bacteria will determine the fitness of the given organism to survive in the gut and may also have therapeutic potential.
APPLICATIONS
1. Aquaculture field:
A. Applications of quorum quenching (As a consequence of disrupted QS) that have been exploited by humans include the use of AHL-degrading bacteria in aquacultures to limit the spread of diseases in aquatic populations of fish, mollusks and crustaceans. This technique has also been translated to agriculture, to restrict the spread of pathogenic bacteria that use quorum sensing in plants.
B. Anti-biofouling is another process that exploits quorum quenching bacteria to mediate the dissociation of unwanted biofilms aggregating on wet surfaces, such as medical devices, transportation infrastructure and water systems.
2. The use of QQ molecules as antibacterial drugs:
Emergence of antibiotic and multi-drug resistant pathogenic bacteria has created the need for new drugs and drug targets. During pathogenesis bacteria release signals which regulate virulence and pathogenicity related genes. Such bacteria co-ordinate their virulent behavior in a cell density dependent phenomenon termed as quorum sensing (QS). In contrast, microbes interfere with QS system by quenching the signals, termed quorum quenching (QQ). As a consequence of disrupted QS, pathogens become susceptible to antibiotics and drugs.
3. For anti-virulence therapy for small risk for the development of resistance
Continuing emergence of antibiotic-resistant pathogens is a concern to human health and highlights the urgent need for the development of alternative therapeutic strategies. Quorum sensing (QS) regulates virulence in many bacterial pathogens, and thus, is a promising target for anti-virulence therapy which may inhibit virulence instead of cell growth and division. This means that there is little selective pressure for the evolution of resistance. Many natural quorum quenching (QQ) agents have been identified. Moreover, it has been shown that many microorganisms are capable of producing small molecular QS inhibitors and/or macromolecular QQ enzymes, which could be regarded as a strategy for bacteria to gain benefits in competitive environments.
