Flagella at the Host-Microbe Interface: Key Functions Intersect With Redundant Responses

Abstract

Many bacteria and other microbes achieve locomotion via flagella, which are organelles that function as a swimming motor. Depending on the environment, flagellar motility can serve a variety of beneficial functions and confer a fitness advantage. For example, within a mammalian host, flagellar motility can provide bacteria the ability to resist clearance by flow, facilitate access to host epithelial cells, and enable travel to nutrient niches. From the host's perspective, the mobility that flagella impart to bacteria can be associated with harmful activities that can disrupt homeostasis, such as invasion of epithelial cells, translocation across epithelial barriers, and biofilm formation, which ultimately can decrease a host's reproductive fitness from a perspective of natural selection. Thus, over an evolutionary timescale, the host developed a repertoire of innate and adaptive immune countermeasures that target and mitigate this microbial threat. These countermeasures are wide-ranging and include structural components of the mucosa that maintain spatial segregation of bacteria from the epithelium, mechanisms of molecular recognition and inducible responses to flagellin, and secreted effector molecules of the innate and adaptive immune systems that directly inhibit flagellar motility. While much of our understanding of the dynamics of host-microbe interaction regarding flagella is derived from studies of enteric bacterial pathogens where flagella are a recognized virulence factor, newer studies have delved into host interaction with flagellated members of the commensal microbiota during homeostasis. Even though many aspects of flagellar motility may seem innocuous, the host's redundant efforts to stop bacteria in their tracks highlights the importance of this host-microbe interaction.

Keywords: DEFA6; IBD; IgA; LYPD8; TLR5; ZG16; fliC; goblet cell.

Figure 1

Bacteria utilize flagellar motility for multiple functions within the host. (A) Bacteria use flagellar motility to resist the flow of intestinal contents, maintaining their location within the host GI tract. In the case of Vibrio cholera, this allows them to maintain stable colonization of the proximal end of the Danio rerio (zebrafish) gut (51, 52). (B) Bacteria use flagellar motility to swim through mucus to reach the epithelium. This is a likely method for microbes that need to reach the epithelium in order to facilitate activities such as adhesion, invasion, and translocation (37, 53). (C) Bacteria use flagellar motility to swim toward chemotactic signals (36, 37, 54, 55). These signals can be products of inflammation or epithelial damage, representing vulnerable areas of the mucosal barrier.

Figure 2

The host prevents microbes from accessing the epithelium. (A) Innate effector molecules and antibody are present in the GI tract lumen and inhibit bacterial motility through various means. ZG16 (blue hexagons) aggregates motile Gram-positive microbes in the outer mucus layer, away from the epithelium (92). Lypd8 (purple squares) inhibits bacterial motility in the colon through a currently unknown, non-agglutinating mechanism (89). Dotted boxes depict examples of SEM micrographs of sIgA (Left) and HD6 (Right) agglutinating bacteria, taken from Levinson et al., 2015 (62, 94) and Chu et al., 2012 (86), respectively. HD6, human α-defensin 6; LYPD8, Ly6/plaur domain containing 8; ZG16, zymogen granular protein 16. (B) Microbicidal effector molecules in the small intestine form a concentration gradient, with the highest concentration located in the crypt (95). It is likely that microbes able to localize closer to the crypts will be subjected to a more inhospitable or fatal environment, as depicted in the magnified image. AMP, Antimicrobial Peptide. (C) Sentinel goblet cells specialized goblet cells located at the entrance of colonic crypts (85). After stimulation with a bacterial ligand such as flagellin or LPS (red lightning bolt), these sentinel goblet cells will secrete mucus into the crypt. This is likely a method to detect microbes that have traveled through the inner mucus layer (possible by flagellar motility) and initiate the secretion of extra mucus (green arrows), thereby effectively flushing the microbes away from the crypt (black arrows). Dotted box depicts a sentinel goblet cell secreting mucus (right) in response to stimulation by a bacterial ligand, compared to an unstimulated sentinel goblet cell (left), taken from Birchenough et al., 2016 (85). (D) IgG-bound bacteria become immobilized in mucus due to the collective minor and non-covalent interactions (green dashed lines) between IgG and mucus (grey mesh) (–98).

Figure 3

The host immune system can inhibit bacterial motility through multiple mechanisms. (A) Host effector molecules can inhibit flagellar motility through non-agglutination means by binding sites on the flagellum other than flagellin. LYPD8 (purple diamond) inhibits the flagellar motility of STM by binding to the flagellum (89). Graphs display ELISA experiments showing that LYPD8 binds the flagella (right), but does not specifically target flagellin (left) [adapted from (89)]. LYPD8, Ly6/Plaur Domain Containing 8. (B) The host can influence the microbiota’s expression of flagellar genes through antibody-mediated pruning (74, 107). A sizable fraction of antibody (green bifurcating structures) in the GI tract bind can bind to the flagella of commensal microbes (left panel), resulting in a significant decrease of flagellated bacteria within the microbiota (right panel). While this antibody-mediated pruning results in a decrease of flagella expression within the microbiota, the overall composition of the microbiota remains largely unchanged. In vitro experiments show that the anti-flagellin antibody causes E. coli to down regulate its expression of flagellin over time (graph). (C) In the GI tract, antibody-mediated agglutination can occur through a process termed enchained growth (62, 94). By this mechanism, bacteria that are actively dividing within the GI tract are coated in antibody (red line structures). Upon successful fission, the daughter cell is immediately linked to the mother cell from antibody crosslinking. Over multiple cycles of division, an agglutinated aggregate is formed consisting of clonal population of bacteria. Fluorescence microscopy micrographs depict examples of these clonal agglutinated aggregates (inset micrographs). (D) Antibody-mucin interactions facilitate the immobilization of motile bacteria within the mucus matrix (green hexagonal structure). In this model, glycans present on the surface of antibody (brown bifurcating structures) and mucin form weak, non-covalent interactions (yellow ovals) (96, 97). When motile bacteria within the mucus are coated by antibody, the numerous antibody-mucus interactions create a cumulatively strong non-covalent interaction that immobilizes the bacteria.