In biology, an autoinducer is a signaling molecule that enables detection and response to changes in the population density of bacterial cells.
[1] They are a key component of the phenomenon of quorum sensing: as the density of quorum-sensing bacterial cells increases, so does the concentration of the autoinducer.
[2][3] Performed by both Gram-negative and Gram-positive bacteria, detection of autoinducers allows them to sense one another and to regulate a wide variety of physiological activities, including symbiosis, virulence, motility, production of antibiotics, and formation of biofilms.
They allow bacteria to communicate both within and between species, and thus to mount coordinated responses to their environments in a manner that is comparable to behavior and signaling in higher organisms.
Not surprisingly, it has been suggested that quorum sensing may have been an important evolutionary milestone that ultimately gave rise to multicellular life forms.
The term autoinduction was first coined in 1970, when it was observed that the bioluminescent marine bacterium Vibrio fischeri produced a luminescent enzyme (luciferase) only when cultures had reached a threshold population density.
This phenomenon was called autoinduction because it involved a molecule (the autoinducer) produced by the bacteria themselves that accumulated in the growth medium and induced the synthesis of components of the luminescence system.
[6] Subsequent research revealed that the actual autoinducer used by V. fischeri is an acylated homoserine lactone (AHL) signaling molecule.
[7] AHLs produced by different species of Gram-negative bacteria vary in the length and composition of the acyl side chain, which often contains 4 to 18 carbon atoms.
For example, some peptide autoinducers are secreted by ATP-binding cassette transporters that couple proteolytic processing and cellular export.
The free-living bioluminescent marine bacterium, Vibrio harveyi, uses another signaling molecule in addition to an acylated homoserine lactone.
[14] AI-2, which is also produced and used by a number of Gram-negative and Gram-positive bacteria, is believed to be an evolutionary link between the two major types of quorum sensing circuits.
[4] Ecologically, V. fischeri is known to have symbiotic associations with a number of eukaryotic hosts, including the Hawaiian Bobtail Squid (Euprymna scolopes).
[19] This molecule is produced in the cytoplasm by the LuxI synthase enzyme and is secreted through the cell membrane into the extracellular environment.
Together this bound complex promotes the expression of virulence factors that are responsible for early stages of the infection process.
[28] It is believed that this control mechanism allows P. aeruginosa to initiate the quorum-sensing cascades sequentially and in the appropriate order so that a proper infection cycle can ensue.
In S. pneumoniae, a number of complex events must occur to achieve a competent state, but it is believed that quorum sensing plays a role.
[36] CSP is produced by proteolytic cleavage of a 41-amino acid precursor peptide (ComC); is secreted by an ABC transporter (ComAB); and is detected by a sensor kinase protein (ComD) once it has reached a threshold concentration.
[46] CSF is secreted into the extracellular environment and is taken back up into cells via the ABC transporter Opp where it acts intracellularly.
CSF inhibits a phosphatase, RabB, which increases the activity of Spo0A, favoring a switch in commitment from competence to the sporulation pathway[41]