Pilus
The pilus (from Latin pilus , `` hair '', in particular `` single hair '', `` fiber ''; plural: pili , also called fimbriae ) is a thread-like appendage consisting of a protein that occurs in prokaryotes and is located outside the cell as a cell process . Pili are typical for gram-negative bacteria which, depending on the individual, have a pilus to many pili. There are several types of pili that differ in their protein, length and diameter, and function. The length varies between about 0.1 and about 20 µm, its diameter 2 to about 20 nm. Examples of functions: attachment to a solid / liquid or gas / liquid interface , attachment to other bacteria, attachment to the surface of animal cells .
The attachment to interfaces is of ecological importance: If the bacteria occur in a flowing medium, they can remain in place if they attach to a solid and are not carried along with the flow of liquid. In this way, the medium flows past them, bringing with it new nutrients and carrying away metabolic waste products. When attached to the air / liquid interface, they can take nutrients from the liquid and oxygen from the air. A dense layer of bacteria (and also other microorganisms) on the surface of liquids (also attached by other means) is called " scum ". Pili, which consist of proteins with an apolar surface, are particularly suitable for attachment to a gas / liquid interface. The attachment to animal cells promotes their pathogenicity in some bacteria .
F-pilus
There are special pili that are used for horizontal gene transfer ( conjugation ). They are called F-Pili (F from English fertility "fertility") or Sexualpili and are relatively thick and hollow, their length is 1-4 µm, their diameter 2–8 nm. Usually only one F-Pilus is formed per individual from the donor bacterium ( donor ). After contact with a recipient individual ( acceptor or recipient ), the pilus is broken down by the donor and the distance between the two cells is reduced. If the distance is small enough, a plasma bridge can be established outside the pilus , via which genetic information in the form of DNA can be exchanged. The exchange of genetic information does not take place via the "tunnel" of the F-Pilus. For example, resistance (R) factors or fertility (F) factors are exchanged across the plasma bridge. The DNA double strand is wound up in single strands and parts of a single strand (it doesn't matter which one) migrate from the donor to the recipient. The plasma bridge then dissolves and both bacteria complete the single-strand area to form a double strand.
Even if genetic material is exchanged during this process ( conjugation ), it is not a real sexual process because bacteria have no gender. The term sex pilus is therefore misleading and should not be used; the correct name is F-Pilus .
Type IV pilus
Some bacteria can move on a solid surface with the help of type IV pili. One speaks of "twitching motility", in English called "twitching motility". The pilus consists of copies of the PilA protein, has a diameter of around 6 nm and is up to 4 µm long. Unlike flagella, it is not hollow. The pili are usually located on both poles of the bacterium.
More types
- T-pilus
- Hrp-Pilus in plant pathogens (main subunit HrpE)
- Type I pilus
- Pap pilus
