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A pilus (Latin for 'hair'; pl.: pili) is a hair-like cell-surface appendage found on many bacteria and archaea. [1] The terms pilus and fimbria (Latin for 'fringe'; plural: fimbriae ) can be used interchangeably, although some researchers reserve the term pilus for the appendage required for bacterial conjugation .
With lengths of 1-2um, the pili can be larger than the diameter of the bacteria itself. [4] The main body of the fimbriae is composed of approx. 1000 copies of the major fimbrial subunit protein PapA, forming a helical rod. [5] The short fimbrial tip is made of the subunits PapK, PapE, PapF and the tip adhesin PapG, which mediates the binding.
The Saf pilin N-terminal extension protein domain helps the pili to form, via a complex mechanism named the chaperone/usher pathway. It is found in all c-u pilins. [8] This protein domain is very important for such bacteria, as without pili formation, they could not infect the host. Saf is a Salmonella operon containing a c-u pilus system. [8]
The F-pili are extremely resistant to mechanical and thermochemical stress, which guarantees successful conjugation in a variety of environments. [10] Several proteins coded for in the tra or trb locus seem to open a channel between the bacteria and it is thought that the traD enzyme, located at the base of the pilus, initiates membrane fusion.
Three types of gliding motility in bacteria are: a) Type IV pili: A cell attaches its pili to a surface or object in the direction it is traveling.The proteins in the pili are then broken down to shrink the pili pulling the cell closer to the surface or object that was it was attached to.
Twitching motility is a form of crawling bacterial motility used to move over surfaces. Twitching is mediated by the activity of hair-like filaments called type IV pili which extend from the cell's exterior, bind to surrounding solid substrates, and retract, pulling the cell forwards in a manner similar to the action of a grappling hook.
The bacteria are found naturally in warm seawater, and can cause a severe and potentially life-threatening illness in humans called vibriosis. Some experts warn it could pose a growing threat.
Most species of pathogenic bacteria express more than one type of chaperone/usher system, for example in Pseudomonas aeruginosa there are five different systems. UPEC expresses both the type 1 fimbriae and the P-pilus (pap) which it expresses sequentially possibly to facilitate migration from the bladder (type I fimbriae) to the kidney (pap).