Fimbriae and pili are hair-like appendages present on the bacterial cell wall similar to flagella. They are shorter than flagella and more in number. They are involved in the bacterial conjugation, attachment to the surface and motility. They are present in both Gram-positive and Gram-negative bacteria but more prevalent in Gram-negative bacteria.
Fimbriae and pili are extensions of the cytoplasmic membrane and are made up of an oligomeric protein known as pilin. The tips of fimbriae and pili are adhesive and the structure corresponds to the glycoprotein receptors of the host cell, which facilitates the attachment. Fimbriae and pili both are involved in adherence but pili are longer and fewer in number than fimbriae. The term pilus is generally used for appendages involved in bacterial conjugation (F or sex pili).
Structure and Function of Fimbriae and Pili
They are hair-like, filamentous, surface appendages present on the bacterial cell. The main characteristics are:
- A bacterial cell may have 1000 fimbriae present on the surface. They can be seen only under the electron microscope.
- They are shorter than flagella. The length is ~0.5 𝝁m and thickness is ~10 nm.
- They originate from the cytoplasmic membrane. They are present on motile as well as non-motile cells.
- They are composed of helically arranged subunits of a protein called pilin, which aggregates to form slender tubes.
- Fimbriae are antigenic and bacteria of different genera may have the same antigen.
- Fimbriae and pili can be classified based on their structure and functions.
- Fimbriae help bacteria in adherence to the host tissue and favourable microenvironments to draw nutrition. E.g. bacteria get attached to the surface by fimbriae of liquid culture media forming a pellicle.
- Sex pili are a special type of fimbriae, present on the ‘male bacterium’ and are longer and fewer (up to 10 in each cell) in number than fimbriae. They are involved in the attachment with the ‘female bacterium’ by forming a conjugation tube for the DNA transfer from the male donor to the female recipient.
- Type IV pili are responsible for twitching motility.
Fimbriae are also called “short attachment pili”. They attach to the host surface and help bacteria colonise and cause infection. They are present on the overall surface or concentrated towards the poles.
Many aerobic bacteria get attached to the surface of the culture medium by fimbriae. This allows them to colonise near the air and also take nutrients from the medium. The attachment helps them to avoid flushing. The thin film at the surface of broth culture is known as a pellicle.
Pili or Conjugative Pili
Pili are generally referred to as the appendages, which are involved in the conjugation. They are also known as long conjugative pili. They are longer than fimbriae and involved in the cell to cell attachment during conjugation for DNA transfer. They are also termed as “sex pili” as they facilitate gene transfer and recombination in the bacterial cell. It is a primitive mode of sexual reproduction in bacteria.
The donor bacterium or ‘male bacterium’ with sex pilus takes in charge and establishes direct contact with the recipient cell by forming a mating bridge. It then transfers its DNA to the recipient cell. The DNA, which gets transferred contains genes for the formation and transfer of pili. Other genes such as antibiotic-resistant genes also get transferred along with it.
Pili are classified based on their susceptibility to bacteriophages, e.g. F-pili, I-pili, etc. Some bacteriophages attach to sex pili before multiplication. They are encoded by a conjugative plasmid. F-pili of E.coli is encoded by F-plasmid.
Type IV Pili
They are responsible for the twitching motility. They adhere to the surface and bring about the movement by contraction. Many archaea contain this type of pili, which help them to adhere to various surfaces.
The disease-causing strains of bacteria possess fimbriae or pili. Pili increases the bacterial ability to adhere to tissues and colonise by multiplying rapidly. Bacteria without fimbriae or pili are generally non-pathogenic. Fimbriae or pili are responsible for virulence through the attachment and also provide resistance to the phagocytosis by white blood cells. Some examples of disease-causing bacteria, which attach to the host tissue by fimbriae or pili are the following:
- Neisseria gonorrhoeae – adherence to the urogenital tract epithelium by fimbriae.
- Streptococcus pyogenes – adherence and resistance to phagocytosis by M-protein associated fimbriae.
- E.coli – adherence to the gastrointestinal tract mucosal epithelium by fimbriae.
This was in brief about the fimbriae and pili. Test your understanding with MCQs on Monera, only at BYJU’S.