T₄ Bacteriophage

Bacteriophages, commonly known as "phages," are viruses that specifically infect and replicate within bacteria. The term "bacteriophage" translates to "bacteria eater," as certain virulent phages can completely lyse susceptible bacterial cultures. As obligate intracellular parasites, phages rely on the host's biosynthetic machinery to multiply within bacterial cells.

1.0Introduction

• Bacteriophages, or "phages," are viruses that specifically infect and parasitize bacteria. They were discovered by Frederick Twort in 1915 in England and independently by Félix d'Hérelle in 1917 at the Pasteur Institute in France. 
• It was d'Hérelle who coined the term "bacteriophage," meaning "bacteria eater," as virulent phages can completely lyse susceptible bacterial cultures.  
• As obligate intracellular parasites, phages replicate within bacteria by utilizing some or all of the host's biosynthetic machinery. 
• They are abundant in nature and can be readily isolated from sources such as feces and sewage. 
• Structurally and genetically diverse, bacteriophages are classified into at least 12 distinct groups.

 Examples of phages

• T-even phages such as T2 , T4 and T6 that infect E.coli . 
• Temperate phages such as lambda and mu. 
• Spherical phages with single stranded DNA such as PhiX174. 
• Filamentous phages with single stranded DNA such as M13.

2.0Characteristics of T₄ Bacteriophage

• Bacteriophage T₄ (phage T₄ ) is a virulent phage; it uses the host cell's metabolic machinery to produce progeny viruses and kills the host in the process. 
• Bacteriophage T₄ is a large & dsDNA virus. 
• T₄ bacteriophage infects the colon bacillus, Escherichia coli bacteria. 
• It does not have a pro-bacteriophage form, 
• The T₄ chromosome is approximately 168,800 base pairs long and contains about 150 characterized genes.

3.0Structure of T₄ Bacteriophage

• With the help of an electron microscope, the bacteriophage's morphology was studied. 
• The T-even phages show complex symmetry. These viruses are generally tadpole-shaped, with a 'head' followed by a 'tail'. 
• The head is hexagonal and outlines like a prism. This shape is also known as an elongated icosahedron. It is 950 A° in length and 650 A° in width.
• The head has a two-layered protein wall that encloses the double-stranded DNA. The wall is 35 A° thick and composed of about 2000 similar capsomeres. DNA is tightly packed in the head and is about 50 µ long. 
• The tail has a complex structure and is proteinaceous. It consists of a cubical, hollow, cylindrical core. 
• This core is 800 A° long, 70 A°in diameter, and has a 25 A° wide central canal. It is surrounded by a contractile sheath, which is 165 A° in diameter.

The Replication cycle of virulent phage is divided into five sequential phases :

• Adsorption 
• Penetration 
• Synthesis of phage components 
• Maturation and assembly 
• Release of progeny viruses

Adsorption

• Phage particles encounter host cells through random collisions, and attachment occurs when the phage's tail fibers recognize and bind to a specific receptor site on the bacterial cell membrane. 
• This adsorption process takes place within minutes of contact. 

Penetration

• Following adsorption to the bacterial cell, the phage's tail sheath contracts, securing the base plate and tail fibers firmly against the host. 
• This action drives the hollow core downward through a weakened section of the cell wall, which has been degraded by a phage-associated muramidase on the base plate. 
• The viral nucleic acid is then injected into the bacterium through the hollow tube, much like a syringe. However, the tube itself does not penetrate the cell wall, and the empty capsid and tail remain outside as a non-functional shell, or "ghost."

Synthesis of phage components

• After the viral nucleic acid enters the bacterial cell, the phage genome commandeers the host's biosynthetic machinery, halting normal cellular functions and redirecting resources toward viral replication. 
• This process starts with the production of early proteins, specialized enzymes required for phage assembly. Later, structural components, including late proteins and subunits of the phage head and tail, are synthesized. 
• While some of these elements form within the bacterial nucleus, others assemble in the cytoplasm.

Maturation and Assembly

• During the maturation phase, phage DNA, head proteins, and tail proteins spontaneously assemble. 
• Each segment of the viral nucleic acid is enclosed within a protective protein coat, and the tail structures are subsequently attached, resulting in the formation of a complete virion—an infective virus particle.

Release

• The lysis of the infected bacterium rapidly releases the progeny phages. The phage enzyme (probably muramidase) weakens the cell wall during phage replication. 
• As a result, the infected bacterium assumes a spherical shape. 
• Muramidase concentration rises in the late stage of the growth cycle, which acts on the already damaged cell wall, causing lysis of the cell with the release of progeny phage.

4.0Biological Importance of Bacteriophages

• Bacteriophages have been used in prophylaxis and medical treatment against several pathogenic bacterial diseases, such as cholera, plague, dysentery, and enteric fever. 
• They are also used to diagnose certain infections like plague, cholera, etc. 
• Bacteriophages feed on pathogenic bacteria present in polluted water. So, they can also be used as scavengers. 
• In many cases, bacteriophages determine the soil's microflora. Thus, they play an important role in agriculture. 
• In space microbiology, lysogenic cultures are used as radiation detectors and are used in USSR spaceship Vostok 2. 
• Bacteriophages are very harmful during the manufacturing process of antibiotics and milk products because they kill beneficial bacteria with their lysogenic activity.