User:Jy865/Swarming motility

Lead
=== Swarming motility is a rapid (2–10 μm/s) and coordinated translocation of a bacterial population across solid or semi-solid surfaces, and is an example of bacterial multicellularity and swarm behaviour. Swarming motility was first reported by Jorgen Henrichsen and has been mostly studied in genus Serratia, Salmonella, Aeromonas, Bacillus, Yersinia, Pseudomonas, Proteus, Vibrio and Escherichia. This multicellular behavior has been mostly observed in controlled laboratory conditions and relies on two critical elements: 1) the nutrient composition and 2) viscosity of culture medium (i.e. % agar). One particular feature of this type of motility is the formation of dendritic fractal-like patterns formed by migrating swarms moving away from an initial location. Although the majority of species can produce tendrils when swarming, some species like Proteus mirabilis do form concentric circles motif instead of dendritic patterns.


 * https://www.nature.com/articles/nrmicro2405
 * Swarming motility is operationally defined as multicellular, flagella-mediated surface migration of bacteria. Swarming requires intercellular interactions, surfactant secretion and an increase in flagellar numbers.
 * Swarming motility has often been genetically bred out of laboratory strains and is best observed in natural isolates. In the laboratory, one must take care to standardize swarming conditions. Although the specific conditions that promote swarming are species dependent, swarming generally occurs on nutrient-rich media solidified by agar concentrations of greater than 0.3%. ===

Background
=== Swarming motility was first reported by Jorgen Henrichsen in [year]   Eventually, x changes occurred in swarming discoveries  Major discoveries in terms of learning about swarming motility include


 * https://www.nature.com/articles/nrmicro2405
 * Bacteria have traditionally been viewed as unicellular organisms that grow as dispersed individuals in a planktonic environment. Recently, this view has begun to change as we have gained an increasing awareness of the role of biofilms, which are communities of sessile organisms that secrete an extracellular matrix and aggregate as multicellular groups. Surface-associated bacteria have another option besides sessile aggregation: sometimes, these bacteria can become highly motile and migrate over the substrate in a process known as swarming. Biofilm research has renewed our interest in bacterial swarming motility, which is often oppositely regulated and antagonistic to biofilm formation1.
 * https://link.springer.com/article/10.1007/s41745-020-00177-2
 * One of the most fascinating sights in nature is to witness certain insects, birds, and fish move together in a very coordinated and precise fashion for food search, to avoid predation and for migration. The collective movement is called swarming. In 1885, Gustav Hauser, a German pathologist discovered collective movement in a bacterium he later named Proteus mirabilis (Armbruster and Mobley in, Nat Rev Microbiol 30: 186–194, 2013).  ===

Characteristics and Behaviors
=== Swarming motility is characterized by


 * https://www.sciencedirect.com/science/article/pii/S1369527499000338?casa_token=4owieG5hqk8AAAAA:KXIynkrmykRVHZzyLb5raWZkcqxVKk12GhKWt-jXcR096DGW5yecFN9fKAE-nT2ujV2sdSfZSWY
 * Swarming has been extensively studied in Proteus mirabilis, in which it is characterised by differentiation of short motile vegetative cells at the colony margin into elongated polyploid hyper-flagellated swarm cells (Figure 1). These differentiated cells align closely along their long axis, forming rafts that migrate as a population by coordinated flagellar action. Regular cycles of mass migration interspersed by population growth without colony expansion (consolidation) result in characteristic large colonies marked by concentric zones, or terraces [4]. Vibrio parahaemolyticus can exhibit similar periodic behaviour (reviewed in [5••]), but other swarming bacteria (e.g. Serratia, reviewed in [6••]) do not usually display pronounced phases.
 * Flagella biogenesis is central to swarming differentiation
 * Hyper-flagellation is the most prominent feature of swarm cells, and differentiation requires efficient flagella assembly 3, 5••, 6••, 44. The close coupling of flagella biogenesis to other aspects of differentiation is clearly seen in the P. mirabilis flhA flagella export mutant and the flgN flagellar assembly (chaperone) mutant, neither of which can hyper-flagellate, elongate or upregulate virulence factors 44, 45, 54•. Expression of the flagellar regulon is governed by the flhDC master operon, which encodes the heterotetrameric FlhD2C2 transcriptional activator 47, 55, and this appears to be the principal regulatory fulcrum during swarm cell differentiation.
 * Swarming bacteria undergo morphological differentiation that distinguish them from their planktonic state. Cells localized at migration front are typically hyperelongated, hyperflagellated and grouped in multicellular raft structures.
 * https://www.nature.com/articles/nrmicro2405
 * A period of non-motility, or a swarm lag, will manifest when cells are transferred from liquid to a solid medium. The lag is thought to indicate a physiological change in cells to become swarming proficient.
 * Some bacteria become elongated during swarming. It is not clear whether cell elongation is required for or simply co-regulated with swarming in these species. The mechanistic connection between swarming motility and cell elongation is unknown, and many swarming bacteria do not become elongated.
 * Swarming motility is operationally defined as a rapid multicellular movement of bacteria across a surface, powered by rotating flagella2 (Fig. 1). Although simple, accurate and mechanistically meaningful, this definition does not do justice to the wide array of phenotypes that are associated with swarming motility, nor does it emphasize all that remains unknown about this behaviour. Furthermore, it is important to acknowledge the common field-specific misnomers (Box 1) and to distinguish swarming from behaviours such as swimming, twitching, gliding and sliding, which can also occur within or on top of solid surfaces3 (Fig. 1). Swarming is the multicellular movement of bacteria across a surface and is powered by rotating helical flagella. Swimming is the movement of individual bacteria in liquid, also powered by rotating flagella. Twitching is surface movement of bacteria that is powered by the extension of pili, which then attach to the surface and subsequently retract, pulling the cell closer to the site of attachment. Gliding is active surface movement that does not require flagella or pili and involves focal-adhesion complexes. Sliding is passive surface translocation that is powered by growth and facilitated by a surfactant. The direction of cell movement is indicated by black arrows, and the motors that power the movement are indicated by coloured circles.
 * Swarming-associated phenotypes:The swarming lag, cell elongation and colony pattern formation are all phenotypes that are associated with swarming motility but that can be abrogated or bypassed without loss of swarming behaviour.
 * The swarming lag. A lag period of non-motile behaviour precedes the initiation of swarming motility when bacteria are transferred from a liquid medium to a solid surface11,61,80,81 (Fig. 5a). The swarming lag is constant for a particular set of conditions but may be shortened by increasing the inoculum density or abolished by using particular mutants11,54,58,82,83,84. The lag is poorly understood, but its occurence indicates that swimming cells must go through a change to become swarming proficient.
 * Cell elongation. It is commonly thought that swarming cells suppress cell division and that cell elongation is either a requirement for or an indicator of swarming motility. The connection between filamentation and swarming motility originates with P. mirabilis, which makes short rods when grown in broth and long filaments with multiple nucleoids when grown on surfaces25,81,87 (Fig. 3b). To date, it is unclear whether elongated cells are required for swarming or whether they simply accumulate at the swarm edge. Despite the importance of elongation in the dogma of swarming motility, no mechanistic or regulatory connection has been elucidated at the molecular level for the control of cell division during swarming. Furthermore, substantial cell elongation is neither a requirement for nor co-regulated with swarming motility in many bacteria11,16,39,40,48,49,90,91.
 * Colony pattern formation. Swarming bacteria form macroscopic colony patterns on solid media. The patterns may take different shapes but the relevance of any particular pattern is unclear. Furthermore, it seems likely that all swarming bacteria can produce a range of patterns depending on the environmental conditions92,93. Therefore, pattern formation may be less of a commentary on swarming regulation and more of an indicator of environmental factors. Featureless swarms are made when cells spread evenly and continuously outward from the point of inoculation, as a monolayer. The monolayer is transparent but may be seen when incident light is reflected off the surface or when oblique light is transmitted through the agar. Cell density in the monolayer is high and approximately uniform throughout the swarm, increasing slightly at the advancing edge36. When the monolayer reaches the boundaries of the plate, the colony grows into a featureless mat11,20 (Fig. 7a). The most famous irregular swarming pattern is the characteristic bull's eye formed by P. mirabilis that results from cyclic and synchronous waves of motility followed by regular periods of swarming cessation81,82,94 (Fig. 7b). Dendrites (also known as tendrils or deep branches) are long, thin regions of colonization emanating from a central origin (Fig. 7c). Some bacteria form spiraling vortices as they travel across the surface of the plate2,62,90,97 (Fig. 7d). These vortices are large, localized groups of cells travelling in a common circular path and have also been referred to as 'wandering colonies' (Ref. 2). Non-swarming cells that are unable to spread across the surface grow as a confined colony in the centre of the plate (Fig. 7e). On prolonged incubation, the colony diameter of a non-swarming strain may increase owing to the contribution of sliding motility.
 * https://enviromicro-journals.onlinelibrary.wiley.com/doi/full/10.1111/j.1462-2920.2008.01747.x?casa_token=s4YDFQ-JwkYAAAAA%3A-G1qTEpYU7thugVSyRKFHFKRYwWSnjfg1trsvWCSpenZto4ae9iemVPwaVl5b7ZRV4WnRCsohPqstn0
 * Swarming migration is preceded by aprofound modification of cell morphology, where shortplanktonic cells differentiate into elongated and multi-flagellated swarm cells (Harshey, 2003). Differentiatedcells keep themselves in close cell–cell contact andmigrate in multicellular rafts along their longitudinal axis(Harshey, 1994). Swarming aspect can vary greatlyaccording to species or even growth conditions used
 * https://www.cell.com/biophysj/pdf/S0006-3495(10)00218-3.pdf
 * When vegetative bacteria that can swim are grown in a rich medium on an agar surface, they become multinucleate, elongate, synthesize large numbers of flagella, produce wetting agents, and move across the surface in coordinated packs: they swarm.
 * https://link.springer.com/chapter/10.1007/978-981-13-2429-1_5
 * Swarming motility is a quick and synchronized movement of a individuals of bacteria through solid or semi-solid surfaces, and is an exemplar of bacterial multicellularity and swarm behavior [29]. Bacterial cells distinguish into a specific state (swarmer cells) which is morphological delineated by hyperflagellation and cell elongation [22, 29]. Swarmer cells require augmented synthesis of some extracellular wetting agents which reduce surface friction and facilitate smooth relocation bacterial cells on viscous surfaces [29]. Swarming warrants intercellular communications, secretion of surfactants and inflates in flagellar numbers. The ecology of swarming is yet mysterious, but often linked with pathogenesis. Swarmer cells also benefit from improved antibiotics resistance and eukaryotic engulfment with gaining better nutrition and obtain aggressive help from secreted surfactants. They often involve the chemotaxis sensory transduction system for tasks that are distinct to chemotaxis [39]. Swarmers display high speed twirls, vortexes and frequently tacits to drive microbial community development at a cost of cell growth. Hence, swarming is divergent from flagella dependent swarming so as to represent individual cell movement in a liquid/solid medium with minor agar concentrations. In order to swarm bacterial cells translocates by extracellular slime (a blend of carbohydrates, proteins, peptides, surfactants, etc.) through which they can disperse a biofilm over a surface. Swarming bacteria are divided into two groups based on ability of flagellar propulsion to surface motility: robust swarmers, (swarm across a solid agar surface) and temperate swarmers, (swarm only on a softer agar surface). Robust swarmers comprise polarly flagellated bacteria, such as Rhodospirillum, Azospirillum, Vibrio and Proteus species. Temperate swarmers comprise E. coli, Pseudomonas, Rhizobium, Bacillus, Serratia, Salmonella and Yersinia species. Conversely, swarming ability is heightened by lipopeptide and rhamnolipid surfactants secreted by the temperate swarmers Bacillus, Serratia, Rhizobium, Pseudomonas and species [11, 24, 29, 39, 66, 88]. Swarming and swimming motility along with extracellular enzyme activities, i.e. nuclease, protease, lipase and haemolysin, are other behaviors that may extensively add to bacterial pathogenesis [31]. This multifaceted multicellular behavior needs the combination of physical and chemical signs, this leads to physiological and morphological characterization of bacteria (normal cell) into swarmer cells. Swarming motility was first documented by Jorgen Henrichse and has been typically studied in genus Salmonella, Serratia, Bacillus, Aeromonas, Pseudomonas, Escherichia, Yersinia, Vibrio and Proteus. However swarming ability is prevalent in various genera of Gram-negative and positive flagellated bacteria and is characteristically evaluated on a solidified agar medium [65]. Swarming motility can also endow with an aggressive advantage in search for nutrient-rich environments and it can be pretentious by bacterial population size, water content and nutrient composition [64]. Swarming motility is a multicellular cooperative way of flagella dependent motility on surface. Biofilm formation forms a sessile bacterial community surrounded by their own extracellular polymeric substances (EPS) matrix. Cell density is a highly indispensable and essential factor to initiate and uphold the swarming process. Therefore, not unexpected that swarming in several bacteria, is coupled to quorum sensing (QS) [12]. QS within bacterial populations can promote pathogenesis, cellular dissemination or dispersal, symbiosis, DNA transfer, microbial biofilm development, in addition to production of antibiotics and other secondary metabolites [74]. QS mechanism also allows bacteria to coordinate swarming, biofilm formation, stress resistance and production of toxins and secondary metabolites. QS is a biochemical communication, that relies on production, secretion and detection of auto inducer (AI) signals to regulate gene expression in response to changes in population density [25]. These molecules are the mediators of QS. This sensing is natural among bacteria and helps to retain the bacteria in a good location. However this assists the bacterial swarm from overcrowding and avoids from several toxic substances. QS permits bacterial colony to control their gene expression in harmony, which is imperative for moving out set of behaviors such as bioluminescence production, biofilm formation, virulence factor and genetic exchange etc. [58]. Nonetheless many bacterial species rely on QS to control important cellular processes which are essential for surveillance, endurance and acclimatization to their changing environments [5]. By monitoring the accumulation of specific AIs, bacteria can even track shifts in population density and species complexity in the vicinity and quickly respond as a group for that reason [59, 86].
 * Hyper flagellation contributes to rapid contamination of host tissues, bond between swarming and virulence is not clearly established in majority of swarming bacterial pathogens. Nevertheless, swarm cell differentiation is frequently complemented by expression of virulence determinants, which might help bacteria in inhabiting new environment. Biofilm formation and EPS production depict an essential part in P.mirabilis infection.
 * https://journals.asm.org/doi/pdf/10.1128/br.36.4.478-503.1972
 * Definition of swarming. Swarming is a kind of surface translocation produced through the action of flagella but is different from swimming. The micromorphological pattern is highly organized in whirls and bands. The movement is continuous and regularly follows the long axis of the cells which are predominantly aggregated in bundles during the movement.
 * Goes into distinctions w other types of bacterial spreading too (6 in total) ===

Causes
=== The causes for swarming motility include


 * In some species, swarming motility requires the self-production of biosurfactant to occur. Biosurfactant synthesis is usually under the control of an intercellular communication system called quorum sensing. Biosurfactant molecules are thought to act by lowering surface tension, thus permitting bacteria to move across a surface.
 * https://www.sciencedirect.com/science/article/pii/S1369527499000338?casa_token=4owieG5hqk8AAAAA:KXIynkrmykRVHZzyLb5raWZkcqxVKk12GhKWt-jXcR096DGW5yecFN9fKAE-nT2ujV2sdSfZSWY
 * Swarming involves differentiation of vegetative cells into hyper-flagellated swarm cells that undergo rapid and coordinated population migration across solid surfaces. Cell density, surface contact, and physiological signals all provide critical stimuli, and close cell alignment and the production of secreted migration factors facilitate mass translocation.
 * Swarming is neither a starvation response nor an obligatory development stage [4]. It is nonetheless a radical and reversible change in behaviour in response to the environment. The social nature of swarming indicates that extracellular and possibly cell–cell signals are central stimuli, as are intracellular physiological parameters and contact with a surface. These signals might be sensed and transmitted by two-component regulatory systems, cytosolic regulators, and even cell-surface flagella.
 * Cell density is critical to swarming as the duration of the lag phase that precedes P. mirabilis migration is strongly influenced by inoculum density 4, 19••, and in Salmonella enteritidis the ability to grow to high densities is correlated to virulence, cell elongation and hyper-flagellation [20].
 * How are extracellular swarm signals sensed? A possible sensory route would be via the chemotaxis system (reviewed in [27]). Chemotaxis components are critical for swarming in P. mirabilis and Serratia marcescens 28, 29.
 * Swarming is dependent on the physiological status of cells since high growth rates on nutrient-rich solid media stimulate differentiation and, in some cases, nutrient concentration profoundly influences colony migration 4, 35. Although metabolic cues are important for initiation of P. mirabilis differentiation, nutrient (glucose) depletion seems not to be decisive since cells at the centre of a swarm colony are growing exponentially even as the second cycle of differentiation is initiated [4]. Cellular components linking physiological signals to differentiation may include the global transcriptional regulator Lrp (leucine-responsive regulatory protein).
 * A pivotal stimulus of swarm cell differentiation is surface contact. Differentiation is induced when viscosity of the growth medium is increased, or when flagella are tethered with antibodies 23, 41.
 * What are the factors controlling the initiation, velocity and duration of migration? P. mirabilis migration requires close cell–cell contact, with cells aligning along their long axis in multicellular rafts 1, 3. Cell–cell contact is stabilised by the production of exopolymers that encapsulate swarm cell rafts 6••, 35, 49 and enhance surface fluidity of the growth medium 6••, 49. Migration is also facilitated by small secreted molecules. Observations on multicellular swarm cell raft assembly, and the lag time preceding migration [4], indicate that cell density is a major factor influencing migration. A mathematical model of P. mirabilis swarm colony development [53••] suggests that swarm cells must reach a minimum age and population density before migration is initiated, and cessation of migration is similarly proposed to be a function of swarm cell age. This model thus predicts that both migration and consolidation are governed by population dynamics rather than by responses to nutrient depletion or accumulation of chemotactic repellents.
 * The flhDC flagellar master operon regulates flagella biogenesis and cell division
 * It seems likely that flhDC is a primary site for the integration of signals inducing swarm cell differentiation, and components have been identified that upregulate the flagellar master operon in P. mirabilis swarm cells.
 * Swarming is a tractable model of bacterial differentiation and multicellularity within a growing colony. Differentiation to swarm cells is based on widely conserved pathways governing flagella biogenesis, motility and septation, rather than the evolution of a distinct developmental programme. Nevertheless, these pathways are subject to modulation during swarming, by altered sensitivity to physiological and environmental signals through known and novel regulators (e.g. RcsBC and RsbA, and the Lrp and UmoA–D proteins) that form an extensive network and act primarily through the flhDC master operon. Swarming cells also require extracellular components (e.g. polysaccharide and surfactants) that allow mass migration of differentiated cells over difficult terrain. The hyper-expression of the flagellar gene hierarchy in Proteus has highlighted induction and negative regulation barely evident in undifferentiated enterobacterial cells, and the coupling of swarming to virulence, whether through an intrinsic role in colonisation or coregulation of motility and virulence genes, adds an additional level of significance. While several signals are believed to induce differentiation (e.g. surface contact, cell density and amino acids) the pathways of signal integration are still poorly understood, in particular the apparent surface contact-sensing by flagellar filaments and the basis of the cell–cell communication assumed to underlie coordinated migration.
 * https://www.nature.com/articles/nrmicro2405
 * Swarming often requires the chemotaxis sensory transduction system for functions that are unrelated to chemotaxis, or directed movement, per se.
 * the requirements that define swarming motility in diverse bacterial model systems, including an increase in the number of flagella per cell, the secretion of a surfactant to reduce surface tension and allow spreading, and movement in multicellular groups rather than as individuals.
 * Lab: Swarming motility generally requires an energy-rich, solid medium, but the specific conditions that support swarming depend on the organism concerned. Some bacteria, such as Bacillus subtilis, swarm on a wide range of energy-rich media, whereas other bacteria, such as Salmonella enterica and Yersinia entercolitica , require the presence of particular supplements (for example, glucose)16,17,18. Swarming is promoted by high growth rates, which may account for the requirement for energy-rich conditions12,19,20. Although some bacteria can swarm over almost any agar surface, most swarming bacteria require soft agar in a narrow range of concentrations. Media that are solidified, with agar concentrations above 0.3%, exclude swimming motility and force the bacteria to move, if possible, over the surface, and agar concentrations above 1% prohibit swarming of many bacterial species. It is conceivable that the standard 1.5% agar that is used to solidify media in the laboratory was specifically chosen for swarming inhibition. When conducting swarming-motility assays, a defined set of conditions must be established and rigorously adhered to21. The water content of the medium is a crucial factor: too little water results in poor swarming, whereas too much water may permit swimming motility. To control the water content, swarm plates are poured to a standard thickness when the agar is relatively cool (∼50 °C), thereby minimizing water loss from condensation on the plate lid. Finally, plates are dried briefly (for ∼15 minutes), open-faced, in a laminar flow hood to remove surface water and minimize the contribution of swimming motility to surface movement12,21.
 * Requirements for swarming motility: Flagella are the most important requirement for swarming motility, along with an increase in flagellar biosynthesis, but this type of movement also requires an increase in cell–cell interactions and the presence of a surfactant.
 * Goes into more detail on how it works w diff genuses
 * When cells transition from swimming to swarming, the number of flagella on the cell surface increases. Organisms with alternative flagellar systems become hyperflagellate in the transition from expression of a single polar flagellum to expression of multiple peritrichous flagella. Species with one flagellar system also seem to increase the number of flagella on the cell surface during swarming6,18,20,25,29,45,46,47. Even P. aeruginosa, which swims with a single polar flagellum, may produce two polar flagella when moving on a surface48,49. Mutations that reduce the expression of flagellar genes reduce flagellar number and reduce or abolish swarming17,20,46,50,51,52,53,54,55,56. Conversely, mutations that enhance the expression of flagellar genes increase flagellar number and enhance swarming47,54,55,57,58,59,60. The reason that swarming requires multiple flagella on the cell surface is unknown.
 * Rafting. Bacteria swim as individuals, but swarming bacteria move in side-by-side cell groups called rafts11,17,20,24,26,29,36,49,61,62,63 (Fig. 3a). Raft formation is dynamic: cells recruited to a raft move with the group, whereas cells lost from a raft quickly become non-motile. The dynamism in cell recruitment and loss suggests that no substance or matrix maintains raft stability, except perhaps the flagella themselves. As with hyperflagellation, the reason that swarming motility requires raft formation is unclear at present.
 * Surfactant synthesis. Many swarming bacteria synthesize and secrete surfactants (short for 'surface-active agent'). Surfactants are amphipathic molecules that reduce tension between the substrate and the bacterial cell and, in doing so, can permit spreading over surfaces. Surfactants often manifest as a clear, watery layer that precedes the cells at the swarm front11,29,45,49,64. Some bacteria fail to make swarming surfactants and will only swarm on special agar with inherently low surface tension owing, perhaps, to the presence of a surfactant in the agar itself9,18,35,40,65. Surfactant production is commonly regulated by quorum sensing68,76,77,78. Surfactants are shared secreted resources and are effective only at high concentration. Therefore, quorum sensing may have evolved to regulate the production of surfactants to ensure that they are made only when there are sufficient bacteria present to make surfactants beneficial.
 * https://www.cell.com/biophysj/pdf/S0006-3495(10)00218-3.pdf
 * We see completely jammed, immobile monolayers only in swarms that fail, which is usually caused by surface dryness or a drop in incubation temperature. For modeling purposes, a swarming cell should probably be treated as a constant force object rather than a constant speed object. A bacterial swarm is a spatially and temporally coordinated system composed of billions of individual cells.
 * Despite the often intricate genetic mechanisms that regulate swarming, there are also several ways in which physico-chemical phenomena could play a part in the dynamics of swarming and biofilm formation [74]. Possible parameters intervening in these are the heterogeneity of substrates, the surface-active nature of signaling molecules 31, 70 and the dependence of viscosity on the concentration of bacteria and its effect on thin film hydrodynamics [75]. Likewise, the extracellular slime is a non-Newtonian fluid, the viscosity of which strongly depends on local deformation rates that will affect the spreading dynamics of the bacterial film [31]. Finally, at high concentrations of bacteria, large-scale coherent movements of bacteria with vortex-like motions might appear because of hydrodynamic coupling, in which the collective motion of bacteria through the viscous slime drives the fluid flow 76, 77. ===

Significance
=== Swarming motility as a phenomenon in the natural world takes place in and can help


 * The fundamental role of swarming motility remains unknown. However, it has been observed that active swarming bacteria of Salmonella typhimurium shows an elevated resistance to certain antibiotics compared to undifferentiated cells.
 * It has been studied in a variety of species thus far, including genus Serratia, Salmonella, Aeromonas, Bacillus, Yersinia, Pseudomonas, Proteus, Vibrio and Escherichia.
 * Different genuses seem to have different stimuli and behaviors.
 * https://www.sciencedirect.com/science/article/pii/S1369527499000338?casa_token=4owieG5hqk8AAAAA:KXIynkrmykRVHZzyLb5raWZkcqxVKk12GhKWt-jXcR096DGW5yecFN9fKAE-nT2ujV2sdSfZSWY
 * Swarming is a powerful means of rapidly colonising nutrient-rich environments, facilitating colony spread and accelerating biomass production.
 * Bacterial swimming motility is influential in many pathogen–host interactions [7], and several pathogens are additionally capable of multicellular swarming migration 1, 3, 8, 9•. Swarming facilitates ascending colonisation of the urinary tract by P. mirabilis [10] and may also be coupled to biofilm formation on catheters [11]. In addition to enabling rapid population migration, differentiation into hyper-flagellated swarm cells is coupled to the ability of P. mirabilis to enter host cells, and to upregulate virulence proteins, including haemolysin, urease and protease 13, 14, 15•.
 * https://www.nature.com/articles/nrmicro2405
 * The ecology of swarming is unknown, but swarming is often associated with pathogenesis. Swarming bacteria also enjoy enhanced resistance to antibiotics and eukaryotic engulfment as well as gaining enhanced nutrition and a competitive advantage from secreted surfactants.
 * Bacterial movement over surfaces may enable pathogenic species to migrate over, adhere to and disperse from sites of infection26,39,126,127. Swarming may protect pathogens from macrophages, as swarm cells were shown to have enhanced resistance to engulfment128. In addition, toxin secretion is often co-regulated with swarming motility126,129. Furthermore, bacteria of diverse species seem to become resistant to a broad range of antibiotics when swarming130,131. The mechanism of generalized multidrug resistance seems to be unrelated to known active antibiotic-efflux systems and is instead likely to be a passive phenomenon resulting from rapid spreading of cells at high density118,130,132. Nonetheless, some bacteria have specialized systems to resist their own secreted surfactants52,132,133. Cationic peptides like polymyxin B have surfactant like structures, and bacteria may express some antibiotic-resistance systems to avoid autotoxicity during swarming134 (Fig. 4). The study of swarming motility promises to yield novel insights into the physiology of multicellular behaviour in bacteria. New swarming-specific genes await discovery and investigation. New biochemical mechanisms are needed to connect swarming phenotypes to other, better-understood cell physiologies. Swarming offers cytological insight into how the number of flagella is controlled. It also provides biophysical models of how flagella function at a surface, as well as being a powerful evolutionary selection pressure. As microbiologists become more interested in life at a surface, bacterial swarming motility will surely move the field forwards.
 * https://www.pnas.org/doi/full/10.1073/pnas.0511323103
 * In theoretical and empirical ecology, competition between species plays a central role in defining community structure and activity. Stable coexistence of diverse organisms in communities is thought to be fostered by individual tradeoffs and optimization of competitive strategies along resource gradients (1). Outside of the laboratory, microorganisms usually coexist in multicellular communities, governed by competition for common nutritional resources with other community members (2). Competitive fitness can be realized simply by occupying a suitable or specialized nutritional niche. Motility provides a mechanism by which microbes continually reposition themselves, adapting to changing nutritional and physical conditions.
 * https://www.sciencedirect.com/science/article/pii/S0966842X0800187X
 * Swarming is the fastest known bacterial mode of surface translocation and enables the rapid colonization of a nutrient-rich environment and host tissues. This complex multicellular behavior requires the integration of chemical and physical signals, which leads to the physiological and morphological differentiation of the bacteria into swarmer cells.
 * Bacteria often thrive in surface-associated multicellular communities that have advantages over individual cells, such as protection against unfavorable environmental conditions (including predation, the presence of antimicrobials and the host immune response). Biofilms are sessile communities with microorganisms embedded within a matrix and attached to a surface. However, motile populations, such as swarming bacteria, can rapidly reach novel niches, which they can colonize; this provides ecological advantages to the bacteria 1, 2. The choice between sessile and motile lifestyles is clearly an important decision to be made by microorganisms that live in varying habitats and requires the integration of many environmental cues. Swarming motility is a process by which bacteria can rapidly (several μm s−1) advance on moist surfaces in a coordinated manner. It requires functional flagella and is coupled to the production of a viscous slime layer. The slime layer is thought to extract water from the agar and keeps the cells in a moist environment. Swarming is a group behavior that requires the cells to reach a certain cell number before the process is initiated. Furthermore, swarmers are often elongated as a result of the suppression of cell division. Swarming is widespread in many genera of Gram-negative and Gram-positive flagellated bacteria and is typically assayed on a solidified medium, containing 0.5–2% agar, from which the bacteria are thought to extract water and nutrients. ===

Applications
=== Scientists and researchers have leveraged swarming motility in


 * https://www.nature.com/articles/nrmicro2405
 * The mechanism of surface sensing (the bacterial 'sense of touch') is unknown, but swarming motility provides a strong model system for its study. Models have been proposed to explain the bacterial response to surface contact, including sensing resistance to flagellar rotation when impeded by surface contact and sensing perturbations in the Gram-negative outer membrane.
 * Swarming motility seems to be narrowly conserved in the bacterial domain and is currently restricted to three families (Fig. 2). The reported number of swarming species is almost certainly an underestimate, because swarming motility is often inhibited by standard laboratory media and genetically abolished during the domestication of commonly-used laboratory strains11,12,13,14. The selection against swarming in these strains may be due to evolutionary forces that act when surface motility provides no advantage, for example in unstructured laboratory environments15. Alternatively, bacteria that spread promiscuously over plates are rarely welcomed by geneticists, and selection against swarming may be artificial in favour of small, compact colonies.
 * https://enviromicro-journals.onlinelibrary.wiley.com/doi/full/10.1111/j.1462-2920.2008.01747.x?casa_token=s4YDFQ-JwkYAAAAA%3A-G1qTEpYU7thugVSyRKFHFKRYwWSnjfg1trsvWCSpenZto4ae9iemVPwaVl5b7ZRV4WnRCsohPqstn0
 * Resistance of bacteria to antibiotics is a major worldwideissue. Multidrug resistance has been mainly attributed toreduced drug intake due to use of low-permeability barri-ers coupled with active drug export mediated by efflux. Biofilms are wellknown for their extremely high tolerance to many antimi-crobial agents such as antibiotics, heavy metals and dyes.Biofilm resistance, while still poorly understood, is consid-ered to be multifactorial and involve diverse mechanisms(Drenkard, 2003). Swarming is a type of social motilityallowing rapid colonization of a surface by a populationof highly differentiated swarm cells. As this is also amulticellular behaviour, we wanted to investigate if swarm-ing motility could be broadly associated with an antibioticresistant phenotype...Although swarm cells display a multiresistant phenotypesimilar to that of biofilm cells, mechanisms used could bedifferent, as suggested by our results with thendvBmutant. Compared with cells in biofilms, swarm cells arenot protected within a complex extracellular matrix, andare very metabolically active, while still presenting a mul-tiresistant phenotype. We therefore propose that swarm-ing motility represents a valuable model for understandingmechanisms underlying resistance of multicellular bacte-rial communities to antimicrobial agents.
 * The swarming behaviour closely resembles the biofilmstate as both represent coordinated social phenomenarequiring some degree of cellular differentiation and regu-lation through cell–cell communication. Resistance to avariety of antibiotics has been recently reported for swarmcells  ofSalmonella  entericaserovar  Typhimurium.This multiresistant phenotype ofSalmonellaswarm cellsappears not to be due to mutations and is linked toengagement into group behaviour (Kim and Surette,2003; Kimet al., 2003). In the effort to understand thehigh resistance of multicellular behaviour, we wanted toinvestigate whether the swarming phenomenon could beassociated with a prevalent and general multiresistantphenotype among bacteria. We challenged planktoniccells and swarming cells of several species with a widerange of antibiotics and other antimicrobials. We observedthat migrating swarm cells display an increased resis-tance to most of the agents tested. Antimicrobial resis-tance could therefore be a general feature of bacterialmulticellular social behaviours. ogetherwith  the  high  resistance  of  biofilms,  these  resultssupport the hypothesis that antimicrobial resistanceis   a   general   feature   of   bacterial   multicellularity.Swarming  motility  might  thus  represent  a  form  ofsocial  behaviour  useful  as  a  model  to  investigatebiofilm antibiotic resistance.

Unknown
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 * https://www.nature.com/articles/nrmicro2405
 * The role of chemotaxis. Chemotaxis is the directed movement of an organism with respect to a chemical gradient. Bacteria mediate chemotaxis by biasing the duration spent in one of two behaviours, either running in a relatively straight line or tumbling erratically to acquire a new direction. Running and tumbling are controlled by the direction in which the flagella rotate. A series of chemotaxis signal transduction proteins detects stimuli in the environment, transduces the stimulus and controls the direction of flagellar rotation99.
 * The mechanism of surface sensing. Swarming motility requires contact with a solid substrate, and interaction with a surface may induce cells to become swarming proficient during the swarming lag. If surface contact is indeed an inducing stimulus, it stands to reason that the cells must contain a signal transduction system to transduce this information. Elucidating the mechanism of surface sensing, or determining the molecular basis for the bacterial sense of touch, is the 'holy grail' of swarming-motility research.The sense of touch is poorly understood for all systems, but it is particularly problematic for bacteria.
 * The mechanism of force generation.
 * Swarming as a developmental state. Swarming motility is a behaviour. The swarm lag indicates that swimming cells must change in order to become swarming proficient, but it is not clear that swarm cells constitute a true developmental state.
 * Swimming in two dimensions? Researchers who study swarming are often asked: “How do you know that swarming is not simply swimming motility constrained in two dimensions?”
 * For those who are convinced that swarming motility is a separate and distinct behaviour, many questions remain.What physiological changes take place during the swarming lag? Is surface contact a direct stimulus and, if so, how is it transduced? Is cell division coupled to swarming and, if so, what is the mechanistic connection? How is force generated and coordinated in multicellular rafts? How many bacterial species are swarming proficient, and how many times has swarming been bred out of laboratory isolates? Finally, what is the ecological relevance of swarming motility? Although the perfect surface of a carefully dried agar plate is never found in the environment, swarming may occur on nutrient-rich, soft substrates such as hydrated soils, plant roots and animal tissues, and swarming cells enjoy various advantages. ===

Biosurfactant, quorum sensing and swarming
=== In some species, swarming motility requires the self-production of biosurfactant to occur. Biosurfactant synthesis is usually under the control of an intercellular communication system called quorum sensing. Biosurfactant molecules are thought to act by lowering surface tension, thus permitting bacteria to move across a surface. ===

Cellular differentiation
=== Swarming bacteria undergo morphological differentiation that distinguish them from their planktonic state. Cells localized at migration front are typically hyperelongated, hyperflagellated and grouped in multicellular raft structures. ===

Ecological significance
=== The fundamental role of swarming motility remains unknown. However, it has been observed that active swarming bacteria of Salmonella typhimurium shows an elevated resistance to certain antibiotics compared to undifferentiated cells. ===