Flagella and fimbriae
Bacteria can have different types of protrusions from the surface, known as flagella and fimbriae. Fimbriae are also called pili.
The primary mission of the flagellum is to provide the bacteria motility. They can then "swim" with the help of their flagella. Motility may be important for the ability of bacteria to cause disease and, therefore, flagella are regarded as a patogenicity factor. The flagellum is typically 10-30 nm in diameter and 5-15 µm in length. Bacteria can be classified by the number of flagella and how the flagella are arranged on the cell surface as follows:
- Monotrichous bacteria have a single flagellum (e.g. Vibrio cholerae), which is said to be polar.
- Lofotrichous bacteria have many flagella, extending from one or two opposing areas on the cell surface.
- Amfitrichous bacteria have flagella on each end of the cell.
- Peritrichous bacteria have flagella scattered all over the cell surface (eg, Escherichia coli).
- Atrichous bacteria are lacking flagella.
The flagellum is made up of three parts, consisting of different proteins:
- The basal body, consisting of a system of rings, which are anchored in the cell envelope. The inner rings (S and M) are the engine which drives flagellar movement.
- The hook, which sits near the cell surface and connects the engine with the long flagellar filamentet.
- The filament, consisting of many subunits of the protein flagellin. Flagellin molecules form a hollow tube through which the new flagellin molecules are transported when the tube is extended. Flagellin is antigenic and is the so-called H antigen.
Bacteria within the phylum Spirochaetota have so-called periplasmic flagella (= axial filaments = endoflagella), which are localized in the periplasmic space and gives these bacteria a very characteristic corkscrew like movement.
Fimbriae (= pili) is another type of hair-like projections which in some exceptional cases (type IV pili) can give bacteria motility (eg in Psueudomonas sp.), but above all contribute to bacterial adhesion. So-called F-pili and sex pili give bacteria the opportunity to exchange genetic material (DNA).