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Quorum sensing : Bacteria talk with each other

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Quorum-sensing bacteria leak into their surroundings a chemical that, like body odor, indicates their presence. As the bacterial population increases, so does the concentration of this chemical, called an autoinducer. When the amount of autoinducer reaches a certain level, the whole bacterial neighborhood is transformed. The once-independent, solitary bacterial cells suddenly become part of a large, multicellular organism.


An everyday example of such bacterial transfiguration is the tenacious slime that can accumulate on teeth, ships, bathtubs, and kitchen drains. These slippery coatings are actually sophisticated microbial communities called biofilms in which bacteria either become part of protective coatings, walls, or nutrient-filled channels, or they swim around inside these structures as highly mobile, independent cells.


Bacteria exist within complex communities termed biofilms on solid surfaces, including plant roots. Members of these communities interact both cooperatively and competitively to enable the inhabitants to perform functions not possible by single organisms.


The discovery that bacteria are able to communicate with each other changed our general perception of many single, simple organisms inhabiting our world. Instead of language, bacteria use signalling molecules which are released into the environment. As well as releasing the signalling molecules, bacteria are also able to measure the number (concentration) of the molecules within a population. Nowadays we use the term 'Quorum Sensing' (QS) to describe the phenomenon whereby the accumulation of signalling molecules enable a single cell to sense the number of bacteria (cell density). In the natural environment, there are many different bacteria living together which use various classes of signalling molecules. As they employ different languages they cannot necessarily talk to all other bacteria. Today, several quorum sensing systems are intensively studied in various organisms such as marine bacteria and several pathogenic bacteria.


QS enables bacteria to co-ordinate their behaviour. As environmental conditions often change rapidly, bacteria need to respond quickly in order to survive. These responses include adaptation to availability of nutrients, defence against other microorganisms which may compete for the same nutrients and the avoidance of toxic compounds potentially dangerous for the bacteria. It is very important for pathogenic bacteria during infection of a host (e.g. humans, other animals or plants) to co-ordinate their virulence in order to escape the immune response of the host in order to be able to establish a successful infection.


Quorum sensing was first observed in Vibrio fischeri, a bioluminiscent bacterium that lives as a symbiont in the light-producing organ of the Hawaiian bobtail squid. When V. fischeri cells are free-living (or planktonic), the autoinducer is at low concentration and thus cells do not luminesce. In the light organ of the squid (photophore), they are highly concentrated (about 1011 cells/ml) and transcription of luciferase is induced, leading to bioluminescence.


In the past 10 years, it has become well established that disease-causing genes of most pathogens are expressed in response to diffusible signals released from the host.


Tapping into the chemical communiques needed to make biofilms has many applications. Biofilm-blocking substances could reduce illness caused by infected catheters and other implanted medical devices. They could prevent bacterial buildup in industrial cooling towers and swimming pools. And, if added to toothpaste or mouthwash, they could battle tooth decay.


By studying the way bacteria join together into organized groups, scientists also learn how humans and other higher organisms developed, Bassler says. After all, our bodies are essentially collections of genetically identical cells that, based on chemical cues, specialized into different tissues (skin, bones, a heart, brain, and so forth). According to Bassler, these tissues probably interact with each other using chemical crosstalk similar to that in bacteria.



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