QUORUM SENSING IN A NUTSHELL

Unicellular organisms like bacteria have a unique system which enable them to act like multicellular organisms by using some special chemical languages to perform several collective functions.

In the same way humans make use of various languages which they understand for communicating among themselves to solve problems and achieve common goals, so do bacteria use chemical languages to communicate with their siblings to carry out several activities such as virulence, sporulation, symbiosis, bioluminescence, motility and antibiotic production.

When bacteria gather in multitudes, they produce special chemical words among themselves, which they recognize, understand and then use it to instruct each other about the activities they would like to carry out in UNISON! This is done using a sysytem called quorum sensing.

A quorum is the minimum number of members needed in a group to officially conduct a business or cast votes. In this manner, bacteria come together to cast their chemical votes, which is eventually counted and then everybody respond to take actions.

Quorum sensing is a communication system in bacteria cells which they carry out by releasing and sensing some chemical words or signalling molecules called autoinducers. They recognize these words (autoinducers) and then they turn on group behaviours that are only successful when all of the cells participate in unison. It is not something that a single cell can carry out alone.

The success of this system depend greatly on the ability of bacterial cells to continuously divide: from one to two, to four, eight and so on, to form colonies that would be enormous enough to conduct the business.

Participating in unison using this system enable them to: control pathogenicity, take up DNA in there surrounding, carry out nitrogen fixation, bioluminescence and so on.

Based on human relationship with bacteria, it is obvious that virulence is one of the most important to us. So can a single pathogenic bacteria find it's way into our body to cause diseases? 

The answer is a simple NO.

We are so huge! therefore, a single bacteria cannot enter our body and cause diseases.

For example, a pathogenic (disease-causing) bacterium that secretes a toxin will have no effect as a single cell; production of toxin by one cell alone would merely waste resources. However, they do what they want by quorum sensing, that is, they come in and multiply until a sufficiently large population of cells is present, enough members to launch their virulence attack. 

Let us take a look at the mechanism of this system:

The pathway to accomplish this consist of bacteria populations, signal molecules, and behavioral genes.

A signal producing protein in individual bacterial cells produce the chemical words or signalling molecules which are called autoinducers.

As the autoinducer diffuses into the surrounding medium, other bacterial cells move toward the source and begin producing autoinducers. The concentration of the inducer increases as cell numbers increase. This, in turn, attracts more cells and initiates synthesis of more autoinducers.

They keep producing these autoinducers among themselves until the autoinducers hit a certain amount which tells the bacteria how many they are. They recognize these molecules (autoinducer) which are like private secret conversation among them and then they all act in synchrony; this tells us that each bacteria has its own specific language or autoinducer which allows it to count its siblings. 

The molecules (autoinducers) are a little bit different in every single species, just the same way various human populations have different languages. Bacteria use these languages to distinguish themselves from others.

For example, the gram negative bacteria have N-acyl homoserine lactone (AHL) as their autoinducer while gram positives use special oligopeptides termed autoinducing peptides (AIPs).

When the autoinducers reach the sufficient amount, they start binding to their partner receptors that are found around each bacterial cells and then information comes into the cells which trigger series of events  which include the transcription of genes encoding virulence factors such as the secretion of toxins in the case of virulence or encoding and expression of genes for several other collective behaviours such as biofilm formation, bioluminescence and nitrogen fixation.

An interesting research about quorum sensing found out that these bacteria can be made deaf and dumb so that they won't be able to recognize their chemical languages (autoinducers) especially now that most of them are becoming resistant to antibiotics. This can be achieved giving them some molecules called antiquorum sensing molecules that look like autoinducers to make them not to hear, talk, nor launch virulence. These molecules compete with the autoinducers to bind to receptors. 

Advances are being made to make these antiquorum sensing molecules medicine-like so that they can also be embedded in materials such as breathing tubes, catheters and shunts to make them resistant to biofilm formation and possible subsequent infections.

Conclusion.

    

Quorum sensing therefore allow individual bacterial cells to communicate among theirselves by sensing and monitoring signal concentration and size of their commmunity which enable them to coordinate and accomplish various collective behaviours which would be impossible for individual cells to carry out alone.