Quorum sensing - communication plan for microbes.

Bacteria do talk with each other. The discovery of Quorum sensing is a closer step in understanding the bacterial talk. It is one of the several density dependent functions exhibited by most bacterial colonies. Bacteria usually change or show different phenotypes depending on the density of bacterial cells in a given area, say for example the agar surface on a Petri dish.

It is in effect a kind of gene regulation in which production of an 'auto inducer' by the bacterial cells switch on or activates the genes which cause visible changes in phenotype. Quorum sensing can have its applications in plant and animal cell culture, control of infectious diseases, apoptosis mechanisms in cancer treatments, regulation of fermentation for production of secondary metabolites etc.

Gene expressions are altered in response to environmental fluctuations which is caused due to the increase in cell density. This is found in both Gram positive as well as Gram negative bacteria for regulation of processes such as motility, spore formation, symbiosis, conjugation, virulence, competence, antibiotic production etc.

These inter as well as intra specific means of bacterial communication is mediated by a group of chemicals called 'autoinducers'. Acylated homoserine lactones are the general autoinducers of Gram negative bacteria whereas oligopeptides are processed by the Gram positive bacteria to regulate quorum sensing. The mechanism of quorum sensing differs in different species of bacteria. However, the process enables them to coordinate in a better way, eliciting specific responses from the host community and therefore determine the behavioral patterns of the entire bacterial colony. This is expressed as phenotypical changes associated with such regulations.

Quorum sensing in bacteria is parallel to communication among higher organisms. Therefore it is considered as the first step in multicellularity in the course of evolutionary history.

The phenomenon was first discovered in certain marine bacterial colonies of V.harveyi and V. fischeri. The individual cells are non luminescent whereas when grown in cultures in laboratory conditions appeared luminescent with a blue green light. The bacteria also expressed the bio luminescence in symbiotic relations with marine organisms such as Japanese pinecone fish and certain squids. These marine organisms were found to have ideal conditions in their light organs for the growth of these bacterial species to such levels needed to express the luminescence. The bacterial species in turn provided the host with the necessary light to feed in dark ocean depths. 3-oxo-hexanoylhomoserine lactones (3-oxo-C6-HSL) was found to be the autoinducer in the regulation of the lux gene responsible for the production of luciferase enzyme involved in this system.

By quorum sensing, each bacterial cell can sense the presence of signaling molecules as well as determine the cell density and elicit specific response which is a cell density dependent function. The term was first described by Fuqua et al. It essentially described the minimum threshold of cell density needed to evoke a concerted response from a population of microorganisms. The signaling molecule or autoinducer is reimportable into the bacterial cells after the critical level is achieved for a specific cell density.

These autoinducers can be broadly classified into two categories.
1. aminoacids and oligo peptide derivatives
2. homoserine lactones

The entire process is simple. Individual cells in a bacterial colony produce signaling molecules at a specific rate and secrete them into the extracellular space. Once the concentration of the autoinducer in the extracellular space reaches a certain threshold level, the molecules reenter the cell via diffusion or active transport. These signaling molecules then interact with an intracellular effector usually a specific receptor which will in turn activate the concerned genes for the desired phenotype. The genes are usually up regulated to activate a signal cascade which result in production of secondary characteristics like antibiotic or biofilm production. The phenotypic effect is in turn sensed by the other cells in the extracellular space.

Quorum sensing is the tool for bacterial attack. Bacteria often thrive at drastic conditions which are prone to constant and rapid fluctuations. QS enable them to adapt better to such changes. The mechanism is especially important to pathogenic bacteria to establish an infection and coordinate their virulence. It also helps to evade the host's immune response and establish an infection.

Similar mechanisms have been discovered in other microorganisms. For example, S.pneumoniae uses a peptide to regulate the genetic transformation mechanism. B.sublitis has two quorum sensing mechanisms to regulate spore formation and genetic competence. Further mechanisms need to be unveiled in this unusual bacterial sensing to develop novel methods of treatment to prevent infection and increase production of secondary metabolites from these microorganisms.
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