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Fimbriae and pili are hair-like appendages on the bacterial cell wall, similar to flagella.They are more numerous and smaller than flagella. They play a role in bacterial conjugation, surface adhesion, and motility. They can be found in both Gram-positive and Gram-negative bacteria, but Gram-negative bacteria are more common.
Fimbriae and pili are cytoplasmic membrane extensions that are made up of the oligomeric protein pilin. Fimbriae and pili tips are sticky, and their structure correlates to the host cell’s glycoprotein receptors, making adhesion easier. Both fimbriae and pili are involved in adhesion, although pili are longer and have a smaller number of them.The term pilus refers to appendages that are involved in bacterial conjugation (F or sex pili).
Fimbriae
Fimbriae is another name of “short attachment pili.” They adhere to the host’s surface, allowing bacteria to colonise and infect the host. They might be found all over the place or concentrated towards the poles.
Fimbriae attach many aerobic bacteria to the surface of the culture medium. This enables them to colonise in close proximity to the air while simultaneously obtaining nutrients from the media. They can avoid flushing by using the attachment. A pellicle is a thin coating that forms on the surface of broth culture.
Pili or Conjugative Pili
Pili are the appendages that are involved in the conjugation process. Long conjugative pili is another name for them. They are longer than fimbriae and have a role in cell-to-cell adhesion during DNA transfer conjugation. Because they assist gene transfer and recombination in the bacterial cell, they are also known as “sex pili.” In bacteria, it’s a rudimentary kind of sexual reproduction.
The donor bacteria, also known as a’male bacterium’ with sex pilus, takes command and forms a mating bridge with the recipient cell. It then gives the recipient cell its DNA. Genes for the creation and transfer of pili are found in the DNA that is transmitted.It also transfers other genes, such as antibiotic-resistant genes.
Pili are divided into groups based on their bacteriophage susceptibility, such as F-pili, I-pili, and so on. Before multiplication, some bacteriophages bind to sex pili. A conjugative plasmid is responsible for their production. E.coli’s F-pili is encoded by the F-plasmid.
Type IV Pili
They are in charge of twitching motility. They cling to the surface and contract, causing the movement. This form of pili may be found in a lot of archaea, and they assist them to stick to different surfaces.
Bacterial strains that cause disease have fimbriae or pili. Pili boosts bacteria’s capacity to adhere to tissues and colonise by rapidly proliferating them. Bacteria that don’t have fimbriae or pili are usually non-pathogenic. Fimbriae, also known as pili, are crucial for virulence via attachment and also give resistance to white blood cell phagocytosis. The following are some instances of disease-causing bacteria that connect to the host tissue via fimbriae or pili:
Neisseria gonorrhoeae – Fimbriae adhesion to the urogenital tract epithelium
Streptococcus pyogenes – adhesion and resistance to phagocytosis by M-protein associated fimbriae
E.coli – Fimbriae adhere to the mucosal epithelium of the gastrointestinal tract.
Structure of Fimbriae and Pili
Pili and flagella differ in that they are thinner and shorter, less stiff and straight, and have a higher number of cells. They can be found at the bacterial cell’s poles or in a wide distribution across the cell’s intestinal surface. The pili has a diameter of around 250 mm and a length of 0.2-20 m.
Pili is genetically governed by plasmids, which range in number from one to ten. Fimbilin has a molecular weight of around 16,000 Daltons. The Long Conjugation Pili are hollow-cored helical tubes. The cylinder of repeating protein units makes up these structures. The plasmid of the bacteria administers the filamentous structure of these. Long conjugation pili have a diameter of 65-135 mm and are close to 20 mm.
Function of Fimbriae and Pili
Fimbriae and pili play a variety of roles; they are involved in a variety of actions. The bacteria with fimbriae are known as fimbriate bacteria. These fimbriae are adhesive in nature, allowing the entity to be attached to either a natural substrate or another entity. Agglutination of blood cells such as leukocytes, epithelial cells, erythrocytes, and others is also caused by the fimbriae.
As a thermolabile nonspecific agglutinogen, the fimbriae are equipped with antigenic characteristics. The metabolic processes are influenced by the fimbriae. Fimbriae-containing cells are referred to as Fim+ cells. In comparison to cells without fimbriae (Fim– cells), they exhibit a high rate of metabolic action. On a static liquid medium, they function as aggregation organelles, forming stellate aggregation.
Difference between Fimbriae and Pili
Pili | Fimbriae |
Pili are fine hair-like microfibers with a thick tube structure | fimbriae are small bristle-like fibres that emerge from the bacterial cells’ surface |
Pili are shorter than Fimbriae Pili are found in less than one to ten cells per cell. | Fimbriae are longer than pili. Fimbriae are found in roughly 200-400 cells per cell |
Fimbriae are found in both gram-positive and gram-negative bacteria | pili are only found in gram-negative bacteria. |
pilin protein makes up the pili. | Fibrillin protein makes up the fimbriae |
Fimbriae help in cell attachment to the surface | pili is critical in bacterial conjugation. |
Bacteria cell Wall
The bacterial cell envelope is a multilayered structure that protects germs from their unpredictable and frequently hostile environment.
The cell envelopes of most bacteria fall into one of two kinds.Gram-negative bacteria have a thin peptidoglycan cell wall, which is bordered by a lipopolysaccharide-containing outer membrane. Gram-positive bacteria do not have an outer membrane, but are surrounded by thicker peptidoglycan layers than gram-negative bacteria. Teichoic acids are lengthy anionic polymers that thread across these layers of peptidoglycan. The content and arrangement of these envelope layers, as well as new insights into cell envelope construction mechanisms, are discussed.
Conclusion
We can conclude that many bacteria have small protein tubes called fimbriae and pili that originate from the cytoplasmic membrane. Pili and fimbriae are both capable of adhering bacteria to surfaces. Fimbriae and pili are two appendages on the bacteria’s cell wall. The cytoplasmic membrane of certain bacteria produces these thin protein tubes, which protrude after passing through the peptidoglycan layer of the cell wall.
In addition to providing overall cell strength, the bacterial cell wall serves various other purposes. It also aids in the maintenance of cell shape, which is critical for the cell’s ability to grow, reproduce, collect nutrients, and move.
