What is the Difference Between Fimbriae and Pili

The main difference between fimbriae and pili is that fimbriae are responsible for the attachment of the cell to its substrate whereas pili are responsible for the attachment and horizontal gene transfer during bacterial conjugation. 

Fimbriae and pili are two filamentous structures that extend from the surface of bacteria. They perform distinct functions in a bacterial cell. Furthermore, fimbriae occur in both Gram-positive and Gram-negative bacteria while pili only occur in Gram-negative bacteria. Moreover, several hundreds of fimbriae occur per cell while the number of pili per cell is less than 10. 

Key Areas Covered 

1. What are Fimbriae
     – Definition Structure, Role
2. What are Pili
     – Definition, Structure, Role
3. What are the Similarities Between Fimbriae and Pili
     – Outline of Common Features
4. What is the Difference Between Fimbriae and Pili
     – Comparison of Key Differences

Key Terms 

Attachment, Bacteria, Filamentous Structures, Fimbriae, Horizontal Gene Transfer, Pili 

What are Fimbriae 

Fimbriae are the short, filamentous structures found on the surface of both Gram-positive and Gram-negative bacteria. Also, they are thin structures whose main function is to attach the bacterium to its substrate. However, fimbriae do not involve in the locomotion of the bacterial cell as the flagella do. Also, they do not contain receptors. Moreover, fimbriae are important in the formation of bacterial clusters.  

Figure 1: Fimbriae

Shigella dysenteriae is an example of bacteria that form fimbriae, which help to attach the bacteria on to the surface of the small intestine.  

What are Pili 

Pili are somewhat long, thick, and rigid filamentous structures only found in the surface of the Gram-negative bacteria. They aid in the bacterial conjugation, which is a sexual reproduction method of bacteria by means of both bacterial attachment and transfer of genes between them. During bacterial conjugation, fewer genes from one bacterium are transferred to a second bacterium through pili. And, this type of gene transfer is known as horizontal gene transfer. In addition, the pili involved in sexual reproduction are called sex pili. On the other hand, some pili contain receptors for viruses. Furthermore, the Type IV pili can generate motile forces, producing a twitching motility. 

Figure 2: Bacterial Conjugation

Escherichia coli and Neisseria gonorrhoeae are two types of bacteria that contain pili. 

Similarities Between Fimbriae and Pili 

• Fimbriae and pili are two filamentous structures extend from the surface of the bacteria. 
• Both are made up of proteins. 
• Also, both are shorter than a flagellum. 
• Furthermore, both structures help in the attachment of the bacterial cell to various structures. 
• Gram-negative bacteria contain both fimbriae and pili. 
• Besides, both fimbriae and pili are antigenic; hence, they can evoke an immune response in the host. 

Difference Between Fimbriae and Pili 

Definition 

Fimbriae refer to tiny, bristle-like fibers arising from bacterial cells while pili refer to hair-like microfibers on the surface of bacteria. Thus, this is the main difference between fimbriae and pili.

Occurrence 

Moreover, fimbriae occur in both Gram-positive and Gram-negative bacteria while pili occur in Gram-negative bacteria. 

Number 

Around 200-400 fimbriae occur per bacterial cell while only 1-10 pili occur per bacterial cell. 

Made up of 

Another difference between fimbriae and pili is that fimbriae are made up of fimbrillin protein while pili are made up of pilin protein. 

Formation 

The genes responsible for the formation of fimbriae occur in the bacterial chromosome while the genes in the plasmids are responsible for the formation of pili. Hence, this is also a difference between fimbriae and pili.

Length 

One other difference between fimbriae and pili is their length; fimbriae are shorter while pili are longer. 

Diameter 

Furthermore, fimbriae are thin while pili are thicker. 

Rigidity 

Rigidity is another difference between fimbriae and pili. Fimbriae are less rigid while pili are more rigid. 

Function 

In addition, a difference between fimbriae and pili based on their function is that the fimbriae attach the bacterium to the substrate while the pili aid in bacterial conjugation. 

Conclusion 

Fimbriae are short, filamentous structures found in both Gram-positive and Gram-negative bacteria. They help to attach bacteria to the substrate. On the other hand, pili are somewhat long filamentous structures found in Gram-negative bacteria, aiding bacterial conjugation. Therefore, the main difference between fimbriae and pili is their function and occurrence.

Reference:

1. Janson, A, et al. “Fimbriae, Pili, Flagella and Bacterial Virulence.” Concepts in Bacterial Virulence. Contrib Microbiol. Basel, Karger, vol. 12, pp. 67–89. Available Here

Image Courtesy:

1. “Fimbriae” By AJC1 (CC BY-SA 2.0) via flickr
2. “Conjugation” By Adenosine – Own work (CC BY-SA 3.0) via Commons Wikimedia