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:
The flagellum is made up of three parts, consisting of different proteins:
Bacteria within the phylum Spirochaetes 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).