Figure 1.
The biophysical properties of flagella, to “twist and stick,” lend themselves towards nonspecific adhesion.
Left, a summary of key characteristics of the flagella apparatus that are advantageous for adherence, and right, specific properties highlighted on the left. (A) Flagellum length results in a long reach towards colonization surfaces as an early-stage anchor—higher affinity binding can occur at closer proximity with specific adhesins and receptors. (B) Flagella rotation generates force that promotes membrane interactions during initial adherence. (C) Flagella are highly repetitive structures, non-specific low affinity binding can result in adhesion at high avidities.
Figure 2.
Cross-kingdom immune recognition of flagellin structures.
Top: backbone of the key residues of flagellin recognized by plant (left) and animal (right) innate immune receptors are highlighted in red. FliC from S. enterica is presented as a “model” flagellin, with reports for recognition by both TLR5 and FLS2 receptors. These residues are superimposed on the solved flagellin structure (PDB# 1UCU) in UCSF Chimera [113]. Surfaces and backbone are coloured according to previously assigned structural domains as indicated below each monomer [57]. Bottom: recognition of flagella filaments by plant (left) and animal (right) innate immune receptors does not occur as key residues (surfaces highlighted in red) are hidden within the filament structure. However, immune recognition still occurs in animals via antibody recognition of the D3 domain.
Figure 3.
A variety of mechanisms employed to “dodge” the flagellin innate immune response.
(A) Flagella filaments degrade, releasing monomeric flagellin, the residues of which are recognised by receptor TLR5, NLRC4, or FLS2, resulting in cytokine release or PTI. The example residues and receptor (right) shown are involved in TLR5 recognition. (B) Flagellin recognition by TLR5, NLRC4, or FLS2 is evaded by variation in key residues involved in flagellin detection, which can necessitate compensatory mutations. (C) Bacteria secrete enzymes that specifically target and degrade monomeric flagellin, preventing its recognition by TLR5, NLRC4, or FLS2. (D) Post-translational glyosylation of flagellin is thought to enhance flagella stability; reduced release of flagellin from flagella filaments will result in reduced recognition by TLR5 or FLS2. (E) Bacteria secrete effector proteins that interfere with TLR5, NLRC4, or FLS2 recognition either by direct inhibition of receptor expression or binding, or by inhibition of downstream signalling pathways. (F) Bacteria down-regulate or switch off flagella expression when motility and/or binding are no longer required.