How bacteria talk to each other:
communication and
quorum sensing in the
bacterial world
For decades microbiologists tended to think of bacterial populations as collections of individuals growing and behaving independently. In this conventional view of prokaryotic existence, bacteria live unicellularly, with responses to external stimuli limited to the detection of chemical and physical signals of environmental origin. This view of bacteriology is now recognized to be overly simplistic, because bacteria
communicate with each other through small 'hormone-like' organic compounds. These bacterial cell-to-cell signaling systems were initially described as mechanisms through which bacteria regulate gene expression via
cell density and, therefore, they have been collectively termed quorum sensing (QS).
Quorum sensing is an example of what might be called multi-cellular behavior in that many individual cells communicate and coordinate their activities to act as a unit. Its primary function is to sense population densities and diffusion rates. Only further experimentation can determine the role of this phenomenon in various microorganisms.
Gene expression in bacteria can be regulated by a wide variety of intra- and extracellular signals. In fact, numerous morphological and physiological changes are induced by chemical and physical changes in the local environment. Such adaptive responses involve chemical perception and information processing that transiently alters gene expression patterns so as to protect against environmental threats. The discovery that bacteria themselves can produce extracellular chemical signals for intercellular communication has evoked a new paradigm for gene regulation.
Now generally termed 'quorum sensing' or autoinduction, bacterial cell-to-cell communication enables population density-based control of gene transcription via the production, release and sensing of low-molecular weight compounds. In the majority of cases, the concentration of extracellular autoinducer increases concomitantly with the bacterial cell density. Upon reaching a 'critical' autoinducer concentration, a signal transduction cascade is triggered that results in expression of a target gene(s).
Recent evidence has demonstrated that such cell-to-cell signaling is a fundamental activity carried out by numerous microorganisms. A number of specialized processes are reported to be regulated by density-dependent signaling molecules including antibiotic production, bioluminescence, biofilm formation, genetic competence, sporulation, swarming motility and virulence. However, a more centralized role for QS is emerging where quorum signaling pathways overlap with stress and starvation circuits to regulate cellular adaptation to changing environmental conditions. In global regulatory system one potential “signal” that prokaryotes can respond to is the presence in their surroundings of other cells of the same species.
Quorum sensing makes good practical sense. It ensures that sufficient cell population is reached before eliciting a particular response. For example a pathogenic bacterium that secretes a toxin can have no effect as a single cell; production of the toxin by that cell would be a waste of resources. However, if there is a sufficiently high population of cells present, the coordinated expression of the toxin may successfully initiate a disease.
An alternative function for the production and release of such signaling molecules is to determine the extent of diffusion and mixing in a cell’s immediate environment. When there is too much mixing and diffusion, it would not make sense to release molecules such as proteases and antibiotics. If the concentration of the signaling compound rapidly dropped after release, it could signal the cell that other molecules should not be secreted.
Thus, quorum sensing systems have evolved as a means for improving a microbe's access to complex nutrients or environmental niches or for collectively enhancing its defense capabilities against other microorganisms or eukaryotic host defense mechanisms.
More abstracts about the Qourum sensing