at all." vibrio species utilize qrr rnas, small non-coding rnas, that are activated by these autoinducers to target cell density master regulators. farnesol is used by the fungus candida albicans as a quorum sensing molecule that inhibits filamentation. a database of quorum-sensing peptides is available under the name quorumpeps. certain bacteria can produce enzymes called lactonases that can target and inactivate ahls. researchers have developed novel molecules which block the signalling receptors of bacteria ("quorum quenching"). mbtl is a compound that has been shown to inhibit quorum sensing and decrease the amount of cell death by a significant amount. additionally, researchers are also examining the role of natural compounds (such as caffeine) as potential quorum sensing inhibitors. research in this area has been promising and could lead to the development of natural compounds as effective therapeutics. sequence analysis the majority of quorum sensing systems that fall under the "two-gene" (an autoinducer synthase coupled with a receptor molecule) paradigm as defined by the vibrio fischeri system occur in the gram-negative pseudomonadota. a comparison between the pseudomonadota phylogeny as generated by 16s ribosomal rna sequences and phylogenies of luxi-, luxr-, or luxs-homologs shows a notably high level of global similarity. overall, the quorum sensing genes seem to have diverged along with the pseudomonadota phylum as a whole. this indicates that these quorum sensing systems are quite ancient, and arose very early in the pseudomonadota lineage. although examples of horizontal gene transfer are apparent in luxi, luxr, and luxs phylogenies, they are relatively rare. this result is in line with the observation that quorum sensing genes tend to control the expression of a wide array of genes scattered throughout the bacterial chromosome. a recent acquisition by horizontal gene transfer would be unlikely to have integrated itself to this degree. given that the majority of autoinducer–synthase/receptor pairs occur in tandem in bacterial genomes, it is also rare that they switch partners and so pairs tend to co-evolve. in quorum sensing genes of gammaproteobacteria, which includes pseudomonas aeruginosa and escherichia coli, the luxi/luxr genes form a functional pair, with luxi as the auto-inducer synthase and luxr as the receptor. gammaproteobacteria are unique in possessing quorum sensing genes, which, although functionally similar to the luxi/luxr genes, have a markedly divergent sequence. this family of quorum-sensing homologs may have arisen in the gammaproteobacteria ancestor, although the cause of their extreme sequence divergence yet maintenance of functional similarity has yet to be explained. in addition, species that employ multiple discrete quorum sensing systems are almost all members of the gammaproteobacteria, and evidence of horizontal transfer of quorum sensing genes is most evident in this class. interaction of quorum-sensing molecules with mammalian cells and its medical applications next to the potential antimicrobial functionality, quorum-sensing derived molecules, especially the peptides, are being investigated for their use in other therapeutic domains as well, including immunology, central nervous system disorders and oncology. quorum-sensing peptides have been demonstrated to interact with cancer cells, as well as to permeate the blood–brain barrier reaching the brain parenchyma. role of quorum sensing in biofilm development when aggregated in high enough densities, some bacteria may form biofilms to protect themselves from biotic or abiotic threats. biofilms may also serve to transport nutrients into the microbial community or transport toxins out by means of channels that permeate the extracellular polymeric matrix (like cellulose) that holds the cells together. finally, biofilms are an ideal environment for horizontal gene transfer through either conjugation or environmental dna (edna) that exists in the biofilm matrix. the process of biofilm development is often triggered by environmental signals, and bacteria are proven to require flagella to successfully approach a surface, adhere to it, and form the biofilm. as cells either replicate or aggregate in a location, the concentration of autoinducers outside of the cells increases until a critical mass threshold is reached. at this point, it is energetically unfavorable for intracellular autoinducers to leave the cell and they bind to receptors and trigger a signaling cascade to initiate gene expression and begin secreting an extracellular polysaccharide to encase themselves inside. archaea methanosaeta harundinacea 6ac methanosaeta harundinacea 6ac, a methanogenic archaeon, produces carboxylated acyl homoserine lactone compounds that facilitate the transition from growth as short cells to growth as filaments. viruses a mechanism involving arbitrium has recently been described in bacteriophages infecting several bacillus species. the viruses communicate with each other to ascertain their own density compared to potential hosts. they use this information to decide whether to enter a lytic or lysogenic life-cycle. plants qs is important to plant-pathogen interactions, and their study has also contributed to the qs field more generally. the first x-ray crystallography results for some of the key proteins were those of pantoea stewartii subsp. stewartii in maize/corn and agrobacterium tumefaciens, a crop pathogen with wider range of hosts. these interactions are facilitated by quorum-sensing molecules and play a major role in maintaining the pathogenicity of bacteria towards other hosts, such as humans. this mechanism can be understood by looking at the effects of n-acyl homoserine lactone (ahl), one of the quorum sensing-signaling molecules in gram-negative bacteria, on plants. the model organism used is arabidopsis thaliana. the role of ahls having long carbon-chains (c12, c14), which have an unknown receptor mechanism, is less well understood than ahls having short carbon-chains (c4, c6, c8), which are perceived by the g protein-coupled receptor. a phenomenon called "ahl priming", which is a dependent signalling pathway, enhanced our knowledge of long-chain ahls. the role of quorum-sensing molecules was better explained according to three categories: host physiology–based impact of quorum sensing molecules; ecological effects; and cellular signaling. calcium signalling and calmodulin have a large role in short-chain ahls' response in arabidopsis. research was also conducted on barley and the crop called yam bean (pachyrhizus erosus) that reveals the ahls determining the detoxification enzymes called gst were found less in yam bean. quorum sensing-based regulatory systems are necessary to plant-disease-causing bacteria. looking towards developing new strategies based on plant-associated microbiomes, the aim of further study is to improve the quantity and quality of the food supply. further research into this inter-kingdom communication also enhances the possibility of learning about quorum sensing in humans. quorum quenching quorum quenching is the process of preventing quorum sensing by disrupting signalling. this is achieved by inactivating signalling enzymes, by introducing molecules that mimic signalling molecules and block their receptors, by degrading signalling molecules themselves, or by a modification of the quorum sensing signals due to an enzyme activity. inhibition closantel and triclosan are known inhibitors of quorum sensing enzymes. closantel induces aggregation of the histidine kinase sensor in two-component signalling. the latter disrupts the synthesis of a class of signalling molecules known as n-acyl homoserine lactones (ahls) by blocking the enoyl-acyl carrier protein (acp) reductase. mimicry two groups of well-known mimicking molecules include halogenated furanones, which mimic ahl molecules, and synthetic al peptides (aips), which mimic naturally occurring aips. these groups inhibit receptors from binding substrate or decrease the concentration of receptors in the cell. furanones have also been found to act on ahl-dependant transcriptional activity, whereby the half life of the autoinducer-binding luxr protein is significantly shortened. degradation recently, a well-studied quorum quenching bacterial strain (km1s) was isolated and its ahl degradation kinetics were studied using rapid resolution liquid chromatography (rrlc). rrlc efficiently separates components of a mixture to a high degree of sensitivity, based on their affinities for different liquid phases. it was found that the genome of this strain encoded an inactivation enzyme with distinct motifs targeting the degradation of ahls. modifications as mentioned before, n-acyl-homoserine lactones (ahl) are the quorum sensing signaling molecules of the gram-negative bacteria. however, these molecules may have different functional groups on their acyl chain, and also a different length of acyl chain. therefore, there exist many different ahl signaling molecules, for example, 3-oxododecanoyl-l-homoserine lactone (3oc12-hsl) or 3-hydroxydodecanoyl-l-homoserine lactone (3ohc12-hsl). the modification of those quorum sensing (qs) signaling molecules is another sort of quorum quenching. this can be carried out by an oxidoreductase activity. as an example, we will discuss the interaction between a host, hydra vulgaris, and the main colonizer of its epithelial cell surfaces, curvibacter spp. those bacteria produce 3-oxo-hsl quorum sensing molecules. however, the oxidoreductase activity of the polyp hydra is able to modify the 3-oxo-hsl into their 3-hydroxy-hsl counterparts. we can characterize this as quorum quenching since there is an interference with quorum sensing molecules. in this case, the outcomes differ from simple qs inactivation: the host modification results in a phenotypic switch of curvibacter, which modifies its ability to colonize the epithelial cell surfaces of h. vulgaris. applications applications of quorum quenching that have been exploited by humans include the use of ahl-degrading bacteria in aquacultures to limit the spread of diseases in aquatic populations of fish, mollusks and crustaceans. this technique has also been translated to agriculture, to restrict the spread of pathogenic bacteria that use quorum sensing in plants. anti-biofouling is another process that exploits quorum quenching bacteria to mediate the dissociation of unwanted biofilms aggregating on wet surfaces, such as medical devices, transportation infrastructure and water systems. quorum quenching is recently studied for the control of fouling and emerging contaminants in electro membrane bioreactors (embrs) for the advanced treatment of wastewater. extracts of several tradtional medicinal herbs display quorum quenching acivity, and have potential antibacterial applications. social insects social insect colonies are an excellent example of a decentralized system, because no individual is in charge of directing or making decisions for the colony. several groups of social insects have been shown to use quorum sensing in a process that resembles collective decision-making. ants colonies of the ant temnothorax albipennis nest in small crevices between rocks. when the rocks shift and the nest is broken up, these ants must quickly choose a new nest to move into. during the first phase of the decision-making process, a small portion of the workers leave the destroyed nest and search for new crevices. when one of these scout ants finds a potential nest, she assesses the quality of the crevice based on a variety of factors including the size of the interior, the number of openings (based on light level), and the presence or absence of dead ants. the worker then returns to the destroyed nest, where she waits for a short period before recruiting other workers to follow her to the nest that she has found, using a process called tandem running. the waiting period is inversely related to the quality of the site; for instance, a worker that has found a poor site will wait longer than a worker that encountered a good site. as the new recruits visit the potential nest site and make their own assessment of its quality, the number of ants visiting the crevice increases. during this stage, ants may be visiting many different potential nests. however, because of the differences in the waiting period, the number of ants in the best nest will tend to increase at the greatest rate. eventually, the ants in this nest will sense that the rate at which they encounter other ants has exceeded a particular threshold, indicating that the quorum number has been reached. once the ants sense a quorum, they return to the destroyed nest and begin rapidly carrying the brood, queen, and fellow workers to the new nest. scouts that are still tandem-running to other potential sites are also recruited to the new nest, and the entire colony moves. thus, although no single worker may have visited and compared all of the available options, quorum sensing enables the colony as a whole to quickly make good decisions about where to move. honey bees honey bees (apis mellifera) also use quorum sensing to make decisions about new nest sites. large colonies reproduce through a process called swarming, in which the queen leaves the hive with a portion of the workers to form a new nest elsewhere. after leaving the nest, the workers form a swarm that hangs from a branch or overhanging structure. this swarm persists during the decision-making phase until a new nest site is chosen. the quorum sensing process in honey bees is similar to the method used by temnothorax ants in several ways. a small portion of the workers leave the swarm to search out new nest sites, and