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What are Pili ?

The term Pili is plural for pilus which is a hair-like appendage that is found to exist on the surfaces of many bacteria. It is often confused with fimbriae which are often used in place of the term Pili. However, certain researchers reserve the term ‘pili’ to correlate them with bacterial conjugation. 

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The pili are composed using oligomeric pilin proteins. Dozens of pili might be present on any bacteria. Some of the bacteriophages, as well as bacterial viruses, tend to attach themselves to the pili receptors in order to start their very own cycle of reproduction. Pili can be considered as an antigenic structure. These structures are also pretty fragile and can be replaced constantly, sometimes with the pili of other compositions. This results in altered antigenicity. 

Classification by Function of Pili

There can be a few different names given to the structure of pili based on the functions that they have. However, the classification doesn’t really overlap all the evolutionary and structural types. The pili can be classified into two different types on the basis of their function.

  • Conjugative Pili 

The conjugative pili have the function of transferring the DNA particles in the bacteria. This process is known as bacterial conjugation. These types of pili are also known as ‘sex pili’ due to the analogy that they have in sexual reproduction. These pili allow the genes to exchange through the creation of mating pairs. F-pilus which is denoted by the F sex factor is one of the common examples of conjugative pili. The size of conjugative pili reaches up to seven nm in diameter. During the process of bacterial conjugation, the pilus that emerges from the bacterium completely overlaps the bacterium recipient. It draws the recipient closer and creates the mating bridge. This results in the establishment of direct contact between the bacteria and there is a controlled pore that is created to facilitate the transferring of the DNA particle from the donor bacterium to the recipient bacterium.

In general, the DNA that is transferred contains all the genes that are required in order to create and transfer the pili. However, there are separate pieces of DNA that are co-transferred and cause the dissemination of different genetic traits throughout the entire population of the bacteria. Antibiotic resistance is an example of one such dissemination caused due to the involvement of other DNA particles. 

  • Fimbriae

Fimbriae are the plural term for fimbria which is used to denote the shorter version of pilus that can help in attaching the bacterium to any surface. It is also known as the attachment pilus in certain cases. The fimbriae are useful in the creation of biofilm as they will attach the bacteria to different host surfaces to facilitate colonization during bacterial infection. The location of the fimbriae is in the poles of the cell or they might be spread evenly over the entire surface of the bacterial cell. Certain types of mutant bacteria don’t possess the fimbriae and hence cannot adhere to the target surfaces that they usually do. Hence, these mutant bacteria are not able to cause any diseases. 

How Can Pili Help Bacteria?

The correlation of pili bacteria can be found from the function that the pili have. The pili tend to extend from the surfaces of different bacterial cells. The Pili are non-helical, hollow, and filamentous appendages of bacteria. One of the most important things to keep in mind is that pili tend to be shorter than the flagella and hence are not really involved in the motility process. The main function of the pili is to ensure that the bacterial cells are properly attached to the host surface in which they are living. The pili are created with a special protein type known as pilin. The true kinds of pili are always present in gram-negative bacteria. Another one of the main functions of pili includes the facilitation of the mating process that happens between the bacterial cells. This process is known as conjugation. 

The Function of Pili and Fimbriae 

There are many different functions of pili and fimbriae as provided below:

The bacteria that contain fimbriae are known as fimbriate bacteria. There are certain adhesive properties of the fimbriae that allow them to attach the bacterial cells to the natural substrate or the host. The fimbriae also have an important role to play in agglutinating the different blood cells such as leukocytes, erythrocytes, as well as epithelial cells. The attachment pili are also equipped with certain antigenic properties because they tend to act as thermolabile nonspecific agglutinogen. They can also affect the activity of metabolism. Sex pili are responsible for creating contact between two bacterial cells and facilitating the process of conjug


Pilus is a component of the bacterial cells that help them to attach to the host organism and facilitate the process of mating in order to share the DNA particles between the cells. Hence, they have an important role to play in the causing of infections on hosts. Students can learn what is pili from this article.

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FAQs on Pili

1. What is pili?

Pili or pilus which is the singular for the former term can be defined as hair-like appendages that are attached to the cells of bacteria. The pili are composed with the help of oligomeric pilin proteins. There might be dozens of pili present in the bacteria. The term 'pili' is used in order to correlate the structure with bacterial conjugation which is their main function. Some of the bacterial viruses as well as bacteriophages tend to attach themselves with the pili of bacterial cells in order to create their very own cycle of reproduction. Pili helps in facilitating the process known as bacterial conjugation or the mating of bacterial cells.

2. What is the difference between pili and fimbriae?

While both fimbriae and pili are protein-filled structures that are found on the surface of the different bacterial cells, there are certain differences between the two. In most cases, the terms are used interchangeably. However, according to certain researchers, the term pilus or pili is strictly used in order to relate to bacterial conjugation. Pili or sex pili help in facilitating the mating process between two bacterial cells which results in the transfer of DNA particles from the donor bacterium to the recipient bacterium. The fimbriae on the other hand are shorter versions of the pili that help in attaching the bacteria to the host surfaces.

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