Bacterial Communication: Quorum Sensing in Bacteria

Many bacteria are able to sense their own population density, communicate with each other by way of secreted chemical factors, and behave as a population rather than as individual bacteria . This is referred to as cell-to-cell signaling or quorum sensing and plays an important role in pathogenicity and survival for many bacteria.

Quorum sensing (def) involves the production, release, and community-wide sensing of molecules called autoinducers (def) that modulate gene expression in response to the density of a bacterial population. When autoinducers produced by one bacterium cross the membrane of another, they bind to receptors in the cytoplasm. This autoinducer/receptor complex is then able to bind to DNA promoters and activate the transcription of quorum sensing-controlled genes. In this way, individual bacteria within a group are able to benefit from the activity of the entire group.

The outcomes of bacteria-host interaction are often related to bacterial population density. Bacterial virulence, that is its ability to cause disease, is largely based on the bacterium's ability to produce gene products called virulence factors that enable that bacterium to colonize the host, resist body defenses, and harm the body. If a relatively small number of a specific bacteria were to enter the body and immediately start producing their virulence factors, chances are the body's immune systems would have sufficient time to recognize and counter those virulence factors and remove the bacteria before there was sufficient quanity to cause harm. Many bacteria are able to delay production of those virulence factors by not expressing the genes for those factors until there is a sufficiently large enough population of that bacterium (a quorum). As the bacteria geometrically increase in number, so does the amount of their secreted autoinducers. When a critical level of autoinducer is reached, the entire population of bacteria is able to simultaneously activate the transcription of their quorum-sensing genes and the body's immune systems are much less likely to have enough time to counter those virulence factors before harm is done.

By detecting population density, bacteria can also sense when a host site has become more or less saturated with bacteria and activate quorum sensing genes, such as those involved in motility, in order to move to and colonize a new site.

For example, Pseudomonas aeruginosa causes severe nosocomial infections, chronic infections in people with cystic fibrosis, and potentially fatal infections in those who are immunocompromised. Its virulence depends on the secretion of a variety of harmful exotoxins and enzymes. If there was an isolated production of these virulence toxins and enzymes by a small number of Pseudomonas, the body's immune responses would most likely be able effectively neutralize these harmful agents with antibodies. However, through a coordination of the expression of the genes coding for these toxins and enzymes by the entire population of bacteria, P. aeruginosa appears to only secrete these extracellular virulence factors when the density of bacteria is large enough that they can be produced at high enough levels to overcome body defenses.

In addition, P. aeruginosa uses quorum sensing genes to form a biofilm that enables the bacteria to adhere to body surfaces, resist phagocytosis, and be less susceptible to antibiotics. However, when that body site becomes saturated with P. aeruginosa, quorum sensing genes for flagella formation become activated allowing some of the Pseudomonas to leave the biofilm and swim to a new location to start another biofilm population.

Bacteria not only produce species or strain specific autoinducers to allow for intraspecies communication (def), they also produce a common autoinducer reconized by other, different bacteria enabling interspecies communication (def). In this way they can detect the density of other populations of bacteria in their immediate environment.

Other quorum sensing-controlled processes in bacteria include, sporulation, antibiotic production, mating, and bioluminescence.

To hear a talk by Dr. Bonnie Bassler, one of the pioneers in bacterial quorum sensing, go to the following link: http://www.ted.com/talks/bonnie_bassler_on_how_bacteria_communicate.html.