Bacterial Fimbriae or Pili : Organ of Adhesion and Gene Transfer

Bacterial fimbriae or pili are short, fine, hair-like appendages that are thinner than flagella and not involved in motility(singular fimbria or pilus).

Pili are made up of protein called pilin.

Bacterial fimbriae are antigenic; however, the antibodies against fimbrial antigens are not protective.

Fimbriae are very small, measuring 0.5μm long and10 nm in thickness. A bacterium can have as many as 1,000 fimbriae.

Functions of Bacterial fimbriae

• Fimbriae are called the organ of adhesion. This property enhances the virulence of bacteria.

• Certain fimbriae called sex pili also help in bacterial gene transfer.

• Fimbriae are not related to motility and can be found both in motile as well as in non-motile organisms.

Types Bacterial fimbriae

Different types of fimbriae are as follows (Fig. 

2.19A)-

• Common pili: There are six types of common pili depending on their morphology, number per cell, adhesive properties and antigenic nature. 

• Sex or F (fertility) pill: 

• Sex pili are special type of large fimbriae present 1-10 per cell (e.g. as found in gonococcus). 

• Sex pili help in bacterial conjugation. 

• They are present in male bacterium; form the conjugation tube in male bacterium and thereby help in transfer of genetic material from male bacteria to the female bacteria via the conjugation tube. 

• Col I (colicin) pili.

 

Detection of Bacterial fimbriae

Electron microscopy is the only method for direct demonstration of fimbriae. However, there are some indirect methods to know the presence of fimbriae such as:

 

Hemagglutination:

• Many fimbriated bacteria (e.g. Escherichia coli, Klebsiella) strongly agglutinate with red blood cells of guinea pigs, fowl, horses and pigs. 

• This property of hemagglutination is a simple method for detecting the presence of fimbriae.  

• In some bacteria, the hemagglutination may be specifically inhibited by D-mannose.
  

 

Surface pellicle:

• Some aerobic fimbriated bacteria form a thin layer at the surface of a broth culture called as pellicle.  

• The pellicle consists of many aerobic bacteria that adhere to the surface by their fimbriae (Fig. 2.198).

• Bacterial Growth Curve 
• Bacterial Flagella
• Bacterial Capsule