# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 6 Filament-Host Interaction
### Flagellar Filament in Biofilm Formation  
The involvement of flagella in biofilm formation is well-documented and many species in which _fliC_ was deleted exhibit significantly reduced biofilm formation on diverse surfaces, suggesting that flagellin _per se_ plays a role in biofilm formation [177, 178, 160, 179, 180, 181]. Besides _fliC_ involvement, studies in _Cronobacter sakazakii_ strain E55, an opportunistic food-borne pathogen, showed that biofilm formation was drastically reduced in _fliD_ and _fli_E mutants, suggesting a direct interaction of flagellar filament and Caco-2 cells [182].  
Beyond the necessity of filaments for bacterial attachment, they may affect biofilm formation through certain specific functions. In the _P. aeruginosa_ PAO1 strain, the absence of phosphorylation at the conserved T27 and S28 residues of FliC does not affect swimming motility, but it does affect the secretion levels of extracellular proteases by the type 2 secretion system (T2 SS) and biofilm formation [183]. Residues T27 and T28 are conserved in the N-terminal domain of flagellin across _Pseudomonas_ sp., _Escherichia_ sp. and _Salmonella_ sp. with the canonical sequence being (T/S)27, (T/S/A)28, implying that flagellin might have some function other than motility, where these phosphorylation sites could play a role. The effects of FliC phosphorylation on biofilm attachment and dispersal indicate that initial attachment and detachment during the dispersal stage are delayed by the loss of FliC phosphorylation in static and dynamic flow biofilms. As each of these processes still proceeded in the absence of phosphorylation, it was suggested that FliC phosphorylation regulates the timing and rate of these processes without affecting biofilm architecture. The FliC-T2 SS interaction is evident during biofilm growth of _P. aeruginosa_. Taken together, these data suggest that FliC phosphorylation could act to integrate environmental cues with the signals for attachment or dispersal of biofilms playing a key role in ecological adaptation of this opportunistic environmental pathogen [183].  
Polar flagella of _Vibrio_ contain multiple flagellin subunits and, in addition to the four that build the filament, two flagellin-homologous proteins (FHPs), FlaE and FlaF, are also present. Neither FlaE nor FlaF, however, are involved in the construction of flagellar filaments and cellular motility. Recently, [PERSON] et al. demonstrated that _V. vulinficus_ mutants defective in both _flaE_ and _flaF_ genes significantly decreased biofilm production compared to wild-type strains, revealing that FlaEF are involved in strengthening the exopolysaccharide (EPS)-enriched biofilm matrix, thus being essential dominant constituents and crucial in the maturation of biofilms [184]. Both proteins are well expressed and secreted into the extracellular milieu through the secretion apparatus for flagellar assembly and significant levels of FlaEF were detected in the extrapolymeric matrix of _V. vulinficus_ biofilm. Although FHPs pass through the same channel as flagellin subunits, FHPs exhibited significantly lower affinities for HAPs, such as FliD and FlgL, and it was assumed that these FHPs are just excreted to extracellular milieu without being assembled into a flagellar structure and without any interaction with flagellins. Other pathogenic _Vibrio_ species also exhibited the presence of FHPs that are involved in biofilm enhancement, which, in turn, may facilitate increased pathogenicity of these bacterial species [184].