Authors Authors and affiliations Alois Nowotny. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. Craigie, J. Koffler, H. Nossal, G. Press Google Scholar. Amino acid substitution enables several animal bacterial pathogens and commensals to reduce activation of TLR5 signalling [ 88 ].
Differences in recognition and subsequent signalling elicited by different flagellins suggests host discrimination between pathogenic and commensal bacteria, as a nonpathogenic strain of E.
Typhimurium [ 89 ]. Divergence in the protofilament number in C. Typhimurium serves to evade the TLR5 recognition, due to substantially different packing of the D1 domains [ 90 ]. Immune activation in plant hosts varies depending on variation in the flg22 sequence of not only phytopathogenic bacteria [ 76 , 77 , 88 , 91 ] but also human pathogenic bacteria. Variation in five amino acids of the flg22 peptide derived from certain S.
Senftenberg isolates induces a reduced pattern-triggered immunity PTI in plants in comparison to that from S. Typhimurium [ 92 ]. Likewise, plant growth-promoting rhizobacteria PGPRs , such as Sinorhizobium meliloti and Agrobacterium tumefaciens have divergent flg22 epitopes that do not elicit any responses [ 75 ].
There is also evidence to suggest that the PGPR Burkholderia phytofirmans has evolved to evade the grapevine immune recognition system via FLS2 altogether [ 93 ]. As an alternative strategy, some bacteria evade immune recognition of flagellin independently of the flg22 peptide Fig. Phytopathogenic bacteria secrete effector proteins that specifically target FLS2 counteracting detection in plants.
FLS2-induced stomatal closure is a characterised response to prevent pathogens from entering internal plant tissue [ 97 ], dependent on the action of oxylipins, rather than the phytohormone abscisic acid ABA [ 98 ].
Some isolates of P. Intriguingly, S. Typhimurium can also delay stomatal closure, although to a lesser extent, via potential effector proteins other than coronatine, which may target the oxylipin pathway. Typhimurium, cannot prevent stomatal closure, suggesting active manipulation of PTI by Salmonella [ ]. This strategy effectively evades the immune recognition in both kingdoms by degrading the natural ligand of TLR5 and FLS2 [ ]. An alternative mechanism of evading recognition may come from modification of flagellin, reported for a number of animal and phytopathogenic bacteria [ , ].
Glycosylation enhances the structural stability of flagellin, preventing exposure of the flg22 region to FLS2 recognition, thereby evading the plant immune response [ ].
There are multiple examples of O -glycosylation, well characterised for the flagellins of C. Other examples include P. However, C. Perhaps it relates more to virulence and adherence, rather than filament stability [ — ]. Interestingly, S. Typhimurium flagellin is methylated at multiple lysine residues, yet this has no known impact on flagellar function [ ]. The role of flagella post-translational modification in immune recognition clearly needs further investigation.
Flagella enable pathogens to exploit or capitalise on various niches associated with the host. Although they display a range of functions, these are intrinsically linked to host colonisation and their own biophysical properties. Flagella are therefore not a virulence factor per se, but rather an early stage colonisation factor. They facilitate individual, pioneering cells to access, bind and invade new plant and animal tissues, and if successful in avoiding host recognition and clearance, to establish new colonies.
More work is needed to understand how bacteria progress from flagella expression to flagella disassembly, both in the context of expression of more specialised colonisation factors that target specific ligands, and in immune recognition.
The location of known ligands and the differences between decoys and membrane bound receptors also need to be addressed. Abstract The flagellum organelle is an intricate multiprotein assembly best known for its rotational propulsion of bacteria. Introduction The prokaryotic flagellum is best known as a motility organelle responsible for bacterial movement and necessary for chemotaxis [ 1 ]. Twist and Stick: Interactions with Host Tissues A role for flagella-mediated adherence has been demonstrated in many different plant species and animal infection models, for both pathogenic and opportunistic bacteria [ 4 , 5 ].
Download: PPT. Figure 1. Adhesion to plant tissues Flagella-mediated adherence to plant tissues has previously been described for phytobacteria, such as Pseudomonas syringae to bean seedlings [ 9 ] and nitrogen-fixing bacteria Azospirillum brasilense to wheat roots [ 10 ].
Adhesion to mammalian tissues The role of flagella in bacterial adherence to mammalian hosts has been demonstrated for various bacterial species in a number of hosts [ 5 ]. Interaction with mucus Epithelial mucosal tissues act as a barrier between the animal host and its environment. Molecular targeting While there are many studies demonstrating a role for flagella in adherence and colonisation of tissues, few have identified or characterised specific targets used by the flagella for binding.
Dodge: Immune Recognition Flagellar-mediated host interactions incur a cost, as conserved regions in flagellin monomers are potent inducers of innate immune responses in vivo, across kingdoms. Structural recognition The flagellin monomer is organised into four connected domains designated D0, D1, D2, and D3 Fig.
Figure 2. Cross-kingdom immune recognition of flagellin structures. Immune evasion by flagella Since flagellin is such an important immunogen, bacteria have evolved multiple strategies to avoid or evade recognition Fig.
Figure 3. Post-translational modification An alternative mechanism of evading recognition may come from modification of flagellin, reported for a number of animal and phytopathogenic bacteria [ , ]. Concluding Remarks Flagella enable pathogens to exploit or capitalise on various niches associated with the host.
Acknowledgments We acknowledge all the additional work that we could not cite for space limitations. References 1. Curr Opin Cell Biol — Egelman EH Reducing irreducible complexity: divergence of quaternary structure and function in macromolecular assemblies. Curr Opin Cell Biol 68— Nature — Environ Microbiol — Biology 2: — Pratt LA, Kolter R Genetic analysis of Escherichia coli biofilm formation: roles of flagella, motility, chemotaxis and type I pili.
Salmonella Enteritidis, however, only rose to its predominant position in man and poultry in the mids as a result of vertical and horizontal transmission within and between large poultry organizations in many parts of the world Calnek et al. The ability of bacterial cells to invade organs after oral inoculation of a host is an indication of virulence. Virulence is a complex phenomenon that is influenced by a number of molecules called virulence factors.
Consequently, epidemics due to virulent organisms occur sporadically, i. Due to the number of virulence factors and the complexity of their expression, it is unusual for a single factor on the outer membrane to be a reliable predictor of virulence. Recent investigations of Salmonella Enteritidis have indicated that the lipopolysaccharide LPS is a virulence factor, and have also indicated that a particular structure of the LPS molecule might be a reliable indicator of virulence potential Barrow ; Rahman et al.
LPS molecules consist of a bisphosphorylated lipid lipid A forming the matrix of the outermost membrane leaflet which is stabilized by divalent cations, and a hydrophilic polysaccharide PS extending outward from the bacterium. The PS consists generally of two distinct regions, a core oligosaccharide containing 10—12 sugars, and a polysaccharide chain of repeating units, the O-specific chain.
The core is covalently bound through an acidic sugar, usually 3-deoxy-D-manno-octulopyranosonic acid Kdo , to the lipid A. Although also present in some capsular polysaccharide structures, Kdo seems to be a characteristic and essential sugar for the great majority of endotoxins. Dideoxyhexoses are paratose serogroup A , abequose serotype B , or tyvelose serotype D1 , and reflect the correlation of serotype classification and chemical structure of the 0-antigenic repeats of Salmonella Steinbacher et al.
Many diagnostic tests have been developed for the detection of salmonella infections in poultry. Conventional methodology for identifying Salmonella is based on the isolation in pure cultures, and biochemical and serological tests Fitzgerald et al. However, culture is expensive and time-consuming and also suffers because individual birds excrete Salmonella Enteritidis intermittently or may eliminate the infection altogether. Serology is the other component of field and laboratory testing of flocks to establish their Salmonella Enteritidis status.
Because of decreased costs and rapid turnaround time, serology has developed into a promising screening tool for flocks. Salmonella Enteritidis is an invasive serotype, and immunoglobulin G IgG responses persist in birds that have been infected with Salmonella Enteritidis.
Therefore, serology would be a superior method for screening birds that are intermittently culture-positive or that have eliminated Salmonella Enteritidis infection McDonough et al. Serological identification and classification techniques of Salmonella are based on O and H antigens according to the Kauffmann—White scheme Fitzgerald et al.
Today, the development of novel reagents for diagnostics has become economically important for the evaluation of more reliable diagnostic kits. The objectives of the present study were to isolation, identification and purification of Salmonella Enteritidis O and H antigens for the various applications of diagnostic tools and to examine the use of these antigens to define the serological specificities. Salmonella Adeyo and Salmonella Oranienburg were used for the isolation of flagella and Salmonella Typhimurium and Salmonella Senftenberg were used for the isolation of LPS in the confirmation studies of the isolated antigens.
Cultures were maintained by three replicating subcultures through a single colony to enhance the purity, identity and homogeneity of the culture.
After that, a streaked single colony checked by Gram stain technique was used in stock culture preparation. Salmonella Adeyo, Salmonella Oranienburg, Salmonella Typhimurium and Salmonella Senftenberg were grown overnight on semi-solid beef extract agar Difco, USA used for the isolation of flagellar antigens. If it is motile, the strain must cover the whole petri. Next, 90— ml of 0. Following the formalin treatment, the flagellin extract was obtained after checking for viability.
The phenol phase was re-extracted with water twice and the combined aqueous phases were evaporated and dialyzed against distilled water for 3 days and freeze-dried. Bacteria were grown on 0. Expression of H antigen was confirmed by routine serological methods which were used for flagellin isolation. After harvesting the bacteria in aq. Briefly, Salmonella cell suspension in aq.
The sediment was resuspended in aq. After centrifugation, sediment was used for isolation of LPS. After coating the well plate with the crude flagellin, the antigenic properties were checked by ELISA against flagella-specific antisera, and the final purity was obtained after column chromatography Sephadex G ; Pharmacia. Fractions were collected every 20 min and the fractions between 10 and 35 were combined. Combined fractions were concentrated with PEG. The protein content was determined by Bradford assay.
Estimation of 3-deoxy-D-mannooctulosonic acid Kdo residues was performed by a modified thiobarbiuric acid TBA assay, and protein content was estimated by a modified Lowry assay as reported by Toman and Skultety Cook DN, et al.
CCR6 mediates dendritic cell localization, lymphocyte homeostasis, and immune responses in mucosal tissue. Kwon JH, et al. Rumbo M, et al.
Lymphotoxin beta receptor signaling induces the chemokine CCL20 in intestinal epithelium. Gomez-Gomez L, Boller T. Flagellin perception: a paradigm for innate immunity. Trends Plant Sci. Lee SK, et al. Helicobacter pylori flagellins have very low intrinsic activity to stimulate human gastric epithelial cells via TLR5. Microbes Infect. Neish AS, et al.
Prokaryotic regulation of epithelial responses by inhibition of I kappa B-alpha ubiquitination. Kelly D, et al. Commensal anaerobic gut bacteria attenuate inflammation by regulating nuclear-cytoplasmic shuttling of PPAR-gamma and RelA.
Figures and Tables. Figure 1 Structure and organization of flagellum and flagellin. Flagella show a hook dark green and a filament, referred to as flagellum yellow. Contribution of flagellum to bacterial pathogenesis in mucosa. Mucosal pro-inflammatory responses to flagellin monomers.
Some bacterial species secrete flagellin monomers red dots in culture due to inefficient capping or flagellum break. This release might more Flagellin-specific regulation of adaptive immunity. Left panel : The model of Parish considers S. Top Abstract Contribution of flagella to pathogenicity Mucosal pro-inflammatory responses to flagellin Conclusions References. Abstract Flagella contribute to virulence of pathogenic bacteria through chemotaxis, adhesion to and invasion of host surfaces.
Teaser Mucosal pathogens use flagella for invasion of host surfaces whereas the host detects and induces defences to pathogen through Toll-like receptor 5 that detects flagellin, the subunit of flagellum.
Contribution of flagella to pathogenicity The prerequisite event for any infection is the encounter of pathogenic bacteria with the target tissue. Motility and pathogenesis The glycocalyx and mucus layer associated to epithelium form inevitable physical and chemical obstacles for pathogens. Similarly, dynamic processes like upward flow of mucus of the bronchial epithelia or peristaltism in the intestine, have to be counterbalanced by pathogens to achieve colonization.
In the host, motility combined to chemotaxis allow the fine tuned access of pathogens to target mucosal tissues Fig. Motility functions of Helicobacter pylori and Pseudomonas aeruginosa are crucial for infection of stomach and lung, respectively [ 8 , 9 ]. Colonization of intestinal mucosa by Vibrio cholerae strictly requires motility [ 7 ].
Colonization of rabbit appendix by S. Since motility increases the occurrence of host-pathogen interactions, this feature contributes to the main role of the flagellum in pathogenesis. Role of flagella in adhesion to and invasion of mucosal surfaces Flagella can participate in the occupancy of a specific niche acting as an adhesin Fig.
Crude flagella from the opportunistic pathogen Clostridium difficile bind to cecal mucus of germ-free mice [ 11 ]. In addition, non-flagellated C. In cystic fibrosis, P. Noticeably, P. Furthermore, enteropathogenic Escherichia coli adhere to the intestinal mucosa or to tissue culture cells via flagellum-dependent mechanism [ 13 ].
The hypervariable region of flagellin D2—D3 is likely bearing the adhesin-like properties Figs. The importance of flagella for the invasion of epithelial cells has been reported for several bacteria like Yersinia enterocolitica [ 5 , 14 ]. In pathogenic bacteria, TTSS serve as molecular syringes required for export and injection of virulence factors into the cytosol of host cells [ 15 ]. Accordingly, pathogens hijack cytosolic pathways to colonize or kill the host cells.
Coordinated expression of virulence genes and genes involved in flagellum synthesis The transcription of about 50 flagellar genes is hierarchically controlled by environmental conditions via the master regulator operon flhDC [ 17 ]. In contrast, the BvgAS system of Bordetella bronchiseptica represses flagellum gene transcription while it activates the expression of virulence factors [ 19 ]. In general, expression of flagella is likely switched off once mucosal pathogenic bacteria disseminate into deeper tissues [ 6 ].
Mucosal pro-inflammatory responses to flagellin Colonization of mucosa is restricted by renewal of epithelial cells, barrier function of the intercellular junctions, andthe production of antimicrobial molecules such as lysozyme and defensins, which clear microbes [ 20 ].
Systemic injection of flagellin induces similar effects in mice [ 22 , 30 ]. In epithelial cells, the flagellins from S. In mucosal tissues, these factors participate in anti-microbial activity, in the recruitment of professional killer as well as antigen-presenting phagocytes, and in the production of inflammatory mediators that set up the platform for phagocyte activation.
Although virulence factors such as invasins were initially found to be required in activation of pro-inflammatory response by pathogenic bacteria in epithelial cells, recent studies showed that this response is fully recapitulated by flagellin [ 25 , 26 ].
Relation between flagellin structure and immunostimulatory activity The hypervariable domain of flagellins from pathogenic E. This is consistent with stimulatory activity of Listeria monocytogenes flagellin that lacks variable region [ 22 ]. Monomers of flagellin induce TLR5 signalling whereas filamentous flagella do not [ 23 ]. Indeed, the signalling moiety of flagellin is hidden in the flagellum but becomes accessible in the monomer Figs.
Most mutations in the conserved D1 domain residues 44— and — of S. The hydrophobic motif 88—97 in S. TLR5 activation is independent of any post-translational modification of flagellin [ 23 , 39 ]. Interestingly, mechanisms developed by bacteria to escape from TLR5 detection can depend on flagellin sequence Box 2.
Together, TLR5 detects a specific conformation of flagellin domain D1, which is required for flagellum formation and function and is exposed only in monomer. Delivery of flagellin monomer in mucosa The mechanisms by which flagellin is released by bacteria during tissue colonisation are crucial for TLR5-mediated responses. Flagellin might be released in mucosa by secretion as observed in vitro Fig. For example, Caulobacter crescentus ejects its flagellum when this organelle is no longer required for the bacterial life cycle [ 45 ].
Alternatively, flagella could be sheared from bacterial surfaces by host proteases or detergents such as bile salts or surfactants. Thus, appropriate antibodies and T cell responses are elicited to eradicate the pathogen. DCs are the key antigen-presenting cells that control the induction of adaptive immunity [ 46 ].
Immature DCs are resident sentinel cells in tissues that are specialized in antigen capture and that can be activated by MAMPs. TLR5: the paradigm of TLRs promoting Th2 and regulatory responses The pioneer studies conducted with flagellin isolated from Salmonella adelaide showed that flagellin is a potent stimulator of antibody responses, a hallmark of Th2 responses for review see [ 47 ] Fig.
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