Fimbriae and Pili

Fimbriae and pili are bacterial appendages. The cytoplasmic membrane of some bacteria produces these thin protein tubes, which protrude after passing through the cell wall’s peptidoglycan layer. Pili are long hair-like tubular microfibers present on the surface of bacteria, whereas fimbriae are bristle-like small fibres that appear on the bacterial surface. Only gram-negative bacteria have pili, but both gram-negative and gram-positive bacteria have fimbriae.

Fimbriae and pili can both attach to the bacterial surface. In comparison to fimbriae, pili are usually longer and fewer in number. They are found in virtually all gram-negative bacteria, but not as frequently in gram-positive bacteria.

Structure of the Fimbriae and Pili

Pili and flagella differ in that they are thinner and shorter, less stiff and straight, and contain a significant number of cells. They can be found at the bacterial cell’s poles or all over the intestinal surface of the cell. The pili are around 250 mm in diameter and 0.2-20 m in length. Pili is genetically governed by plasmids, which range in number from 1 to 10. Fimbilin has a molecular weight of about 16,000 Daltons. Long conjugation pili are hollow-cored helical tubes. The cylinder of repeating protein units is what these structures are called. The filamentous structure of these is governed by the bacterium’s plasmid. Long conjugation pili have a diameter of 65-135 mm and a length of close to 20 m, which is greater than fimbriae.

Fimbriae

Fimbriae assist bacteria to colonise cells or surfaces in the environment and resist flushing. The pili or fimbriae protein in the fimbriae is made up of close to 163 amino acids and is 100 percent protein. Having the shaft Pilin is made up of adhesive tip structures that have the same shape as certain glycolipid receptors on the host cell.

Pili

Pili can be classified into two types: long conjugation pili and short attachment pili. Long conjugation pili (F or sex pilli) are long and infrequent. Conjugation is made easier with the help of these pili. The transfer of DNA from a male bacterium to a female bacterium allows for genetic recombination in gram-negative bacteria.

Fimbriae and Pili: Their Roles

On the surface of the bacterial cell, they are hair-like filamentous appendages. The following are the primary traits: 

• 1000 fimbriae can be found on the surface of a bacterial cell. Only an electron microscope is capable of detecting them.

• They are smaller than flagella and have a shorter lifespan. It measures 0.5 m in length and 10 nm in thickness.

• The cytoplasmic membrane is where they begin. Both motile and non-motile cells have them.

• They’re formed up of helical pilin subunits that combine to form slender tubes.

• Fimbriae bacteria are antigenic, and antigens from various genera can coexist.

• Fimbriae aid bacteria in adhering to host tissue and obtaining nourishment in favourable microenvironments. Bacteria, for example, cling to the surface of liquid culture media via fimbriae, generating a pellicle.

• Sex pili are a form of fimbriae found on the’male bacteria.’ They are longer and fewer in number (up to ten in per cell) than fimbriae. They help construct a conjugation tube for the DNA transfer from the male donor to the female recipient by attaching to the ‘female bacteria.’

• Twitching motility is caused by type IV pili.

Fimbriae vs. Pili: What’s the difference?

The following are the differences between the fimbriae and the pili:

1. Pili are fine hair-like microfibers with a thick tube structure, whereas fimbriae are small bristle-like fibres that emerge from the bacterial cell surface.

2. Fimbriae are shorter than pils.

3. Fimbriae occur 200-400 times per cell, while pili occur one to ten times each cell.

4. Both gram-negative and positive bacteria have fimbriae, however only gram-negative bacteria have pili.

5. Fibrillin protein is found in the fimbriae, while pilin protein is found in the pili.

6. Compared to the pili, the fimbriae are less stiff.

7. The bacterial genes in the nucleoid area control the creation of fimbriae, whereas plasmid genes control the formation of pili.

8. Fimbriae help cells adhere to surfaces, while pili help bacteria conjugate.

Conclusion 

Hence, Fimbriae and pili, analogous to flagella, are hair-like appendages on the bacterial cell wall. They’re smaller and more numerous than flagella. Bacterial conjugation, surface adhesion, and motility are all affected by them. Gram-positive and Gram-negative bacteria both include them, however Gram-negative bacteria have a higher prevalence. 

Fimbriae and pili are oligomeric proteins that are extensions of the cytoplasmic membrane. The adhesive ends of fimbriae and pili correspond to the host cell’s glycoprotein receptors, making attachment easier. Both fimbriae and pili play a role in adhesion, however pili are longer and have a smaller number of them. Appendages involved in bacterial conjugation are referred to as pilus (F or sex pili).