Apilus (Latin for 'hair';pl.:pili) is a hair-likecell-surface appendage found on manybacteria andarchaea.[1] The termspilus andfimbria (Latin for 'fringe'; plural:fimbriae) can be used interchangeably, although some researchers reserve the termpilus for the appendage required forbacterial conjugation. All conjugative pili are primarily composed ofpilin –fibrous proteins, which areoligomeric.
Dozens of these structures can exist on the bacterial and archaeal surface. Some bacteria,viruses orbacteriophages attach toreceptors on pili at the start of theirreproductive cycle.
Pili areantigenic. They are also fragile and constantly replaced, sometimes with pili of different composition, resulting in altered antigenicity. Specific host responses to old pili structures are not effective on the new structure. Recombination between genes of some (but not all) pili code for variable (V) and constant (C) regions of the pili (similar toimmunoglobulin diversity). As the primary antigenic determinants, virulence factors and impunity factors on the cell surface of a number of species ofgram-negative and somegram-positive bacteria, includingEnterobacteriaceae,Pseudomonadaceae, andNeisseriaceae, there has been much interest in the study of pili as an organelle of adhesion and as a vaccine component. The first detailed study of pili was done by Brinton and co-workers who demonstrated the existence of two distinct phases within one bacterial strain: pileated (p+) and non-pileated)[2]
A few names are given to different types of pili by their function. The classification does not always overlap with the structural or evolutionary-based types, asconvergent evolution occurs.[3]
Conjugative pili allow for the transfer ofDNA between bacteria, in the process ofbacterial conjugation. They are sometimes called "sex pili", in analogy tosexual reproduction, because they allow for the exchange of genes via the formation of "mating pairs". Perhaps the most well-studied is the F-pilus ofEscherichia coli, encoded by theF sex factor.
A sex pilus is typically 6 to 7nm in diameter. During conjugation, a pilus emerging from the donor bacterium ensnares the recipient bacterium, draws it in close, and eventually triggers the formation of amating bridge, which establishes direct contact and the formation of a controlled pore that allows transfer of DNA from the donor to the recipient. Typically, the DNA transferred consists of the genes required to make and transfer pili (often encoded on aplasmid), and so is a kind ofselfish DNA; however, other pieces of DNA are often co-transferred and this can result in dissemination of genetic traits throughout a bacterial population, such asantibiotic resistance. The connection established by the F-pilus is extremely mechanically and thermochemically resistant thanks to the robust properties of the F-pilus, which ensures successful gene transfer in a variety of environments.[5] Not all bacteria can make conjugative pili, but conjugation can occur between bacteria of different species.[6][7]
Hyperthermophilicarchaea encode pili structurally similar to the bacterial conjugative pili.[8] However, unlike in bacteria, where conjugation apparatus typically mediates the transfer of mobile genetic elements, such as plasmids or transposons, the conjugative machinery of hyperthermophilic archaea, called Ced (Crenarchaeal system for exchange of DNA)[9] and Ted (Thermoproteales system for exchange of DNA),[8] appears to be responsible for the transfer of cellular DNA between members of the same species. 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.[8]
Fimbria (Latin for 'fringe',pl.:fimbriae) is a term used for a short pilus, anappendage that is used to attach the bacterium to a surface, sometimes also called an "attachment pilus"[10] oradhesive pilus. The term "fimbria" can refer to many different (structural) types of pilus. Indeed, many different types of pili have been used for adhesion, a case ofconvergent evolution.[3] TheGene Ontology system does not treat fimbriae as a distinct type of appendage, using the generic pilus (GO:0009289) type instead.
This appendage ranges from 3–10 nanometers in diameter and can be as much as several micrometers long. Fimbriae are used by bacteria to adhere to one another and to adhere to animal cells and some inanimate objects. A bacterium can have as many as 1,000 fimbriae. Fimbriae are only visible with the use of anelectron microscope. They may be straight or flexible.
Fimbriae possessadhesins which attach them to some sort of substratum so that the bacteria can withstandshear forces and obtain nutrients. For example,E. coli uses them to attach tomannose receptors.
Someaerobic bacteria form a very thin layer at the surface of abroth culture. This layer, called apellicle, 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 ofbiofilm, as they attach bacteria to host surfaces for colonization during infection. Fimbriae are either located at the poles of a cell or are evenly spread over its entire surface.
This term was also used in a lax sense to refer to all pili, by those who use "pilus" to specifically refer to sex pili.[11]
This sectionis missing information about mention of other types: variouschaperone-usher fimbriae built by T7SS,extracellular nucleation-precipitation pili built by T8SS (includingcurli), LPXTG including type 3 pilus (T3P; spaHIG)[3]. Please expand the section to include this information. Further details may exist on thetalk page.(December 2020) |
The Tra (transfer) family includes all known sex pili (as of 2010). They are related to thetype IV secretion system (T4SS).[3] They can be classified into the F-like type (after the F-pilus) and the P-like type. Like their secretion counterparts, the pilus injects material, DNA in this case, into another cell.[12]
Some pili, calledtype IV pili (T4P), generatemotile forces.[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. Then, when the pili contract, they pull the bacterium forward like a grappling hook. Movement produced by type IV pili is typically jerky, so it is calledtwitching motility, as opposed to other forms of bacterial motility such as that produced byflagella. However, some bacteria, for exampleMyxococcus xanthus, exhibitgliding motility. Bacterial type IV pili are similar in structure to the component proteins ofarchaella (archaeal flagella), and both are related to theType II secretion system (T2SS);[15] they are unified by the group ofType IV filament systems. Besides archaella, many archaea produce adhesive type 4 pili, which enable archaeal cells to adhere to different substrates. The N-terminal alpha-helical portions of the archaeal type 4 pilins and archaellins are homologous to the corresponding regions of bacterial T4P; however, the C-terminal beta-strand-rich domains appear to be unrelated in bacterial and archaeal pilins.[16]
Genetic transformation is the process by which a recipient bacterial cell takes up DNA from a neighboring cell and integrates this DNA into its genome byhomologous recombination. InNeisseria meningitidis (also called meningococcus), DNA transformation requires the presence of shortDNA uptake sequences (DUSs) which are 9-10 monomers residing incoding regions of the donor DNA. Specific recognition of DUSs is mediated by a type IVpilin.[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. ComP displays an exquisite binding preference for selective DUSs. The distribution of DUSs within theN. meningitides genome favors certain genes, suggesting that there is a bias for genes involved in genomic maintenance and repair.[18][19]
This family was originally identified as "type IV fimbriae" by their appearance under the microscope. This classification survived as it happens to correspond to a clade.[20] It has been shown that some archaeal type IV pilins can exist in 4 different conformations, yielding two pili with dramatically different structures.[21] Remarkably, the two pili were produced by the same secretion machinery. However, which of the two pili is formed appears to depend on the growth conditions, suggesting that the two pili are functionally distinct.[21]
Another type are called type 1 fimbriae.[22] They contain FimH adhesins at the "tips". Thechaperone-usher pathway is responsible for moving many types of fimbriae out of the cell, including type 1 fimbriae[23] and theP fimbriae.[24]
"Gram-negative bacteria assemblefunctional amyloid surface fibers calledcurli."[26] Curli are a type of fimbriae.[22] Curli are composed of proteins called curlins.[26] Some of the genes involved areCsgA,CsgB,CsgC,CsgD,CsgE,CsgF, andCsgG.[26]
Pili are responsible for virulence in the pathogenic strains of many bacteria, includingE. coli,Vibrio cholerae, and many strains ofStreptococcus.[27][28] This is because the presence of pili greatly enhances bacteria's ability to bind to body tissues, which then increases replication rates and ability to interact with the host organism.[27] If a species of bacteria has multiple strains but only some are pathogenic, it is likely that the pathogenic strains will have pili while the nonpathogenic strains do not.[29][30]
The development of attachment pili may then result in the development of further virulence traits. Fimbriae are one of the primary mechanisms ofvirulence forE. coli,Bordetella pertussis,Staphylococcus andStreptococcus bacteria. Their presence greatly enhances the bacteria's ability to attach to the host and cause disease.[31] Nonpathogenic strains ofV. cholerae first evolved pili, allowing them to bind to human tissues and formmicrocolonies.[27][30] These pili then served as binding sites for thelysogenic bacteriophage that carries the disease-causingtoxin.[27][30] The gene for this toxin, once incorporated into the bacterium's genome, is expressed when the gene coding for the pilus is expressed (hence the name "toxin mediated pilus").[27]
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