# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 1 Introduction  
The bacterial flagellum is one of the most complex and dynamic biological nanomachines known and has attracted attention since its discovery in the late nineteenth century. The history of flagellar research is an excellent example of the gradual transition of biological studies, that were purely morphological, to biochemical and biophysical studies at the atomic level, allowing current understanding to the point of being able to modify it for various purposes.  
Bacterial flagella are appendages on the cell body that provide motility. The number of flagella per cell varies depending on the species. Monotrichous species have only one flagellum at the pole (e.g., _Pseudomonas aeruginosa_); loophroitchous have multiple flagella at the same pole (e.g., _Helicobacter pylori_); amphitrichous have one flagella at each pole (e.g., _Campylobacter jejuni_); and peritrichous bacteria have multiple flagella in all directions (e.g., _Escherichia coli_, _Salmonella enterica_) [1]. Flagella consist of three morphologically distinct sections: a membrane-embedded basal body, a hook and a filament [2]. The basal body is a complex assembly that functions as a motor powered by a proton gradient; its rotation generates torque. This torque is transmitted through the hook to the filament that serves as propeller, creating thrust and pushing bacteria through liquid environments [3]. Counter-clockwise rotation of the motor provides smooth forward swimming, and in peritrichous bacteria filaments form a bundle on one pole of the cell [4]. Clockwise rotation of some of the flagella results in unraveling of the bundle and specific tumbling movement. Filaments are much longer than the cell body and during smooth swimming they adopt a left-handed supercoiled corkscrew shape, while clockwise rotation changes the handedness [5, 6].  
In species such as _Vibrio cholerae_ and _H. pylori_, flagella are enclosed by a sheath--a membranous structure contiguous with the outer membrane with various proposed functions, such as the prevention of filament disintegration in gastric acid, immune evasion and the place of origin of the outer-membrane vesicles that protect bacteria from bacteriophages [7]. Spirochetes are a special case since they have periplasmic flagella, the rotation of which causes undulation of the cells [8].  
The most extensively studied flagellar systems by far are those of _E. coli_ and _S. enterica_ ser. Typhimurium and, with some exceptions, they serve as models to explain flagellar architecture in general. During the assembly of the flagellum, integral membrane proteins of the basal body use the Sec pathway for insertion into the membrane. Structural components of the periplasmic rod, the hook and the filament are transported by a type 3 secretion system (T3 SS) specifically adapted to carry the distal flagellar components across the membrane. The flagellar T3 SS is located at the base of the flagellum and it is one of the earliest structures of the assembly, together with the MS ring of the basal body. The structure and mechanism behind basal body function were a subject of several recent reviews [9; 10; 11; 12], and they will not be considered further here. Instead, we will focus on the various aspects of the flagellar filament, its components and the role it plays in host-pathogen interactions.