Fimbriae and Pili

Fimbriae and pili, both are appendages on the cell wall of the bacteria. These thin protein tubes originate from the cytoplasmic membrane of several bacteria, protruding out after it penetrates the peptidoglycan layer of the cell wall. While the fimbriae are bristle-like short fibers occurring on the bacterial surface, Pili are long hair-like tubular microfibers found on the surface of bacteria. The pili are found in some gram negative bacteria only, whereas the fimbriae are found in both the gram negative and gram positive bacteria.

Both fimbriae and pili are capable of sticking to the bacterial surface. Pili are usually longer and less in number compared to the fimbriae. Virtually, they are found in all the gram-negative bacteria but not as much in gram-positive bacteria. Fimbriae enable the bacteria in colonization of environmental cells or surfaces and help in resisting flushing.

The fimbriae comprises 100% protein known as pilin or fimbrilin consisting of close to 163 amino acids. The shaft having Pilin consists of adhesive tip structures towards its terminal with a shape that corresponds to that of particular glycolipid receptors on the cell of the host. Pili are of two types – the long conjugation pili and the short attachment pili.

The F or sex pilli (Long conjugation pili) are long and a few in number. These conjugation pilus facilitates conjugation. Genetic recombination is enabled in gram negative bacteria through the transfer of DNA from the male bacterium to a female bacterium.

Fimbriae and Pili – Structure

Pili and flagella are different in being thinner and shorter, less rigid and straight and are large in number. Their occurrence is either at the poles of a bacterial cell or are even in distribution over the cell’s enteric surface. The pili have a diameter of close to 250 Å and lengthwise they are 0.2-20 µm.

Plasmids genetically govern Pili and vary in the number, ranging between 1 and 10. The molecular weight of Fimbilin is close to 16,000 Daltons. The Long conjugation pili are helical tubes having a hollow core. These structures are the cylinder of repeating protein units. The filamentous structure of these are administered by the plasmid of the bacterium. The diameter of the Long conjugation pili is 65-135 Å and close to 20 µm lengthwise, which is greater in comparison to fimbriae.

Fimbriae and Pili – Function

The role of fimbriae and pili are not limited, they are involved in many activities. The Fimbriate bacteria are the bacterium having fimbriae. These fimbriae are adhesive in nature attaching the entity to the substrate that naturally occurs or to any other entity. Additionally, the fimbriae cause agglutination of the blood cells such as leukocytes, epithelial cells, erythrocytes, etc.

The fimbriae are armed with antigenic traits as they serve as thermolabile nonspecific agglutinogen. The fimbriae affects the metabolic processes. The cells that contain fimbriae are referred to as Fim+ cells. They have a high rate of metabolic action compared to the cells which do not contain fimbriae (Fim– cells). They operate as aggregation organelles, forming stellate aggregation on a static liquid medium.

These pili connect two cells due to their hollow cores serving as conjugation tubes. The female cell is identified by the tip of the pilus from where the genetic content of the donor cell transfers to the female cell.

Difference between Fimbriae and Pili

The fimbriae differ from the pili in the following ways:

  • Pili are fine hair like microfibers having pilin – a thick tubular structure while the fimbriae are tiny bristle-like fibers emerging from the surface of the bacterial cells
  • Pili are longer than fimbriae
  • Occurrence of the fimbriae in each cell is about 200-400 while the occurrence of pili are lesser than one to ten every cell
  • Fimbriae are found in the both the gram negative and positive bacteria both, pili are present in the gram negative bacteria only
  • The fimbriae are composed of fibrillin protein while the pilin protein makes up the pili
  • The fimbriae are less rigid compared to the pili
  • The formation of fimbriae is administered by the bacterial genes in the nucleoid area while the pili is administered by the plasmid genes
  • Fimbriae plays a role in attaching cells to the surface while pili are critical in bacterial conjugation

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