# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 2 Historical Overview  
During his work, which confirmed the germ theory, [PERSON] also made the first micrographs of the bacterial flagella [13]. By the 1930s there was a consensus among bacteriologists that these were locomotory organelles and a brief controversy caused by the South African bacteriologist [PERSON] in the mid-1940s sparked a series of biochemical and electron microscopy studies that finally settled the question of the role of bacterial flagella [14].  
The first suggestion that flagella are of a proteinaceous nature came from [PERSON] and [PERSON] in 1938, based on the observed insolubility of the H antigen (a historical designation of the flagellar antigen) in trichloroacetic acid, together with its thermolability and sensitivity to alcohol [15]. The majority of the early biochemical studies of flagella were conducted by [PERSON], a student of [PERSON] at the University of Uppsala at the time. Using a purification protocol developed by his colleague [PERSON] that he further improved, he mainly studied flagellar filaments of _Proteus vulgaris_ and _Bacillus subtilis_, combining various approaches such as paper chromatography, electrophoresis, ultracentrifugation and light absorption [16; 17; 18; 19]. His analyses showed that flagella are protein structures that could be decomposed by acid treatment into smaller particles of elongated shape and molecular weight of approximately 41 kDa [16]. [PERSON] and coworkers at Purdue University took an interest in the matter and confirmed those findings [20; 21; 22]. [PERSON] and [PERSON] performed the first X-ray analysis of flagella at the University of Leeds in 1949 and obtained a diffraction pattern characteristic for fibrous proteins [23]. In 1955, they proposed the name \"flagellin\" that was adopted for the protein molecules that make up the flagellar filament [24].  
In the following years, several groups reported successful attempts to repolymerize flagellin into native-like filaments by varying pH, salt concentration and temperature [25; 26; 27]. Additionally, [PERSON] et al. noticed that the filament elongation in vitro occurs only at the distal end, making the filament growth unidirectional [28]. The same principle was also observed in living cells, raising the question of how this process occurs and what is the energy source that drives it [29].  
During the 1960s, much attention was directed to the structural organization of the filament. [PERSON] et al. observed that the filament is a hollow structure with the flagellin molecules packed in a helical array [30]. During this time, several models of filament organization were proposed based on the interpretation of electron microscopy and X-ray diffraction data with varying numbers of longitudinal rows of flagellin positioned around the central channel [30; 31; 32]. Analyzing optical diffraction of the electron micrographsof the \(S\). Typhimurium filaments, [PERSON] and [PERSON] finally proposed a correct model for \(S\). Typhimurium in 1972 with 11 longitudinal rows of flagellin, now referred to as protofilaments, positioned around a central hole [33].  
Another distinctive feature of the filament was noticed in 1985. [PERSON] et al. were studying intact flagella in _Salmonella_ and observed that their ends are tapered and blunt [34]. On many, they noticed a thin protein layer that looked like a cap. Using immunogold labeling, it was shown that the cap structure at the tip of the filament is made of FilD (also called hook-associated protein 2 or HAP2) [35], a protein isolated and identified as a minor component of the hook several years before [36]. These studies led to the establishment of a model in which the FliD complex initially assembles at the tip of the hook before any of the flagellin molecules are secreted, thus preventing the leakage of flagellin and assisting in the initiation of filament formation. Therefore, although initially part of the hook junction region, with the formation and elongation of the filament beneath the FliD cap, the cap becomes localized to the tip of the filament.