Pilus

Some bacteria, viruses or bacteriophages attach to receptors on pili at the start of their reproductive cycle.

As the primary antigenic determinants, virulence factors and impunity factors on the cell surface of a number of species of gram-negative and some gram-positive bacteria, including Enterobacteriaceae, Pseudomonadaceae, and Neisseriaceae, there has been much interest in the study of pili as an organelle of adhesion and as a vaccine component.

They are sometimes called "sex pili", in analogy to sexual reproduction, because they allow for the exchange of genes via the formation of "mating pairs".

During conjugation, a pilus emerging from the donor bacterium ensnares the recipient bacterium, draws it in close, and eventually triggers the formation of a mating bridge, which establishes direct contact and the formation of a controlled pore that allows transfer of DNA from the donor to the recipient.

It has been suggested that in these archaea the conjugation machinery has been fully domesticated for promoting DNA repair through homologous recombination rather than spread of mobile genetic elements.

Fimbriae possess adhesins which attach them to some sort of substratum so that the bacteria can withstand shear forces and obtain nutrients.

This layer, called a pellicle, consists of many aerobic bacteria that adhere to the surface by their fimbriae.

Thus, fimbriae allow the aerobic bacteria to remain both on the broth, from which they take nutrients, and near the air.

Fimbriae are required for the formation of biofilm, as they attach bacteria to host surfaces for colonization during infection.

[14] The external ends of the pili adhere to a solid substrate, either the surface to which the bacterium is attached or to other bacteria.

Besides archaella, many archaea produce adhesive type 4 pili, which enable archaeal cells to adhere to different substrates.

[17] Menningococcal type IV pili bind DNA through the minor pilin ComP via an electropositive stripe that is predicted to be exposed on the filament's surface.

[20] It has been shown that some archaeal type IV pilins can exist in 4 different conformations, yielding two pili with dramatically different structures.

[26] Some of the genes involved are CsgA, CsgB, CsgC, CsgD, CsgE, CsgF, and CsgG.

Fimbriae are one of the primary mechanisms of virulence for E. coli, Bordetella pertussis, Staphylococcus and Streptococcus bacteria.

[31] Nonpathogenic strains of V. cholerae first evolved pili, allowing them to bind to human tissues and form microcolonies.

Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell. 4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.
Escherichia coli undergoing conjugation . Bacteria produce long extracellular appendages called sex pili, which connect two neighbouring cells and serve as a physical conduit for transfer of DNA. Adapted from [ 4 ]
Proposed conjugation mechanisms between donor and recipient cells in archaea (left) and bacteria (right). The schematic shows how ssDNA substrates are generated by the HerA-NurA machinery in the donor archaeal cells and by the plasmid-encoded relaxosome in bacteria. The figure is reproduced from [ 8 ]
Escherichia coli .
Type IV Pilus Twitching Motility 1. Pre-PilA is made in the cytoplasm and moves into the inner membrane. 2. Pre-PilA is inserted into the inner membrane. 3. PilD, a peptidase , removes a leader sequence, thus making the Pre-PilA shorter and into PilA, the main building-block protein of Pili. 4. PilF, a NTP -Binding protein that provides energy for Type IV Pili Assembly. 5. The secretin protein, PilQ, found on the outer membrane of the cell is necessary for the development/extension of the pilus. PilC is the first proteins to form the pilus and are responsible for overall attachment of the pilus. 6. Once the Type IV Pilus attaches or interacts with what it needs to, it begins to retract. This occurs with the PilT beginning to degrade the last parts of the PilA in the pilus. The mechanism of PilT is very similar to PilF. 7. Degradation of the pilus into the components to be utilized and synthesized into PilA again. [ 13 ]
Type IVa pilus machine architectural model
This figure depicts fimbriae adhesion. In this process the fimbriae of a bacterial cell (right) adhere to specific proteins, called receptors, found on the outer membrane of a host cell (left). They do this by a specific interaction between the receptors of the host cell and the perfectly matched adhesions found on the bacteria's fimbriae. This process of bacteria adhering to a host cell can result in the colonization of that host cell as more and more bacteria collect around it, and is integral to the continued survival of the bacteria, enabling them to infect tissues and entire organs. [ 25 ]