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Pili bacteria

Pili bacteria are those bacteria that have the small hair-like appendages on the surface of bacteria.

Characteristics of pili:

The pili have the following characteristics:

  • Pili are composed of pilin protein that is synthesized by the bacteria.
  • Pili are responsible for the reproduction of bacteria e.g. conjugative pili.
  • Pili are immunogenic in nature. They can induce the production of antibodies in the host.
  • They are fragile protein and 6-7 nm in diameter.
  • These are the hair-like appendages present on the surface of bacteria

Types of pili:

There are two types of pili.

  • Conjugation pili
  • Type IV Pili

Conjugation pili:

Conjugation pili are those that are responsible for the exchange of genes between the two bacteria. They are also called “sex pili” and responsible for sexual reproduction.

During conjugation, two bacteria having sex pili come closer and stimulate the formation of the mating bridge. The bridge is responsible for the exchange of genetic material from the donor to the recipient. Genes that are transferred to the recipient are the genes that are encoded by plasmid. These genes are responsible for antibiotic resistance, drug resistance and insect resistance.

Examples are Escherichia coli

Type IV Pili

Type IV pili are those pili that help bacteria to adhere to the surfaces. This pili also produce motile forces. The grappling hook is produced that pull the bacteria forward when bacterial pili contract. This pili also produce gliding motility and twitching movement. These movements are opposite to the movement produced by flagella. These pili resemble the protein of archeal flagellin.

Examples are Myxococcus Xanthus that shows the gliding movement.

How conjugation occur?

For the conjugation process, both donor and recipient bacteria should have sex pili. Donor bacteria donate F+ plasmid to the F bacteria. First, donor and recipient bacteria come close to each other and form a mating bridge. Double-stranded plasmid breaks and converts into the single-stranded plasmid. The single stranded plasmid is transferred into the recipient bacteria. DNA polymerase enzyme of bacteria starts to add nucleotides in the single-stranded plasmid to form a double stranded plasmid. Now, both the bacteria have F+ plasmids.

Functions of pili:

The functions of the pili are as follows:

  • Conjugative pili allow the transfer of genetic material between two bacteria.
  • The presence of pili is responsible for the virulence of pathogenic bacteria.
  • It adheres with surfaces and thus allows the attachment of bacteria with the surfaces.
  • These pili allow the genetic variation in the bacteria.


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