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In biology, quorum sensing or quorum signaling (QS) is the ability to detect and respond to cell population density by gene regulation. Quorum sensing is a type of cellular signaling, and more specifically can be considered a type of paracrine signaling. However, it also contains traits of both autocrine signaling: a cell produces both the autoinducer molecule and the receptor for the autoinducer. As one example, QS enables bacteria to restrict the expression of specific genes to the high cell densities at which the resulting phenotypes will be most beneficial, especially for phenotypes that would be ineffective at low cell densities and therefore too energetically costly to express. Many species of bacteria use quorum sensing to coordinate gene expression according to the density of their local population. In a similar fashion, some social insects use quorum sensing to determine where to nest. Quorum sensing in pathogenic bacteria activates host immune signaling and prolongs host survival, by limiting the bacterial intake of nutrients, such as tryptophan, which further is converted to serotonin. As such, quorum sensing allows a commensal interaction between host and pathogenic bacteria. Quorum sensing may also be useful for cancer cell communications.
Pseudomonas aeruginosa is a common encapsulated, Gram-negative, aerobic–facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, P. aeruginosa is a multidrug resistant pathogen recognized for its ubiquity, its intrinsically advanced antibiotic resistance mechanisms, and its association with serious illnesses – hospital-acquired infections such as ventilator-associated pneumonia and various sepsis syndromes.
The gene rpoS encodes the sigma factor sigma-38, a 37.8 kD protein in Escherichia coli. Sigma factors are proteins that regulate transcription in bacteria. Sigma factors can be activated in response to different environmental conditions. rpoS is transcribed in late exponential phase, and RpoS is the primary regulator of stationary phase genes. RpoS is a central regulator of the general stress response and operates in both a retroactive and a proactive manner: it not only allows the cell to survive environmental challenges, but it also prepares the cell for subsequent stresses (cross-protection). The transcriptional regulator CsgD is central to biofilm formation, controlling the expression of the curli structural and export proteins, and the diguanylate cyclase, adrA, which indirectly activates cellulose production. The rpoS gene most likely originated in the gammaproteobacteria.
The PrrF RNAs are small non-coding RNAs involved in iron homeostasis and are encoded by all Pseudomonas species. The PrrF RNAs are analogs of the RyhB RNA, which is encoded by enteric bacteria. Expression of the PrrF RNAs is repressed by the ferric uptake regulator (Fur) when cells are grown in iron-replete conditions. Under iron limitation, the PrrF RNAs are expressed and act to negatively regulate several genes encoding iron-containing proteins, including SodB and succinate dehydrogenase. As such, PrrF regulation "spares" iron when this nutrient becomes scarce.
The rsmY RNA family is a set of related non-coding RNA genes, that like RsmZ, is regulated by the GacS/GacA signal transduction system in the plant-beneficial soil bacterium and biocontrol model organism Pseudomonas fluorescens CHA0. GacA/GacS target genes are translationally repressed by the small RNA binding protein RsmA. RsmY and RsmZ RNAs bind RsmA to relieve this repression and so enhance secondary metabolism and biocontrol traits.
RyhB RNA is a 90 nucleotide RNA that down-regulates a set of iron-storage and iron-using proteins when iron is limiting; it is itself negatively regulated by the ferric uptake repressor protein, Fur.
Autoinducers are signaling molecules that are produced in response to changes in cell-population density. As the density of quorum sensing bacterial cells increases so does the concentration of the autoinducer. Detection of signal molecules by bacteria acts as stimulation which leads to altered gene expression once the minimal threshold is reached. Quorum sensing is a phenomenon that allows both Gram-negative and Gram-positive bacteria to sense one another and to regulate a wide variety of physiological activities. Such activities include symbiosis, virulence, motility, antibiotic production, and biofilm 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 and allows bacteria 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.
Lactonase (EC 3.1.1.81, acyl-homoserine lactonase; systematic name N-acyl-L-homoserine-lactone lactonohydrolase) is a metalloenzyme, produced by certain species of bacteria, which targets and inactivates acylated homoserine lactones (AHLs). It catalyzes the reaction
Pyocyanin (PCN−) is one of the many toxic compounds produced and secreted by the Gram negative bacterium Pseudomonas aeruginosa. Pyocyanin is a blue secondary metabolite, turning red below pH 4.9, with the ability to oxidise and reduce other molecules and therefore kill microbes competing against P. aeruginosa as well as mammalian cells of the lungs which P. aeruginosa has infected during cystic fibrosis. Since pyocyanin is a zwitterion at blood pH, it is easily able to cross the cell membrane. There are three different states in which pyocyanin can exist: oxidized (blue), monovalently reduced (colourless) or divalently reduced (red). Mitochondria play an important role in the cycling of pyocyanin between its redox states. Due to its redox-active properties, pyocyanin generates reactive oxygen species.
Bacterial small RNAs (bsRNA) are small RNAs produced by bacteria; they are 50- to 500-nucleotide non-coding RNA molecules, highly structured and containing several stem-loops. Numerous sRNAs have been identified using both computational analysis and laboratory-based techniques such as Northern blotting, microarrays and RNA-Seq in a number of bacterial species including Escherichia coli, the model pathogen Salmonella, the nitrogen-fixing alphaproteobacterium Sinorhizobium meliloti, marine cyanobacteria, Francisella tularensis, Streptococcus pyogenes, the pathogen Staphylococcus aureus, and the plant pathogen Xanthomonas oryzae pathovar oryzae. Bacterial sRNAs affect how genes are expressed within bacterial cells via interaction with mRNA or protein, and thus can affect a variety of bacterial functions like metabolism, virulence, environmental stress response, and structure.
Rhamnolipids are a class of glycolipid produced by Pseudomonas aeruginosa, amongst other organisms, frequently cited as bacterial surfactants. They have a glycosyl head group, in this case a rhamnose moiety, and a 3-(hydroxyalkanoyloxy)alkanoic acid (HAA) fatty acid tail, such as 3-hydroxydecanoic acid.
CrcZ is a small RNA found in Pseudomonas bacteria, which acts as a global regulator of carbon catabolite repression. In P. aeruginosa, CrcZ is responsible for sequestering the protein Crc. Crc is an RNA-binding global regulator, which acts by inhibiting the translation of the transcriptional regulator AlkS.
Carbon storage regulator A (CsrA) is an RNA binding protein. The CsrA homologs are found in most bacterial species, in the pseudomonads they are called repressor of secondary metabolites. The CsrA proteins generally bind to the Shine-Dalgarno sequence of messenger RNAs and either inhibit translation or facilitate mRNA decay.
The Catabolite repression control (Crc) protein participates in suppressing expression of several genes involved in utilization of carbon sources in Pseudomonas bacteria. Presence of organic acids triggers activation of Crc and in conjunction with the Hfq protein genes that metabolize a given carbon source are downregulated until another more favorable carbon source is depleted. Crc-mediated regulation impact processes such as biofilm formation, virulence and antibiotic susceptibility.
Everett Peter Greenberg is an American microbiologist. He is the inaugural Eugene and Martha Nester Professor of Microbiology at the Department of Microbiology of the University of Washington School of Medicine. He is best known for his research on quorum sensing, and has received multiple awards for his work.
SrbA (sRNA regulator of biofilms A) is a small regulatory non-coding RNA identified in pathogenic Pseudomonas aeruginosa. It is important for biofilm formation and pathogenicity. Bacterial strain with deleted SrbA had reduced biofilm mass. As the ability to form biofilms can contribute to the ability a pathogen to thrive within the host, the C. elegans hosts infected with the srbA deleted strain displayed significantly lower mortality rate than the wild-type strain. However, the deletion of srbA had no effect on growth or antibiotic resistance in P. aeruginosa.
The ivy-DE RNA motif is a conserved RNA structure that was discovered by bioinformatics. ivy-DE motifs are found in the genus Pseudomonas.
Kalai Mathee is a professor at Florida International University, joint editor-in-chief of the Journal of Medical Microbiology, and an elected fellow of the American Academy of Microbiology. She is known for her research on bacterial infections caused by Pseudomonas aeruginosa.
The molecule 2-heptyl-3-hydroxy-4-quinolone, also named the Pseudomonas quinolone signal (PQS), has been discovered as a cell-to-cell signal that encodes for and controls expression of virulence factors, including the two major quorum sensing systems of P. aeruginosa; the las and rhl systems. Pseudomonas aeruginosa is a gram-negative bacteria and opportunistic human pathogen that can cause serious and sometimes fatal infections in humans. Similar to many other bacterial species, P. aeruginosa uses quorum sensing (QS) systems to communicate between cells in a population. This allows coordination of gene expression in a population based on changing cell densities, abundance of nutrients, and other environmental factors.
References
- ↑ Sonnleitner, E.; Sorger-Domenigg, T.; Madej, M. J.; Findeiss, S.; Hackermuller, J.; Huttenhofer, A.; Stadler, P. F.; Blasi, U.; Moll, I. (2008). "Detection of small RNAs in Pseudomonas aeruginosa by RNomics and structure-based bioinformatic tools". Microbiology. 154 (10): 3175–3187. doi:10.1099/mic.0.2008/019703-0. PMID 18832323.
- ↑ Sonnleitner, E.; Gonzalez, N.; Sorger-Domenigg, T.; Heeb, S.; Richter, A. S.; Backofen, R.; Williams, P.; Hüttenhofer, A.; Haas, D.; Bläsi, U. (2011). "The small RNA PhrS stimulates synthesis of the Pseudomonas aeruginosa quinolone signal". Molecular Microbiology. 80 (4): 868–885. doi:10.1111/j.1365-2958.2011.07620.x. PMID 21375594. S2CID 45876403.