are chemical signaling molecules that are produced and used by bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
participating in quorum sensing
Quorum sensing is a system of stimulus and response correlated to population density. Many species of bacteria use quorum sensing to coordinate gene expression according to the density of their local population. In similar fashion, some social insects use quorum sensing to determine where to nest...
. Quorum sensing is a phenomenon that allows both Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
bacteria to sense one another and to regulate a wide variety of physiological activities. Such activities include symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
Motility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to single-celled and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in...
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
production, and biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
formation. Autoinducers come in a number of different forms depending on the species, but the effect that they have is similar in many cases. Autoinducers allow bacteria to communicate both within and between different species. This communication alters gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
and allows bacteria to mount coordinated responses to their environments, in a manner that is comparable to behavior
Behavior or behaviour refers to the actions and mannerisms made by organisms, systems, or artificial entities in conjunction with its environment, which includes the other systems or organisms around as well as the physical environment...
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
Vibrio fischeri is a gram-negative rod-shaped bacterium found globally in marine environments. V. fischeri has bioluminescent properties, and is found predominantly in symbiosis with various marine animals, such as the bobtail squid. It is heterotrophic and moves by means of flagella. Free living...
produced a luminescent enzyme (luciferase
Luciferase is a generic term for the class of oxidative enzymes used in bioluminescence and is distinct from a photoprotein. One famous example is the firefly luciferase from the firefly Photinus pyralis. "Firefly luciferase" as a laboratory reagent usually refers to P...
) only when cultures had reached a threshold population density. At low cell concentrations, V. fischeri
did not express the luciferase gene. However, once the cultures had reached exponential growth phase, the luciferase gene was rapidly activated. This phenomenon was termed “autoinduction” because it involved a molecule (autoinducer) that accumulated in a growth medium and induced the synthesis of components of the luminescence system. Subsequent research revealed that the actual autoinducer used by V. fischeri
is an acylated homoserine lactone (AHL) signaling molecule.
In the most simplified quorum sensing systems, bacteria only need two components to make use of autoinducers. They need a way to produce a signal and a way to respond to that signal. These cellular processes are often tightly coordinated and involve changes in gene expression. The production of autoinducers generally increases as bacterial cell densities increase. Most signals are produced intracellularly and are subsequently secreted in the extracellular environment. Detection of autoinducers often involves diffusion back into cells and binding to specific receptors. Usually, binding of autoinducers to receptors does not occur until a threshold concentration of autoinducers is achieved. Once this has occurred, bound receptors alter gene expression either directly or indirectly. Some receptors are transcription factors themselves, while others relay signals to downstream transcription factors. In many cases, autoinducers participate in forward feedback loops, whereby a small initial concentration of an autoinducer amplifies the production of that same chemical signal to much higher levels.
Acylated Homoserine Lactones
Primarily produced by Gram-negative bacteria, acylated homoserine lactones (AHLs) are a class of small neutral lipid molecules composed of a homoserine lactone ring with an acyl chain. 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. AHLs are synthesized by AHL synthases. They diffuse in and out of cells by both passive transport
Passive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy, because, unlike in an active transport, the transport across membrane is always coupled with the growth of entropy of the...
and active transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
mechanisms. Receptors for AHLs include a number of transcriptional regulators called “R proteins,” which function as DNA binding transcription factors or sensor kinases.
Gram-positive bacteria that participate in quorum sensing typically use secreted oligopeptides as autoinducers. Peptide autoinducers usually result from posttranslational modification
Posttranslational modification is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis, and thus gene expression, for many proteins....
of a larger precursor molecule. In many Gram-positive bacteria, secretion of peptides requires specialized export mechanisms. For example, some peptide autoinducers are secreted by ATP-binding cassette transporters that couple proteolytic processing and cellular export. Following secretion, peptide autoinducers accumulate in extracellular environments. Once a threshold level of signal is reached, a histidine sensor kinase protein of a two-component regulatory system detects it and a signal is relayed into the cell. As with ASLs, the signal ultimately ends up altering gene expression. Unlike some ASLs, however, most oligopeptides do not act as transcription factors themselves.
Furanosyl borate diester
The free-living bioluminescent marine bacterium, Vibrio harveyi
Vibrio harveyi is a species of Gram-negative, bioluminescent, marine bacteria in the genus Vibrio. V. harveyi are rod-shaped, motile , facultatively anaerobic, halophilic, and competent for both fermentative and respiratory metabolism. They do not grow at 4°C or above 35°C. V...
, uses another signaling molecule in addition to an acylated homoserine lactone. This molecule, termed Autoinducer-2
Autoinducer-2 , a furanosyl borate diester, is a member of a family of signaling molecules used in quorum sensing. AI-2 is unique in that it is one of only a few known biomolecules incorporating boron. First identified in the marine bacterium Vibrio harveyi, AI-2 is produced and recognized by many...
(or AI-2), is a furanosyl borate diester. 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.
Autoinducers in gram-negative bacteria
As mentioned, Gram-negative bacteria primarily use acylated homoserine lactones (AHLs) as autoinducer molecules. The minimum quorum sensing circuit in Gram-negative bacteria consists of a protein that synthesizes an AHL and a second, different protein that detects it and causes a change in gene expression. First identified in V. fischeri
, these two such proteins are LuxI and LuxR, respectively. Other Gram-negative bacteria use LuxI-like and LuxR-like proteins (homologs), suggesting a high degree of evolutionary conservation. However, among Gram-negatives, the LuxI/LuxI-type circuit has been modified in different species. Described in more detail below, these modifications reflect bacterial adaptations to grow and respond to particular niche
Niche may refer to:*Niche , an exedra or an apse that has been reduced in size;*Niche , Colombian/Spanish football player, full name Víctor Manuel Micolta Armero*Niche , a British Thoroughbred racehorse...
Vibrio fischeri: bioluminescence
Ecologically, V. fischeri
is known to have symbiotic associations with a number of eukaryotic hosts, including the Hawaiian Bobtail Squid (Euprymna scolopes
). In this relationship, the squid host maintains the bacteria in specialized light organs. The host provides a safe, nutrient rich environment for the bacteria and in turn, the bacteria provide light. Although bioluminescence can be used for mating and other purposes, in E. scolopes
it is used for counter illumination to avoid predation.
The autoinducer molecule used by V. fischeri
is N-(3-oxohexanoyl)-homoserine lactone. This molecule is produced in the cytoplasm by the LuxI synthase enzyme and is secreted through the cell membrane into the extracellular environment. As is true of most autoinducers, the environmental concentration of N-(3-oxohexanoyl)-homoserine lactone is the same as the intracellular concentration within each cell. N-(3-oxohexanoyl)-homoserine lactone eventually diffuses back into cells where it is recognized by LuxR once a threshold concentration (~10 μg/ml) has been reached. LuxR binds the autoinducer and directly activates transcription of the luxICDABE
operon. This results in an exponential increase in both the production of autoinducer and in bioluminescence. LuxR bound by autoinducer also inhibits the expression of luxR
, which is thought to provide a negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
compensatory mechanism to tightly control levels of the bioluminescence genes.
Pseudomonas aeruginosa: virulence & antibiotic production
is an opportunistic human pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
associated with cystic fibrosis
Cystic fibrosis is a recessive genetic disease affecting most critically the lungs, and also the pancreas, liver, and intestine...
. In P. aeruginosa
infections, quorum sensing is critical for biofilm formation and pathogenicity. P. aeruginosa
contains two pairs of LuxI/LuxR homologs, LasI/LasR and RhlI,RhlR. LasI and RhlI are synthase enzymes that catalyze the synthesis of N-(3-oxododecanoyl)-homoserine lactone and N-(butryl)-homoserine lactone, respectively. The LasI/LasR and the RhlI/RhlR circuits function in tandem to regulate the expression of a number of virulence genes. At a threshold concentration, LasR binds N-(3-oxododecanoyl)-homoserine lactone. Together this bound complex promotes the expression of virulence factors that are responsible for early stages of the infection process.
LasR bound by its autoinducer also activates the expression of the RhlI/RhlR system in P. aeruginosa
. This causes the expression of RhlR which then binds its autoinducer, N-(butryl)-homoserine lactone. In turn, autoinducer-bound RhlR activates a second class of genes involved in later stages of infection, including genes needed for antibiotic production. Presumably, antibiotic production by P. aeruginosa
is used to prevent opportunistic infections by other bacterial species. Interestingly, N-(3-oxododecanoyl)-homoserine lactone prevents binding between N-(butryl)-homoserine lactone and its cognate regulator, RhlR. 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.
Other gram-negative autoinducers
- P. aeruginosa also uses 2-heptyl-3-hydroxy-4-quinolone (PQS) for quorum sensing. This molecule is noteworthy because it does not belong to the homoserine lactone class of autoinducers. PQS is believed to provide an additional regulatory link between the Las and Rhl circuits involved in virulence and infection.
- Agrobacterium tumefaciens
Agrobacterium tumefaciens is the causal agent of crown gall disease in over 140 species of dicot. It is a rod shaped, Gram negative soil bacterium...
is a plant pathogen that induces tumors on susceptible hosts. Infection by A. tumefaciens involves the transfer of an oncogenic plasmid from the bacterium to the host cell nucleus, while quorum sensing controls the conjugal transfer of plasmids between bacteria. Conjugation
Conjugation or conjugate may refer to:* Conjugation , the modification of a verb from its basic form* Conjugate , used to rationalize the denominator of a fraction...
, on the other hand, requires the HSL autoinducer, N-(3-oxoctanoyl)-homoserine lactone.
- Erwinia carotovora
Pectobacterium carotovorum is a bacterium of the family Enterobacteriaceae, which is a plant pathogen with a diverse host range. It causes soft rot and blackleg of potato and vegetables as well as slime flux on many different tree species....
is another plant pathogen that causes soft-rot disease. These bacteria secrete cellulases and pectinases, which are enzymes that degrade plant cell walls. ExpI/ExpR are LuxI/LuxR homologs in E. carotovora believed to control secretion of these enzymes only when a high enough local cell density is achieved. The autoinducer involved in quorum sensing in E. carotovora is N-(3-oxohexanoyl)-L-homoserine lactone.
Autoinducers in gram-positive bacteria
Whereas Gram-negative bacteria primarily use acylated homoserine lactones, Gram-positive bacteria generally use oligopeptides as autoinducers for quorum sensing. These molecules are often synthesized as larger polypeptides that are cleaved post-translationally to produce “processed” peptides. Unlike HSLs that can freely diffuse across cell membranes, peptide autoinducers usually require specialized transport mechanisms (often ABC transporters). Additionally, they do not freely diffuse back into cells, so bacteria that use them must have mechanisms to detect them in their extracellular environments. Most Gram-positive bacteria use a two-component signaling mechanism in quorum sensing. Secreted peptide autoinducers accumulate as a function of cell density. Once a quorum level of autoinducer is achieved, its interaction with a sensor kinase at the cell membrane initiates a series of phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
events that culminate in the phosphorylation of a regulator protein intracellularly. This regulator protein subsequently functions as a transcription factor and alters gene expression. Similar to Gram-negative bacteria, the autoinduction and quorum sensing system in Gram-positive bacteria is conserved, but again, individual species have tailored specific aspects for surviving and communicating in unique niche environments.
Streptococcus pneumoniae: competence
is human pathogenic bacterium in which the process of genetic transformation
-Mathematics:* Transformation * Transformation * Integral transform* Data transformation * Transformation matrix-Natural science:* Phase transformation, a physical transition from one medium to another...
was first described in the 1930s. In order for a bacterium to take up exogenous DNA from its surroundings, it must become competent. 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. Competence stimulating peptide (CSP) is a 17-amino acid peptide autoinducer required for competency and subsequent genetic transformation. 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. Detection is followed by autophosphorylation of ComD, which in turn, phosphorylates ComE. ComE is a response regulator responsible for activating transcription of comX
, the product of which is required to activate transcription of a number of other genes involved in the development of competence.
Bacillus subtilis: competence & sporulation
B. subtilis is a soil-dwelling microbe that uses quorum sensing to regulate two different biological processes: competence and sporulation. During stationary growth phase when S. pneumoniae are at high cell density, approximately 10% of the cells in a population are induced to become competent. It is believed that this subpopulation becomes competent to take up DNA that could potentially be used for the repair of damaged (mutated) chromosomes. ComX (also known as competence factor) is a 10-amino acid peptide that is processed from a 55-amino acid peptide precursor. Like most autoinducers, ComX is secreted and accumulates as a function of cell density. Once a threshold extracellular level is achieved, ComX is detected by a two-component ComP/ComA sensor kinase/response regulator pair. Phosphorylation of ComA activates the expression of comS
gene, ComS inhibits the degradation of ComK, and finally ComK activates the expression of a number of genes required for competence.
Sporulation, on the other hand, is a physiological response of B. subtilis
to depletion of nutrients within a particular environment. It is also regulated by extracellular signaling. When B. subtilis
populations sense waning conditions, they respond by undergoing asymmetric cell division. This ultimately produces spores that are adapted for dispersal and survival in unfavorable conditions. Sporulation in B. subtilis
is mediated by CSF (sporulation factor), a pentapeptide cleaved from the precursor peptide PhrC. CSF is secreted into the extracellular environment and is taken back up into cells via the ABC transporter Opp where it acts intracellularly. While low internal concentrations of CSF contribute to competence, high concentrations induce sporulation. CSF inhibits a phosphatase, RabB, which increases the activity of Spo0A, favoring a switch in commitment from competence to the sporulation pathway