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
Types of Bacteriophages
Significance of Bacteriophages
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:
Collar
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.
Sheath
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.
Baseplate
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. 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: The T4 bacteriophages are lytic phages.
2. Lysogenic Cycle
It involves the following steps: The lambda (λ) phages are a typical example of lysogenic phage. 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.
