A pilus is an adhesin that allows bacteria to attach to cells, helping to colonize the host. Gram-negative bacteria in particular are typically equipped with a pilus or multiple pili. The presence of pili can significantly increase the pathogenicity of a pathogen and is considered a virulence factor.
What is a pilus?
The pilus, or fimbriae, is a component of prokaryotes. It is a filamentous protein that lies outside cells as a cell extension and belongs to the adhesins. Adhesins are surface components of bacteria that enable the microorganism to attach to biological structures of its host. Thus, adhesins correspond to so-called virulence factors, as they are a basic prerequisite for colonization. In a narrower sense, these are bacterially produced factors that enable the bacterium to attach to structures or cells of the host. For example, in adhesion by adhesins, the bacterium does not simply wash off. The expression of adhesins varies in some bacterial species depending on environmental conditions. Different types of pili in the sense of an adhesin exist. The cell appendages differ in protein, length, and diameter. Their function may also be subject to slight variations from bacterium to bacterium. The length can vary from 0.1 to 20 micrometers. The diameter of the pili ranges from two to about 20 micrometers. In addition to adhesion to solid-liquid or gas-liquid interfaces, pili also allow bacteria to attach to other bacteria and fix to animal epithelial cells. In addition, the appendages are sometimes involved in bacterial DNA exchange. Unlike bacterial flagella, pili are short and rigid. They project intracellularly and extracellularly.
Significance and function
Pili are most typical of bacteria with Gram-negative staining behavior. These types of bacteria possess one to four of these cell processes, depending on the individual. Pili allow water-colonizing bacteria to attach to solids and in this way remain in the same place in liquid media. The medium washes new nutrients past them and washes away the breakdown products of their metabolism. Moreover, by attaching themselves between air and liquid via their pilus or pili, bacteria can extract nutrients from the liquid medium and draw oxygen from the air at the same time. The attachment of a dense bacterial layer to the surface of liquid media is also known as a scum layer. Some types of pili are used for horizontal gene transfer. These pili are called F-pili or sex pili. They are relatively thick and hollow appendages that have only donor bacteria or donors. The recipient individual is called the acceptor or recipient and the pilus is degraded after contact with it. This automatically decreases the cell distance between acceptor and donor. Outside the pilus, a plasma bridge can thus be established at an extremely small distance, which serves to transfer genetic information. Resistance (R) factors and fertility (F) factors are brought to exchange via the plasma bridge. As part of this process, the double strand of DNA is unwound into single strands, after which portions of the strand migrate from the donor to the recipient. This is followed by the dissolution of the plasma bridge. The bacteria then complete the single strand to form the double strand. Still other bacteria possess so-called type IV pili, which allow them to move on a solid surface. Their pili consist of PilA protein copies and are not hollow. They are usually located at both poles of a bacterium equipped with them. Other types of pilus include the Hrp pilus, found primarily in plant pathogens, the type-I pilus, the type-IV pilus, and the Pap pilus. The commonality of the pili lies in their construction protein, which corresponds to the so-called pilin. In addition, most pili are tubular in shape.
Diseases and disorders
In many bacteria, pathogenicity is enhanced by being equipped with pili. That is, in many cases, a bacterium with pilus is more pathogenic than a bacterium without pilus. In this case, pili not only assume the role of an adhesin, but also the role of a virulence factor. In this context, virulence factors are all properties of a microorganism that make its pathogenic effect and thus its virulence determinable.In addition to pili for attachment to cells, instruments for penetration into cells and mechanisms for cell destruction play a role for the virulence factors of a particular microorganism. Virulence factors are often structural elements like the pili, but can also correspond to metabolites of the microorganism. In many genera of bacteria, the pili are the critical structural element for colonizing a host organism. If a bacterium cannot attach to its host, it is generally less able to invade the host organism. As long as the bacterium does not invade, it cannot multiply in the host and thus cannot cause a pathological condition in the host’s body. In most cases, pili react in specific or nonspecific ways with individual receptors within the target cell membrane to anchor the bacterium within it. Specialized pili for DNA exchange between bacteria also broadly promote the aggressiveness of a pathogenic agent. The faster a pathogen can spread throughout the host’s body, the more aggressive and rapid the resulting infection.
