QUORUM SENSING-Bio Chemistry

QUORUM SENSING-Bio Chemistry

QUORUM SENSING-Bio Chemistry

Quorum sensing is a phenomena used by many species of bacteria to describe cell to cell communication and trigger expression of particular gene according to density of their local population.QUORUM SENSING-Bio Chemistry

Quorum sensing play significant role to perform diverse biological functions in bacteria such as formation of biofilm, virulence and antibiotic resistance based on population density. Some social insects also use quorum sensing in order to determine where to nest.

Historical background:

This mechanism was first discovered in Vibrio fischeri mutualistic symbiont in the photophore (or light-producing organ) when it was observed that the cultures of bacteria produced light when large number of bacteria were present, for example ants and honey bees.

Mechanism of quorum sensing:

By using the phenomena of quorum sensing, bacteria produce certain signalling molecules that are called autoinducers or pheromones. These molecules bind to the receptors sites of these bacteria’s that have ability to detect these signalling molecules. It activates the transcription of certain genes depending upon their cell density.

This is the basic mode of quorum sensing present in both Gram-positive and Gram- negative bacteria.

There are varieties of different molecules that can be used as signals. Common classes of such signalling molecules are oligopeptides in Gram-positive bacteria, N-Acyl Homoserine Lactones (AHL) in Gram-negative bacteria. A family of autoinducers known as autoinducer-2 (AI-2) is present in both Gram-negative and Gram-positive bacteria.

Quorum sensing in bacterial biofilms:

Cells take the form of biofilms when their colonies surpass a certain density of individuals. The greater the number of bacterial cells, the richer its environment is in quorum-sensing signals. Above a certain density, quorum-sensing signals switch on number of genes. Biofilm organisms are significantly more resistant to environmental stresses and antibiotics. So, they can stay for longer period in their host organisms.

Examples:

Vibrio cholera:

This bacteria is the causative agent of human disease cholera, uses the mechanism of quorum sensing for cell-to-cell communication and detection of signalling molecules. When the cell density is low, Vibrio cholera activates the secretion of signalling molecules and starts the formation of biofilm. When cell density is high, the reverse mechanism occurs. The accumulation of quorum sensing autoinducers is inhibited which supresses these traits.

Escherichia coli & Salmonella enterica:

E.coli and Salmonella enterica, both are commonly found in Gram-negative bacteria. They do not produce AHLs rather they have receptors that detect AHLs from other bacteria and change the expression of genes according to their population.

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