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 involves the production, release, and community-wide sensing of molecules called auto inducers that modulate gene expression in response to the density of a bacterial population. When auto inducers produced by one bacterium cross the membrane of another, they bind to receptors in the cytoplasm. This auto inducer/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 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 quantity 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 auto inducers. 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. As some pathogenic organisms are increasingly resistant to traditional antibiotics, medical researchers around the world are trying to find other ways of fighting infection. Using the ‘quorum quenching’ compounds which interfere with bacterial signaling and disrupt their social life can be really helpful.