Bacteria can have one flagellum or several, and they can be either polar one or several flagella at one spot or peritrichous several flagella all over the bacterium. In addition to motility, flagella possess several other functions that differ between bacteria and during the bacterial life cycle: a flagellum can, for example, participate in biofilm formation, protein export, and adhesion. Duan et al.
Over 60 structural and regulatory proteins are required for flagellum assembly and function. Flagellum consists of a cytoplasmic export apparatus, a basal body embedded in the cell membrane CM , a hook that connects the basal body to the filament, and a filament that functions as a propeller Figure 1 A.
Flagellar assembly starts with the CM-associated components of the basal body and the secretion apparatus, through which the other flagellar proteins are then secreted, first the remaining basal body components, then the hook and the hook-filament junction proteins.
Filament assembly starts after the hook FlgE molecules has been completed and the filament capping proteins have been positioned. The filament is composed of about 20, flagellin FliC proteins that are incorporated below the distal pentameric FliD cap, which functions as a plug and is required for assembly of nascent monomeric flagellin. A two-component signaling cascade involving chemotaxis-related proteins affects flagellar rotation, which is facilitated by the engine consisting of the basal body-associated stator proteins and the basal body, which functions as a rotor.
Some proteins relevant to this review and their functions are listed in Table 1. For a detailed description of flagellar assembly, see Chevance and Hughes [ 4 ]. For a review on chemotaxis, the reader is referred to Sourjik and Wingreen [ 5 ]. The designations used are mainly the names in enteric bacteria; for homologs in other species, see [ 8 ]. A Schematic presentation of bacterial flagellum structure. The pentameric FliD cap at the distal end of the filament, the hollow filament composed of about 20, identical flagellin subunits, the junction zone between filament and hook, and the hook connecting the filament to the basal body, represent extracellular parts green shades of the flagellum.
The basal body grey in the cell wall consists of a centrally located hollow rod that connects different rings embedded in the outer membrane OM , the peptidoglycan layer, and the cytoplasmic membrane CM. Stator complexes dark grey , composed of membrane proteins MotA and MotB, are associated with the CM-bound ring and the cytoplasmic ring below the CM, and provide motility-required energy. The cytoplasmic export machinery black that secretes the extracellular subunits is located within the cytoplasmic ring.
Note that OM-associated parts of the basal body are absent in the flagella of Gram-positive bacteria. B Schematic presentation of flagellin monomer upper panel and flagellin polymerization lower panel.
The variable, exposed, globular domains of flagellin are shown in green. The conserved N - and C -terminal regions involved in flagellum polymerization are indicated blue, light blue, yellow, red as well as the regions binding to TLR5 light blue, yellow and those involved in inflammasome formation blue, red.
Bacterial adhesion is a critical initiation step in bacterial colonization and persistence, both for pathogens and commensals. Bacteria express various adhesive surface structures such as capsule, fimbriae or pili, and several surface proteins for examples the reader is referred to Klemm et al.
Typically the adhesive structures are not expressed at the same time as the flagellum, so that movement and attachment occur one at a time. Thus, bacteria switch from motile to sessile lifestyle and vice versa , and these changes are triggered by different environmental conditions—such as temperature, osmolarity, and pH—which regulate the expression of the flhDC flagellar master operon [ 2 ].
The regulation of flagellar expression occurs temporally at both the level of transcription and assembly [ 2 , 4 ]. The flagellum has also been shown to function as an export apparatus that mediates extracellular secretion of non-flagellar virulence-associated effector proteins and biotechnologically important heterologous polypeptides [ 1 , 10 , 11 ]. From the mammalian host perspective, the flagellum is relevant for immune defense: The immune system recognizes flagellin, which triggers adaptive and innate immune response.
The conserved N - and C -termini of monomeric flagellin Figure 1 B involved in flagella assembly interact with cytoplasmic NOD-like receptors in eukaryotic cells and induce the formation of inflammasome, which leads to pyroptosis [ 12 , 13 , 14 ]. Flagellin belongs to molecules containing a pathogen-associated molecular pattern PAMP that is recognized by toll-like receptor 5 TLR5 [ 12 ].
TLR5 is mostly expressed at the basolateral surface of intestinal epithelial cells and by monocytes and fibroblasts, and binds the conserved termini of flagellin, which leads to the activation of cytokine secretion in host cells [ 12 , 15 ].
The central region of FliC is variable in sequence, and this region is exposed in native flagellin [ 16 , 17 ]. Sequence variation explains the observed differences in e. The bacterial flagellum thus affects bacterial virulence in various ways, i. In many bacterial species, the flagellum is an acknowledged virulence factor, and non-flagellated strains have in several cases been observed to be less virulent. The flagellum can act directly as an adhesin, as detailed in Section 3 and Section 4 , but can also affect virulence by other means.
Motility towards a host cell is a prerequisite for adhesion and invasion, and flagella can play an essential role in colonization by facilitating bacterial motility even if the flagella do not directly participate in the adhesion or invasion. Flagella can also contribute to virulence by regulating the expression of other virulence factors [ 2 ] and, as discussed below, the flagellum in some cases affects virulence in more than one manner.
Early studies have shown that the single polar flagellum of Vibrio cholerae , the causative agent of cholera, is crucial for its virulence: non-motile V. However, non-flagellated mutants of S. Enteritidis have been shown to cause significantly less of the typical invasion-associated membrane ruffling than the wild-type strain in cultured human Hep-2 and avian Div-1 epithelial cells, indicating that flagella are involved in the early events of S. Enteritidis invasion in a still uncharacterized manner [ 54 ].
Recently, Olsen and colleagues [ 40 ] revealed serovar-specific differences in the involvement of flagella and chemotaxis genes in the adhesion and invasion of Salmonella. In the cattle-adapted Salmonella enterica serovar Dublin, the flagella filament was required for adhesion and invasion of Intestine cells, whereas adhesion was unaffected in the constitutively tumbling cheB mutant and the constitutively smoothly swimming cheA mutant indicating that targeted motility is not essential for the adhesive capacity.
In the broad-host-range Salmonella enterica serovar Typhimurium, adhesion to Intestine was decreased by the deletion of flagellin and chemotaxis-related genes. Co-infection of mice with wild-type and mutant strains of both serovars revealed that the chemotaxis-related genes were dispensable in vivo in both serovars.
The virulence of the S. Dublin fliC mutant was decreased with oral but not with intraperitoneal administration indicating that flagellin is important at the early colonizations stages of S. Dublin infection. Typhimurium, deletion of the flagellin genes fliC and fljB rendered the mutant more virulent when administered intraperitoneally, whereas no effect was observed with oral infection indicating that flagellin is not required at the systemic phase of S.
Typhimurium infection. Cronobacter spp. A study of a random transposon mutant library of the clinical Cronobacter sakazakii strain ES5 showed that peritrichous flagella are involved in biofilm formation on polystyrene and adhesion to Caco-2 cells [ 42 ]. In the study, five mutants carried transposons in flagellum-associated genes fliD , flgJ , and flhE.
Phenotypically the fliD mutant had shorter flagella than the wild-type, the flgJ mutant lacked flagella, and the flhE mutant had flagella morphologically similar to the wild-type. Motility could not be examined, but the biofilm formation was drastically reduced in fliD and flhE mutants.
Dramatic decrease in adhesion to Caco-2 cells was observed with flgJ mutant, and flhE mutant adhered as well as the wild-type, whereas the adherence of the fliD mutant was not measured. The results apparently indicate that motility is relevant in C. Contradictory findings of the importance of flagella in P. Polar flagella are important virulence factors for Helicobacter pylori , a cause of gastric ulcers, because motility enables the bacteria to reach the gastric epithelium, adhere to it with several adhesins, and colonize the epithelium reviewed by Sheu et al.
Studies with a non-motile fliD mutant showed that FliD and thus a functional flagellum is required for the colonization of mice [ 64 ]. Clyne et al.
In several bacterial species, such as E. A comprehensive review of studies on the direct role of the flagellum as an adhesin is presented below. The Gram-negative enterobacterial species E. Two major groups of pathogenic E. Human pathogenic E. IPEC strains, which originate in domestic animals and infect human beings mainly via contaminated food or water, are divided into six main pathovars: enterohaemorrhagic E. The term Shiga-toxin producing E. One pathovar may contain strains of different H serotypes and similar H serotypes can be found in more than one pathovar [ 69 ].
Experiments performed by inactivation of the flagellin gene and assessment of adhesive properties of the mutant in comparison to the wild-type strain indicated that the flagella of an APEC isolate did not significantly affect adhesion to Hep-2 cells and non-mucus-secreting HTA cells. Conclusively, these aforementioned findings regarding APEC and especially Shiga-toxin producing EHEC OH21 indicate that flagellum-mediated motility is crucial in invasion, but that flagellin is not required for adhesion.
Several reports, however, show that flagella of some E. Insertional inactivation of the fliC H6 gene in EPEC significantly decrease the adhesion, whereas the flagellated but non-motile isogenic motB mutant adheres to HeLa as efficiently as the wild-type strain thus demonstrating that the flagellar filament indeed is involved in adhesion, not only in the movement towards a target. Complementation of the fliC mutation with plasmid-cloned fliC H6 partially restored the adhesiveness of the mutant, and expression of FliC H6 in a non-adhesive laboratory strain of E.
The interaction with HeLa cells was also demonstrated with purified H6 flagella, whereas purified H7 flagella did not bind to the cells. It was also shown that flagella expression in all motile EPEC strains tested expressing e. Flagella of aEPEC OH40 were also observed to be adhesive: the wild-type aEPEC strain adhered to the colorectal cell line T, and invaded T and Caco-2 cells, whereas an isogenic fliC mutant showed a significantly reduced adhesive and invasive capacity [ 27 ].
The authors speculate that a soluble molecule of eukaryotic origin regulates flagella production and thereby flagellum-mediated adhesion in EPEC. Using immunodot analysis, Erdem and colleagues [ 24 ] showed that purified flagella and denatured monomeric flagellin of both EPEC OH6 and EHEC OH7 bound to immobilized commercially available bovine and porcine mucins, as well as to mucus prepared from bovine colon in a dose-dependent manner.
Preincubation of purified flagella with commercial mucin followed by molecular exclusion chromatography demonstrated simultaneous elution of flagella and mucin, thus verifying the binding of flagella to mucin. Differences between H6 and H7 flagella in the interactions with host molecules were also observed: Purified H6 flagella bound to extracellular matrix proteins collagen and laminin, whereas H7 flagella did not interact with these proteins.
The hemagglutination HA was inhibited by anti-H7 antibodies and mucins see above , whereas a number of characterized carbohydrates and glycoproteins had no inhibitory effect.
The authors speculate that the observed mucin-binding property of H6 and H7 flagella may favor intestinal colonization, whereas the adhesion to the extracellular matrix proteins by H6 flagellum may contribute to colonization at sites where the intestinal barrier is disrupted. The relevance of the flagella-mediated HA property of EHEC remains to be elucidated, but apparently it reflects the receptor specificity of H7 flagella. The wild-type adhesion was blocked by anti-FliC H7 antiserum and by purified H7 flagella.
EHEC OH7 expressed flagella when applied onto BRECs, but indirect immunofluorescence microscopy and in-cell Western assay revealed a temporal expression of H7 flagella; the flagella were present during early colonization and on bacteria not associated with epithelial cells, but expression of flagella was repressed on bacteria in microcolonies or bacteria associated with the typical attaching and effacing lesions formed by EHEC on BRECs.
The results indicate that H7 flagella act as adhesins at the initiating phase of the infection, but are not required at later stages of EHEC infection. The preceding reports further demonstrate the relevance of the appropriate selection of host cells prior to the analysis of flagellar adhesion.
In contrast to IPECs, NMEC strains are associated with high mortality rates, are genetically closely related, express a number of well-characterized fitness factors and belong to only a few serotypes of which OK1:H7 is widely spread and the most studied serotype [ 71 ]. Further analysis indicated that the H7 flagellum of E. In the AIEC type strain LF82, the flagellar master operon flhDC reduces bacterial adhesion to Intestine cells indirectly by down-regulating the expression of adhesive Type 1 fimbriae, but flagella also affect bacterial invasion in a manner not restricted to motility or the presence of a flagellar filament [ 55 ].
Similarly, the flhC gene, but not the flagellin gene, reduces in an unknown manner the colonization of cattle by Shiga-toxin-producing EHEC OH7 [ 56 ]. Claret and colleagues [ 57 ] have shown that the flagellar sigma factor FliA regulates the Intestine adhesion and invasion by strain LF82 via a cyclic-di-GMP-dependent pathway that down-regulates expression of Type 1 fimbriae. In addition to the indirect effect, the flagellum can also directly be involved in AIEC adhesion and invasion.
Conclusively, AIEC flagella contribute to bacterial adhesion and invasion indirectly by regulating expression of other adhesins but also directly by facilitating FliC-mediated adhesion in a strain-dependent manner.
Porcine pathogenic E. Recently, the role of flagellum as a virulence factor has been studied in more detail in porcine isolates. The adhesive phenotype of the mutants was restored by complementation of the deletion with plasmid-cloned fliC , and specific inhibition of bacterial adhesion by purified flagella further demonstrated the adhesive role of flagella.
The results thus show that flagella of porcine-pathogenic E. Bacterial adhesion to host surfaces is not always related to infection. In the human intestine, the microbiota and to some extent also probiotic bacterial strains protect the host surfaces against harmful intruders [ 74 ].
In order to successfully colonize the gut, probiotic bacteria should be able to adhere to intestinal surfaces. More detailed analysis of the adhesive properties of Nissle indicated that the strain attached in a FliC-dependent manner to mucin-producing LST cells, but not to Caco-2 and T24 cells nor to immobilized murine mucus. Adhesion of wild-type Nissle and the hyperflagellated variant to intestinal cryosections was efficiently inhibited by mucin II, and flagella isolated from the wild-type strain bound to mucin II as well as human mucus.
For identification of the receptor molecule of Nissle flagellum, isolated flagella were preincubated with carbohydrates known to be constituents of mucin II, and assessed for binding to immobilized mucin II and human mucus. In both cases, the binding was reduced only in the presence of gluconate. The adhesion of wild-type and hyperflagellated Nissle to human intestinal cryosections and immobilized mucin II was also inhibited by mM gluconate, indicating that gluconate functions as a receptor for Nissle flagella in mucus.
The results show that not only pathogenic but also probiotic E. Flagella can also mediate bacterial adhesion indirectly via other molecules. EtpA is a secreted ETEC adhesin shown to bind to Caco-2 cells and mucin-expressing regions of mouse small intestine [ 32 ].
Complementation of the fliC deletion with plasmid-cloned fliC restored the adhesive capacity to the level of the isogenic wild-type ETEC strain. Purification of EtpA revealed co-isolation of flagellin, regardless of flagellar serotype, indicating an interaction between the two types of molecules.
Pull-down experiments showed that the toxin recognized the conserved N -terminus of monomeric FliC and bound to it and in situ immune electron microscopy revealed localization of EtpA to broken flagellar tips lacking FliD.
Conclusively, EtpA and intact flagella together form a prerequisite for efficient adhesion of ETEC to intestinal cells and mouse small intestine. As evident from the examples above, flagella of E. The role of P. However, as FliF is embedded in the outer membrane, it cannot act as an adhesin, and thus the adhesion-enabling function must be indirect rather than FliF-mediated. Later Arora et al. Wild-type P. Also in this study, complementation with fliD restored adherence while complementation with fliC was not performed.
However, as flagellar biosynthesis requires FliD for the filament polymerization, it remains unclear whether FliD alone is responsible for adhesion or whether FliC or some other flagellar component has a role in adhesion as well. Direct interaction between FliD and mucins was not shown [ 36 , 38 ]. FliD of P. Lillehoj et al. Purified flagellin and flagellin antiserum inhibited adhesion of P. Surprisingly however, deletion of fliD did not affect binding, although flagellin filament cannot polymerize without FliD.
It is therefore possible that P. Purified flagellin of P. In a recent study, P. This was demonstrated with bacteria and flagella-coated fluorescent beads. The same study also showed evidence for the adherence of Type IV pili Tfp to N -glycans at the apical surface of the polarized lung epithelial cells. Subsequently, the adhesion of P.
The flagellum-mediated adherence at the basolateral surface occurred via the epidermal growth factor receptor that is phosphorylated by flagellum, but Tfp-mediated adherence is independent of it. The ability to adhere and invade at both the basolateral and apical surfaces of epithelial cells may be crucial in the pathogenesis of P. Flagellar component s responsible for the interaction with heparin sulfate remain s to be determined.
The anaerobic spore-forming Gram-positive bacterium C. Thus, current results regarding the role of C. The early report by Tasteyre and colleagues [ 21 ] showed that C. On the other hand, Dingle and colleagues [ 22 ] demonstrated, using wild-type C.
In a hamster model FliC and FliD were nonessential for cecal colonization, but as bacterial growth curves indicated that the mutants grew more slowly and produced larger amounts of toxin than the wild-type strains, final conclusions regarding the role of C. Recent transcriptional analysis of the genome-sequenced C. The authors speculated that motility may be down-regulated, but adhesion enhanced during infection.
It is obvious that much of the data in the older papers is misleading. In addition to the bacterial species already mentioned, the role of flagella in adhesion has been studied for example in Burkholderia pseudomallei that causes melioidosis, Burkholderia cepacia , an opportunistic pathogen, Bordetella bronchiseptica , which can colonize the respiratory tract and cause canine and porcine bronchitis, Bordetella pertussis , which causes whooping cough, the gastroenteritis-causing Vibrio vulnificus and Campylobacter jejuni , and the opportunistic pathogen Stenotrophomonas maltophilia.
The flagellum has been shown to mediate adhesion of B. Other studies have also been performed to investigate the adhesive properties of B.
For example, flagella facilitated the invasion of B. The same study also compared the FliC of B. Thus it must be noted that invasion ability seems to be correlated with motility, but other proteins than flagella probably mediate adhesion and invasion by B. The invasiveness of B. Bordetella species are pathogens, which colonize the respiratory tract, and their virulence factors, including flagella, are regulated by the bvg locus. Flagella of B.
The results thus indicate that the flagellum plays a role in V. Motility was suggested to be important for invasion into Intestine cells, but direct adhesive function of flagellum was not observed [ 84 ]. According to Grant et al. This might be explained by FlaC, a protein secreted via flagella and found to increase invasion of C. FlaC is homologous to flagellin proteins FlaA and FlaB at N - and C -terminal regions but lacks the central domain and is not required for the expression of the functional flagellum [ 86 ].
The flagellum has an important role in biofilm formation, and biofilms on abiotic surfaces pose a remarkable health threat in cases such as clinical catheters and cooling systems.
To explore in detail the adhesion and thereby formation of biofilms on catheters, the mechanism of flagellum binding was studied by Friedlander and colleagues [ 87 ]. UPEC were cultured on flat silicon surfaces as well as on patterned surfaces covered with bumps and submicrometer crevices too narrow to fit the bacterial cells, and the surfaces were analyzed for bacterial attachment and presence of flagella by scanning electron microscopy.
During the first two hours after inoculation, adhesion to flat surfaces was more efficient, but after a longer incubation period, the colonization of the patterned surface was more efficient. Adherent bacterial cells were surrounded by flagella, as demonstrated by site-specific mutagenesis and scanning electron microscopy. The results indicate that flagella reach crevices, grasp to improve bacterial adhesion, and are able to penetrate substructures not accessible to the bacterial cells, as was described also for EPEC adhesion to HeLa cells and EHEC interaction with BRECs [ 23 , 26 ].
As Salmonella enterica serovar Typhi is frequently associated with cholesterol-rich gallstones in chronic carriers, the adhesion of Salmonella and formation of biofilm on cholesterol have been studied in more detail [ 88 ].
When a library of random transposon mutants of S. Typhimurium was assessed for adherence to cholesterol, mutants carrying transposons in flagellum-related genes showed reduced adhesion to cholesterol in comparison to the wild-type strain.
Further, formalin-killed wild-type flagellated S. Typhimurium adhered to immobilized cholesterol as efficiently as live bacteria and functioned as a scaffold for biofilm formation of living flagellated as well as non-flagellated Salmonella.
Conclusively, flagella composed of FliC-type flagellin mediate adhesion to cholesterol in S. Typhimurium and promotes the early formation of biofilm. Flagella composed of polymeric flagellin do not bind to TLR5, whereas monomeric flagellin induces a TLR5-mediated inflammatory response [ 12 ].
Interestingly, Subramanian and Quadri [ 89 ] reported that host cell-produced lysophospholipids induced the secretion of biologically active, monomeric flagellin in S. Typhi and S. The results were further supported by the observations that externally added lysophospholipid triggered secretion of monomeric flagellin in salmonellae and flagellin secretion was reduced in Salmonella if host cells were pretreated with inhibitors of lysophospholipid synthesis.
The secretion of monomeric flagellin was not due to depolymerization of flagella filaments but a result of eukaryotic cell-induced flagellin expression in S. Typhi, and the results demonstrated that secretion of monomeric flagellin was dependent on cAMP-dependent signaling. The authors speculate that during intestinal infection, Salmonella can sense lysophospholipids produced by host epithelial cells, activate the export of monomeric flagellin, and thereby modulate the TLR5-mediated innate immune response, which could promote bacterial dissemination.
Gangliosides have been reported to act as receptors for flagellin, but also appear to facilitate flagellin-mediated signaling in eukaryotic cells. Purified P. After addition of P.
TLR5 is predominantly expressed basolaterally, but after a prolonged incubation time, TLR5 was found mainly at the apical surface of 16HBE cells co-localized with flagellin [ 91 ].
An analysis of the role of asialo-GM1 and TLR5 in binding of flagellin and in flagellin-mediated signaling suggests that signaling down-stream of gangliosides is TLR-dependent and that the two flagellin receptors co-operate in activation of signaling pathways in epithelial cells [ 93 ].
Thus, the adhesive capacity of flagellin in combination with host ganglioside and TLR5 appears as an efficient mechanism in host defense in the intestine as well as lungs but may also enable bacterial migration. Flagella have also been reported to function in bacterial symbiosis.
Bacteria persist in multifaceted environments containing variable and dense bacterial populations, and they frequently cooperate in different aspects, like in the degradation of organic matter.
Earlier reports have indicated that unidentified extracellular filaments are involved in the initiation of symbiosis based on nutritional cooperation, also called syntrophy, between the fermentative bacterium Pelotomaculum thermopropionicum and the methanogenic archaea Methanothermobacter thermautotrophicus [ 94 ].
Shimoyama and colleagues [ 95 ] observed putative flagella and fimbriae gene clusters in the genomes of M. Extracellular filaments found in a monoculture of P. Flagellum was shown to connect M. Transcriptome analysis verified the results and revealed that FliD up-regulated more than 50 genes encoding e. The authors speculate that flagellum of P. Flagella are involved in bacterial adhesion and invasion both indirectly, i. However, only a few receptors for flagellar adhesion have so far been convincingly revealed: gangliosides GM1 and GD 1a , asialo-GM1, blood-group-antigen-related Lewis x glycotype, and heparan sulfate in the case of P.
Flagellum-mediated adhesion and invasion have been mostly studied with whole flagella, or alternatively with purified flagellin FliC or flagellar cap FliD proteins. With the exception of the N - and C -terminal regions of FliC involved in innate immunity [ 12 , 13 , 14 , 15 , 16 ] and binding to the EtpA adhesin of ETEC [ 32 ], information on receptor-binding sites in flagella proteins is completely missing.
Reports on other flagellar subunits, e. The existing knowledge on flagella adhesion is summarized in Figure 2. Summary of the bacterial flagellum as an adhesin. Flagellum can mediate bacterial adhesion to eukaryotic cells indirectly via motility 1 , or by binding directly to epithelial cells either on apical 2 or basolateral surface 3. Flagellar target receptors include mucus and mucins 4 , different glycans on cells or in mucus gluconate, heparan sulfate proteoglycans, Lewis x glycotype, GM1, asialo-GM1, GD 1a 5 , extracellular matrix ECM proteins 6 , or bacterial-secreted EtpA, which in turn adheres to host cells 7.
FliC binding also induces TLR5 expression at the apical surface. In addition to various epithelia, flagella may also adhere to amoebae 9 or connect two bacterial species Functions of flagella have mainly been studied in bacterial pathogens and from the viewpoint of bacterial virulence due to the various potential applications that exist for flagella and flagellin such as in vaccine development and diagnostics.
For example, flagellin has been used for a generation of vaccines against Salmonella enterica serovar Paratyphi A, P. Due to its high immunogenicity, flagellin has also been used as a vaccine adjuvant together with poorly immunogenic antigens, and FliC or anti-FliC antibodies have been used in diagnostics of e.
Interestingly, flagellin has also been shown to suppress apoptosis and protect against radiation-related damage [ ]. More recently, it has been noted that probiotic bacteria may also benefit from the flagellum, and that flagella can also be beneficial in symbiotic relationships between bacterial species. Thus many interesting aspects regarding the roles of flagella remain to be investigated in more detail.
We thank Ladan Cockshut for help with language editing. National Center for Biotechnology Information , U. Journal List Biology Basel v. Biology Basel. Published online Oct Author information Article notes Copyright and License information Disclaimer.
This article has been cited by other articles in PMC. Abstract The bacterial flagellum is a complex apparatus assembled of more than 20 different proteins.
Keywords: bacterial flagella, flagellin, FliD, adhesion, invasion. Table 1 Overview of flagella proteins relevant for the review a. Open in a separate window. Figure 1. Table 2 Direct and indirect roles of flagella in bacterial adhesion. Motility and Virulence In many bacterial species, the flagellum is an acknowledged virulence factor, and non-flagellated strains have in several cases been observed to be less virulent.
Flagellum Affects Virulence Mainly by Facilitating Motility Early studies have shown that the single polar flagellum of Vibrio cholerae , the causative agent of cholera, is crucial for its virulence: non-motile V.
However, some cilia can be found in plants in the form of gametes. Cilia are made of microtubules in an arrangement called the ciliary axoneme, which is covered by the plasma membrane.
The cell body makes ciliary proteins and moves them to the tip of the axoneme; this process is called intraciliary or intraflagellar transport IFT. Currently, scientists think approximately 10 percent of the human genome is dedicated to cilia and their genesis.
Cilia range from 1 to 10 micrometers long. These hair-like appendage organelles work to move cells as well as to move materials. They can move fluids for aquatic species such as clams, to allow for food and oxygen transport. Cilia help with respiration in the lungs of animals by preventing debris and potential pathogens from invading the body. Cilia are shorter than flagella and concentrate in much larger numbers. They tend to move in a quick stroke almost at the same time in a group, constituting a wave effect.
Cilia can also aid in the locomotion of some types of protozoa. Two types of cilia exist: motile moving and non-motile or primary cilia, and both work via IFT systems. Motile cilia reside in airway passages and lungs as well as inside the ear.
Non-motile cilia reside in many organs. Flagella are appendages that help move bacteria and the gametes of eukaryotes, as well as some protozoa. Flagella tend to be singular, like a tail. They typically are longer than cilia. In prokaryotes, flagella work like small motors with rotation. In eukaryotes, they make smoother movements. Cilia play roles in the cell cycle as well as animal development, such as in the heart. Cilia selectively allow certain proteins in to function properly.
Cilia also play a role of cellular communication and molecular trafficking. Motile cilia use their rhythmic undulation to sweep away substances, as in clearing dirt, dust, micro-organisms and mucus, to prevent disease. This is why they exist on the linings of respiratory passages.
Motile cilia can both sense and move extracellular fluid. Non-motile, or primary, cilia do not conform to the same structure as motile cilia. They are arranged as individual appendage microtubules without the center microtubule structure.
They do not possess dynein arms, hence their general non-motility. For many years, scientists did not focus on these primary cilia and therefore knew little of their functions.
Non-motile cilia serve as sensory apparatus for cells, detecting signals. They play crucial roles in sensory neurons. Non-motile cilia can be found in the kidneys to sense urine flow, as well as in the eyes on the photoreceptors of the retina. In photoreceptors, they function to transport vital proteins from the inner segment of the photoreceptor to the outer segment; without this function, photoreceptors would die.
When cilia sense a flow of fluid, that leads to cell growth changes. Cilia provide more than clearance and sensory functions only. They also provide habitats or recruitment areas for symbiotic microbiomes in animals. In aquatic animals such as squid, these mucus epithelial tissues can be more directly observed as they are common and are not internal surfaces.
Two different kinds of cilia populations exist on host tissues: one with long cilia that wave along small particles like bacteria but exclude larger ones, and shorter beating cilia that mix environmental fluids.
These cilia work to recruit microbiome symbionts. They work in zones that shift bacteria and other tiny particles to sheltered zones, while also mixing fluids and facilitating chemical signals so that bacteria can colonize the desired region.
Therefore cilia work to filter, clear, localize, select and aggregate bacteria and control adhesion for ciliated surfaces. Cilia have also been discovered to participate in vesicular secretion of ectosomes. More recent research reveals interactions between cilia and cellular pathways that could provide insight into cellular communication as well as into diseases. Flagella can be found in prokaryotes and eukaryotes. They are long filament organelles made of several proteins that reach as much as 20 micrometers in length away from their surface on bacteria.
Typically, flagella are longer than cilia and provide movement and propulsion. Bacterial flagella filament motors can spin as fast as 15, revolutions per minute rpm.
The swimming capability of flagella aids in their function, whether it be for seeking food and nutrients, reproduction or invading hosts.
In prokaryotes such as bacteria, flagella serve as propulsion mechanisms; they're the chief way for bacteria to swim through fluids. A flagellum in bacteria possesses an ion motor for torque, a hook that transmits motor torque, and a filament, or a long tail-like structure that propels the bacterium. The motor can turn and affect the behavior of the filament, changing the direction of travel for the bacterium. If the flagellum moves clockwise it forms a supercoil; several flagella can form a bundle, and these help propel a bacterium on a straight path.
When rotated the opposite way, the filament makes a shorter supercoil and the bundle of flagella disassembles, leading to tumbling. Due to a lack of high resolution for experiments, scientists use computer simulations to predict flagellar motion. The amount of friction in a fluid affects how the filament will supercoil.
Bacteria can host several flagella, such as with Escherichia coli. Flagella allow bacteria to swim in one direction and then turn as needed. This works via the rotating, helical flagella, which uses various methods including pushing and pulling cycles. Another method of movement is achieved by wrapping around the cell body in a bundle. In this manner, flagella can also help to reverse motion. When bacteria encounter challenging spaces, they can change their position by enabling their flagella to reconfigure or disassemble their bundles.
This polymorphic state transition allows different speeds, with the push and pull states typically being faster than the wrapped states.
This aids in different environments; for example, the helical bundle can move a bacterium through viscous areas with a corkscrew effect.
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