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Fimbriae and pili Structure, Functions, Differences.

Fimbriae and pili are interchangeable words employed to indicate short, hair-like structures on the outsides of procaryotic cells. Same as flagella, they...

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This article writter by SouravBio on January 11, 2021

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Fimbriae and pili
Fimbriae and pili

Fimbriae and pili are interchangeable words employed to indicate short, hair-like structures on the outsides of procaryotic cells. Same as flagella, they are made of protein. 

Fimbriae are smaller and harder as compared to flagella, and lightly smaller in diameter. They emerge from the bacterial cell surface and involved in different functions such as adhesion, attachment, and assisting in genetic change.


  • The fimbriae or fimbria (Singular) are bristle-like short fibers occurring on the surface of several gram-positive and gram-negative bacteria.
  • It helps in attachment of bacterial cells on the surface of host cell and on some inanimate objects. For example, E. coli utilizes them to get attached to the mannose receptors.
  • It is also known as “attachment pilus” and mainly found on Gram-negative and Gram-positive bacteria.
  • A single bacteria may contain 1,000 fimbriae.
  • They can be visualized by using only an electron microscope. 
  • Fimbriae enables the formation of pellicles or biofilms. Pellicles is a thin sheet of cell which is formed on the surface of liquid.
  • Curli is an example of Fimbriae. It is a functional amyloid surface fibers of gram-negative bacteria and is made of curlins protein. 
  • Fimbriae is an important virulence factor of different bacteria such as E. coli, Bordetella pertussis, Staphylococcus, and Streptococcus.

Structure of Fimbriae

  • These are composed of a protein, known as fibrillin.
  • Fimbriae are thinner and shorter as compared to flagella.
  • Fimbriae has a molecular weight of 16,000 Daltons.
  • They are 3–10 nanometers in diameter and are 0.03 to 0.14 micrometers in length.

Function of Fimbriae

  • Helps in attachment of bacterial cell on a solid surface or host cell surface.


  • Pili or Pillus (Plural) are hair-like appendages which are seen on the surface of many gram-negative bacteria and archaea.
  • A special type of pili known as sex pili helps in bacterial conjugation.
  • They are antigenic and also fragile and constantly replaced.
  • These are visible under electron microscope.
  • The length of a pill is ranges from 0.5- 2 um and the diameter ranges from 5-7
  • The total number of pili in a single cell is limited, it is ranges from 1 – 10 per cell.
  • These are only found in gram-negative bacteria.
  • There are two types of Pili such as short attachment pili and long conjugation pili. The short one is known as fimbriae and the long one is known as “F” or sex pili.

Structure of Pili

  • The length of a pill is ranges from 0.5- 2 um and the diameter ranges from 5-7.
  • These are hollow and tubular structures.
  • Pili is made of a protein known as pilin, which is an oligomeric protein.

Function of Pili

  • Helps in Bacterial conjugation. It involves gene transfer and attachment.

Type IV pili

  • Some of the pili is known as type IV pili (T4P), it generate motile forces.
  • The exterior ends of the pili adhere to a solid substrate, either the solid surface to which the bacterium is appended or to another bacteria. Then, during the pili contract, they forced the bacterium ahead same as a grappling hook. Action generated by type IV pili is typically jerky, so it is termed as twitching motility, as exposed to other sorts of bacterial motility for example that generated by flagella. 
  • The structure of type IV pili is similar to the component flagellins of archaella (archaeal flagella) and both of them are associated to the Type II secretion system (T2SS).

Mechanism of Type IV Pilus Twitching Motility

  1. Pre-PilA is occurred within the cytoplasm and then passes through the inner membrane.
  2. Pre-PilA is entered within the inner membrane.
  3. Next, a peptidase called PilD, separates a leader sequence, therefore producing the Pre-PilA shorter and into PilA, the central building-block protein of Pili.
  4. A NTP-Binding protein known as PilF supply energy for Type IV Pili Assembly. 
  5.  PilQ, a secretin protein, located on the outer membrane of the cell is required for the construction/enlargement of the pilus. PilC is the primary protein to create the pilus and is accountable for the overall affection of the pilus. 
  6. Once the Type IV Pilus connects or associates with what it demands to, it starts to retreat. This happens with the PilT working to diminish the end parts of the PilA in the pilus. The processes of PilT is very related to PilF. 
  7. Degradation of the pilus within the parts to be used and synthesized into PilA repeatedly.
Fimbriae and Pilli: Type IV Pilus Twitching Motility Steps
Type IV Pilus Twitching Motility Steps| Image modified from Wiki by

Differences Between Fimbriae and Pili

DefineFimbriae are tiny bristle-like fibers arising from the surface of bacterial cells.Pili are hair like microfibers that are thick tubular structure made up of pilin.
DiameterThese are comparatively thinner in diameter.These are thicker than fimbriae. The diameter is ranges from 5-7.
LengthThe length id ranges from 0.03 to 0.14 um.Length is ranges from 0.5 – 2um.
NumberThe number of Fimbriae per cell ranges from 200 -400.Number of pili per cell ranges from 1-10.
Composed of Fimbriae is made up of Fimbrillin protein.Pili is made up of Pilin protein.
FunctionHelps in attachment of bacterial cell to the host cell surface or solid surface.The sex pili Involve in bacterial conjugation.
LocationFimbriae are mainly found in both gram positive and gram negative bacteria.Pili is mainly found in gram negative bacteria and in some archae.
FormationIs governed by bacterial genes in the nucleoid region.Is governed by plasmid genes.
RigidityFimbriae are Less rigid as compared to pili.Pili are More rigid than fimbriae.
MotilityDon’t involve in Motility.The type IV pili shows twitching type of motility.
ReceptorsNo receptors.Functions as receptor for certain viruses.
StructureThese are solid structure.These are hollow tubular structure.
Distribution of Cell Surface.These are evenly distributed on the entire cell surface.These are randomly distributed on surface of cell.
ExamplesSalmonella typhimurium, Shigella dysenteriae.
Shigella dysenteriae
uses its fimbriae to attach to the intestine and then produces a toxin that causes diarrhea.
Escherichia coli, Neisseria gonorrhoeae.
Neisseria gonorrhoeae
, the cause of gonorrhea, uses pili to attach to the urogenital and cervical epithelium when it causes disease.



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