In microbiology, fimbriae are filamentous, protein structures that bacteria have and that differ from flagella by their diameter (less than 8 nm in most cases) and by not having a helical structure. The term is also used in other sciences to define a terminal portion or border of an organ divided into very fine segments.
These anatomical structures do not fulfill mobility functions, are highly variable and apparently are not vital for the bacteria that possess them. This means that if by some physical, chemical or biological factor the bacteria lose their fimbriae, it would not imply the death of the cell or interruptions of their biological cycles.
Fimbriae are characteristic appendages of Gram-negative bacteria (those that do not react to Gram stain).Very few Gram-positive bacteria (those bacteria that stain blue or violet on the Gram stain) are known to possess these structures, however they have been observed in streptococci, corynebacteria and species of the genus Actynomycetes .
They have straight and rigid filamentous shapes, are shorter and thinner (3-10 nm in diameter) than flagella. Most are made up of a single type of highly hydrophobic globular protein called pilin.
They are about 17-25 kilodaltons (kDa) and their subunits are arranged in a helical matrix that leaves a small central hole.
The fimbriae are implanted at the level of the cytoplasmic membrane of the bacteria. Their number varies according to the species and between individuals of the same population, species that can present from a few fimbriae to several hundred or thousands per individual have even been observed.
Fimbriae can be seen around the entire cell perimeter, including the ends of the cell, also called polar regions.
Several types of fimbria are known, but generally two main types are referred to: adhesive and sexual.
They are villi of the cell between 4 and 7 nm in diameter, their number and distribution depend on the species.
They are similar in shape and size, about 1 to 10 per cell. They are wider than adhesive, approximately 9 to 10 nm in diameter. They are genetically determined by sexual factors or conjugative plasmids.
The role of fimbriae in many types of bacteria is not known with certainty. Even so, it seems that in some groups they favor fixation or adherence to different substrates, allow the formation of biofilms that also favor adhesion, aggregation, coaggregation and fixation to liquid surfaces in which they form veils.
The functionality of these fimbriae is adherence to specific and superficial receptors. This is of utmost importance, since adherence to living or inert substrates plays a fundamental role in the colonization of the different habitats or the host depending on the species.
The adhesive peculiarity (adhesin function) is not due to the pilin protein that mainly constitutes the fimbria, but to a glycoprotein called lectin, located at the distal end of the appendix.
This protein is capable of binding with high affinity to polysaccharide side chains present in the cytoplasmic membrane of the cells to which it adheres.
They are necessary for bacterial conjugation, that is, for the exchange of genetic information between a donor and a recipient cell.
Fimbriae are protein in nature. Some authors mention that the protein that compose them is a protein subunit called fimbrilin (FimA), of 17 to 20 kDa and is encoded by the fimA gene.
However, others refer to pilin, a protein that is characterized by the presence of a very short leader peptide, with 6 to 7 residues, followed by a methylated N-terminal phenylalanine residue and by a highly conserved sequence of approximately 24 hydrophobic residues, of the NMePhe pilin type.
The binding of bacteria (with adhesive fimbriae) to specific receptors on the human cell is the first step for the establishment of infections in the body; such as the formation of dental plaque, by coaggregation of individuals of different species on the tooth and tissue colonization factors, by Neisseria gonorrhoeae and uropathogenic strains of Escherichia coli .
The role of fimbriae as a virulence factor in Gram-negative bacteria has been extensively studied in Neisseria gonorrhoeae and N. meningitidis bacteria .
These pathogenic species produce similar fimbriae from a structural and antigenic point of view. Virulent varieties of N. gonorrhoeae show surface fimbriae of 16.5-21.5 kDa and are capable of firmly adhering to cells of the mucosa of the genital tract.
Although Gram-positive bacteria with fimbriae are rare, facultative bacilli of this group have been found in the oral cavity. They show two types of fimbriae:
- Type 1, mediating adherence to the tooth surface through interaction with proline-rich acid proteins in saliva.
- Type 2 fimbriae, which mediate the adherence of bacteria to oral streptococci.
Gram-positive Actynomycetes species bind differently than Gram-negative ones. These are covalently attached to the peptidoglycan layer of the cell wall.
The ability of buccal Actynomycetes species to adhere to mucosal cells and congregate with cariogenic streptococci facilitates the formation of a biofilm and the initiation of dental plaque.
Fimbriae or pili?
Both terms have been used synonymously by some authors, while for others they are not identical, and they only call adhesive fimbriae, while they call sexual fimbriae pili (see characteristics).
Even sexual fimbriae can be found in texts and research as sexual hair, sexual hair, or sexual pili. Any term used is valid and its use depends on the microbiological school of training.
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