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Bacteriophages, also known as phages, are those viruses that infect and reproduce within bacteria. They are ubiquitously found and are the most abundant biological agent on Earth. The most commonly studied bacteriophage, the T4 phage, is a virus that infects E. coli.

They were independently discovered by Frederick W. Twort in 1915 and Félix d’Hérelle in 1917. D’Hérelle coined the term ‘bacteriophage,’ meaning ‘bacteria eater.’

Structure of Bacteriophages

Like other viruses, bacteriophages vary in size and morphology. However, under an electron microscope, they comprise a nucleic acid molecule surrounded by a protein structure. There is a tail attached to its head through a neck.

A T4 bacteriophage consists of the following parts:

1. Head (Capsid)

The head of bacteriophages, more commonly known as the capsid, forms the protective casing that encloses the genetic material of the phage. This genetic material is either DNA or RNA, depending on the type of bacteriophage.

Capids are made of protein subunits known as capsomeres arranged in polyhedral or helical forms. The structural integrity of the capsid is vital for protecting the enclosed genetic material from external factors, ensuring its safe delivery into the bacterial host.

2. Tail

The tail extends from the head, resembling the lander of a lunar spacecraft. It is not only a physical appendage but facilitates the initial attachment to the surface of a bacterial cell, initiating the infection process.

It consists of the following parts:


The collar is a specialized region at the base of the tail, connecting it to the head. It acts as a transition zone, facilitating the transfer of genetic material from the head to the bottom during infection. The collar thus ensures the seamless coordination of the various components of the phage.


The sheath is a tubular structure extending from the collar, surrounding and protecting the tail tube. It is critical in injecting the phage’s genetic material into the bacterial host.

As the sheath attaches to the bacterial cell, it contracts, propelling the tail tube and genetic material into the host cell. It is a mechanism similar to an injection syringe that injects fluids into our body.


At the lower end of the tail, the baseplate serves as the anchoring point for the tail fibers, with a spike in the center. It is a complex structure with receptor-binding proteins that interact specifically with surface receptors on the bacterial cell.

The sheath contracts on touching the host cell, driving the spike into the cell membrane

Tail Fibers

Tail fibers extend from the baseplate and function as molecular sensors. These fibers play a pivotal role in recognizing and binding to their specific receptors on the surface of the bacterial cell. The tail fibers confer specificity to the infection process, ensuring that the T4 bacteriophage selectively targets its intended host. It is thus crucial for the successful infection of a virus particle.

Types of Bacteriophages

Bacteriophages, like other viruses, must infect a host cell to reproduce. The process that includes the steps of infection is called the lifecycle of the phages.

Based on their style of reproduction, the bacteriophages are of two types: virulent phages and lysogenic phages.  The lifecycle of a virulent phage is called the lytic cycle. In contrast, those found in lysogenic phages are called the lysogenic cycle.

1. Lytic Cycle

It consists of the following stages:

  1. Adsorption: It involves the attachment of the phage to the specific bacterial cell surface using its tail fibers.
  2. Penetration: The phage injects its genetic material into the bacterial cell, leaving the empty capsid.
  3. Replication: Upon penetration, the phage takes control of the bacterial machinery, forcing it to replicate phage DNA and produce phage components.
  4. Maturation: New phage particles assemble from the replicated components.
  5. Release: The host cell bursts open (lyses), releasing many phage particles to infect other bacteria. A single bacterium on lysis may release 50 to 200 phages, which can start a fresh infection cycle.   

The T4 bacteriophages are lytic phages.

2. Lysogenic Cycle

It involves the following steps:

  1. Adsorption:  Like the lytic cycle, the lysogenic phage attaches to the bacterial cell on its receptors. 
  2. Penetration: The phage then injects its genetic material into the host cell.
  3. Integration: Instead of immediately taking over the host, the phage DNA integrates with the bacterial chromosome, becoming a prophage.
  4. Replication: The bacterial cell divides, and the prophage is replicated along with the bacterial DNA.
  5. Induction: Under certain conditions, the prophage can exit the bacterial chromosome and initiate the lytic cycle.

The lambda (λ) phages are a typical example of lysogenic phage.

Significance of Bacteriophages

Bacteriophages have widespread applications in the study of biotechnology and medicine.

In Genetic Engineering

Due to their relatively simple structures and lifecycles, bacteriophages are ideal model organisms for studying fundamental genetic processes.

Their ability to infect bacteria allows scientists to introduce specific genes into bacterial cells, facilitating the production of desired proteins or modifying genetic material.

Alternative to Antibiotics

With rising antibiotic resistance, bacteriophages are garnering attention as potential alternatives to traditional antibiotics. Phage therapy involves using specific bacteriophages to target and eliminate bacterial infections, offering a tailored and potentially more effective approach.

Controlling Bacterial Population

Bacteriophages are also used to maintain the health of an ecosystem. It does so by controlling the bacterial population in the ecosystem under study.

In agriculture, bacteriophages have shown promise in controlling the unwanted bacterial population to increase the yield of crops. 

Article was last reviewed on Friday, October 27, 2023

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