A bacteriophage, commonly known as a phage, is a virus that infects and replicates within bacteria. Discovered independently by Frederick W. Twort and Felix d'Herelle in 1915 and 1917, respectively, bacteriophages are composed of proteins encapsulating a DNA or RNA genome, with structures ranging from simple to elaborate. Their genomes can encode as few as four genes or as many as hundreds. Widely distributed in locations with bacteria, bacteriophages are prevalent viruses and play a crucial role in the biosphere, found wherever bacteria exist. They are essential components of ecosystems, contributing to the control of bacterial populations.
Bacteriophages have different sizes and shapes. T4 is among the largest phages. In their appearance the phage resembles a tadpole or spermatozoid. It is differentiated into a head and tail.
Head: All phages contain a head structure which can vary in size and shape. Some are
Isohedral others are filamentous. The head or capsid is composed of many copies of one or more different proteins. Inside the head is found the nucleic acid. The head acts as the
protective covering for the nucleic acid.
Tail: The bacteriophage's tail is a hollow tube facilitating the passage of nucleic acid during infection. Encased in a contractile sheath that contracts upon infecting the bacterium, the tail has a base plate at its end with attached tail fibers. The base plate and tail fibers play a crucial role in binding the phage to the bacterial cell. Tail fibers serve two main functions: (i) aiding in the adsorption of phage particles onto bacterial surfaces, and (ii) secreting enzymes that contribute to the lysis of the bacterial cell wall.
Lytic cycle :
In the lytic cycle, the bacteriophage takes over the host bacterium's cellular machinery to produce new phage particles, ultimately leading to the lysis or destruction of the host cell.Here are the key stages of the lytic cycle:
Adsorption or infection: Virus attachment to the host cell, known as adsorption or infection, involves viral proteins interacting with host cell receptors like proteins, carbohydrates, glycoproteins, lipids, or lipoproteins. These receptors, inherent to the host's surface, play a crucial role in determining cell susceptibility to infection.
Penetration or Injection: The tail-fibers on the host cell surface bend, allowing the end-plate to contact the cell wall. Subsequently, the protein sheath of the tail contracts, pushing the central tubular core through the cell wall pore. This process releases the phage DNA into the host cell cytoplasm. Once the DNA is released, the empty protein coat becomes obsolete.
Synthetic phase : After the release of DNA into the host cell cytoplasm, phage DNA, containing glucosylated hydroxymethyl cytosine instead of cytosine, avoids degradation by host nucleases. This unique modification protects against genetically similar phages and enables the phage DNA to take control of the host cell machinery. This parasitic relationship suppresses cellular activities temporarily, including DNA, RNA, and protein synthesis. The viral DNA swiftly initiates the synthesis of early proteins, serving as enzymes for viral DNA synthesis. The newly formed viral DNA then directs the production of late proteins, mainly viral coat proteins, along with phage lysozymes. These late proteins contribute to the formation of the phage sheath and assist in the injection process during subsequent infections.
Maturation: The process of assembling various components to create a new phage particle within the host cell is referred to as 'maturation.' Head and tail formation initiate independently, with protein components surrounding the DNA to shape the phage's head. Initially, the end-plate is formed, followed by the creation of the tubular core, and later, the development of tail fibers.
Lysis: After the producing new bacteriophages, the host bacterial cell bursts open, and the phage particles are released. The bursting open of the host bacterial cell is called ‘lysis’.
The lysogenic cycle is a unique phase in the life cycle of a bacteriophage, a virus that infects bacteria. Unlike the lytic cycle, where the host cell is quickly destroyed, the lysogenic cycle involves the integration of the phage genetic material into the bacterial DNA. After entering the host cell, the phage DNA becomes a prophage, merging with the bacterial chromosome. During replication, the prophage is faithfully copied along with the bacterial DNA, passing on to daughter cells. This integrated state, called lysogeny, allows the phage to stay dormant within the host without causing immediate harm. Under specific conditions, like stress, the prophage can be triggered to leave the bacterial chromosome and start the lytic cycle, producing new phage particles and causing cell lysis.
The International Committee on Taxonomy of Viruses (ICTV) classifies phages based on morphology and nucleic acid. Currently, there are 19 recognized families target bacteria and archaea. Among these, only two families have RNA genomes, and five families are enveloped. Concerning DNA genomes, only two families have single-stranded genomes. Additionally, eight families have circular DNA genomes, while nine have linear DNA genomes. Nine families exclusively infect bacteria, nine infect archaea, and one family (Tectiviridae) infects both bacteria and archaea.
Bacteriophages, or simply phages, are classified based on various characteristics such as the nature of their genetic material and morphology. Here's a general overview of the classification of bacteriophages:
DNA Phages:
RNA Phages:
Morphology:
Icosahedral Phages: Have a roughly spherical shape, e.g., φx 174.
Helical (Filamentous) Phages: Have a long, coiled shape, e.g., fd, M13.
Tailed Phages: Have a head and a tail, e.g., T-even phages like T4.
Examples:
E. coli Phages: Include T-even phages (T2, T4, T6), λ phage, fd, M13.
Bacillus Phages: Include phages like SPO1, SPO2, PBS1.
Salmonella Phages: Include P22, PI.
(Session 2026 - 27)