Living things can be classified into two groups: unicellular organisms, formed by a single, independent cell (e.g. bacteria); and multicellular organisms, formed by a set of cells that are interdependent, since each cell type has a specialized function (e.g. humans).
In multicellular beings, the communication between different cells is continuous in order to maintain balance and homeostasis in all of the organism’s functions. When we receive a stimulus like high temperatures, specialized cells in our body release molecules to promote sweating in the appropriate cells and so reduce the body temperature. This communication is so vital that any fault in transmitting the message will affect the organism’s balance and the individual’s survival will be threatened. In the case of unicellular organisms, it was thought that within a population each cell acted independently from the rest and could not be influenced by neighboring cells. However, these cells have been found to maintain communication mechanisms in order to perform joint functions to support development of the whole population.
This mechanism is known as quorum sensing (QS) and has permitted certain types of pathogenic bacteria to develop resistance to antibiotics, reducing the efficiency of treatments and increasing development of the disease. Knowing how QS works will help in identifying the vulnerable points in the process where new anti-infectious therapies can be applied. Moreover, understanding how QS is organized allows it to be applied in other areas, for example, as a tool in the organization of computer networks and robotic systems.
What is quorum sensing?
QS is a mechanism that regulates gene expression according to cellular density. This mechanism operates through the release of signal molecules known as ‘autoinducers’, so called because they can act on the same cell that releases them, and are able to trigger gene expression throughout the population, causing a global response.
The concentration of the autoinducer is decisive in starting the QS: the higher the population density (greater number of individuals), the higher the autoinducer’s concentration in the external environment stimulating gene expression.
QS was discovered in a marine bacteria that colonizes a squid symbiotically. This bacteria releases molecules from the autoinducer that accumulate in the external environment during the phase of greatest growth; the high autoinducer concentration allows expression of the gene that encodes the luciferase, the enzyme responsible for the oxidation of luciferin to oxyluciferin; finally an amount of light is emitted when the oxyluciferin loses its excited state, a phenomenon known as bioluminescence.
When the bacterial population density is very low, the autoinducer does not reach concentrations significant enough to trigger gene expression, so the bacteria does not produce light. By contrast, in high bacterial densities the autoinducer concentration also increases and is able to spread inside the bacteria, triggering bioluminescence.
Types of quorum sensing
Different forms of QS have been discovered in various bacterial genera. The difference between them lies in the type of autoinducer molecule used and the global response triggered in the bacteria. A wide variety of responses can be generated, including bioluminescence, virulence, swarming, sporulation and biofilm formation.
The two most common QS types
- Biofilms: Microorganisms using this type of QS usually grow silently while the population density is low, allowing them to go unnoticed by the immune system. Once a high population density is reached, the autoinducers start to release polymers, forming a network that allows the entire population to be grouped and adhered to the living tissue. Tooth decay is a classic example of biofilm.
Biofilms cause around 80% if infectious diseases and are a 1000 times more resistant to antibiotics than when in their free form .
- Swarming: This is really a phenomenon of collective movement in a solid culture when high cellular density is reached. This phenomenon can be observed directly without the aid of devices, appearing as concentric circles around the colony extended over the entire plate.
Finally, the different QS mechanisms in pathogen bacteria have given them capacity to resist antibiotics. However, study is now focused on a new class of antibiotic molecules called Quorum Sensing Inhibitors, which could act in specific points of the gene expression process, preventing its completion.
Omar Santiago Pillaca Pullo
Chemical and Pharmaceutical
 Vipin C. K. Quorum sensing vs Quorum Quenching: A battle with No End in Sight. 2015. Springer India. DOI: 10.1007/978-81-322-1982-8_1