- Bacteriophages or simply called phages are bacterial viruses that infect bacterial cells.
- They are obligate intracellular parasites that multiply inside bacteria by making use of some or all of the host biosynthesis machinery.
- With over 5000 isolates of known morphology, phages constitute the largest of all viral groups.
- Much of our understanding of viruses along with many fundamental concepts of molecular biology has emerged from investigation of the bacteriophage.
- Bacteriophages occur in over 140 bacterial genera and many different habitats.
- They are associated with transmission of genetic material from one bacterium to another.
- The activities of bacteriophage were first described by Twort in 1915, who described it as an infectious agent that distorted the appearance of the colonies of staphylococci.
- Subsequently, d’ Herelle in 1979 demonstrated the lytic activities of the culture filtrate on bacterial colonies. He suggested that the lytic agent was a virus and gave it a name bacteriophage (phage: to eat).
- The best studied phages are the T-even phages (T2, T4, T6, etc.) and M13 that infect the bacteria Escherichia coli.
- Phages show high host specificity. They usually inhabit the intestinal microbial flora of humans and other animals.
- They can pass through the filters, which hold back the bacteria. They are inactivated by boiling.
Morphology of a Bacteriophage:
- Bacteriophages exhibit a wide variety of morphologies.
- Most of the phages are tadpole-shaped with a hexagonal head and a cylindrical tail.
- Other phages appear cubic, filamentous or pleomorphic.
- Large phages usually consist of a head and a tail.
- Many phages contain specialized syringe-like structures (i.e. tails) that bind to the receptors on the cell surface and inject the phage nucleic acid into a host cell.
- Head:
- It is icosahedral (20 sides) or filamentous in shape.
- The head measures between 28 and 100 nm in length.
- The head is made up of protein coat known as a phage capsid that surrounds the genome or nucleic acid molecule.
- Most of the bacteriophages or phages consist of single, linear and double-stranded DNA genome whereas others contain double stranded RNA, single-stranded RNA, or single-stranded DNA.
- Some group of phages that specifically infect positive strains of coli consist of only RNA as a genome and some others
- This DNA is protected against degradation by environmental nucleases with the help of capsid.
- The capsid or protein coat is composed of individual protein units known as capsomeres.
- Tail
- Many but not all phages have tails attached to the phage head.
- A short collar connects head with the tail.
- The tail consists of a hollow tube through which the nucleic acid passes into the bacterial cell during infection.
- The tail of complex phage like T4 is surrounded by a contractile sheath called a tail sheath.
- The tail consists of a terminal tail plate or basal plate which is normally hexagonal in shape.
- From each vertex of the hexagonal basal plate, a tail fiber and spike originate.
- These tail fibers and spikes specifically bind to the receptors on the outer membrane of the host cell wall, the process called attachment or adsorption.
- The tail sheath acts like a syringe and contracts to inject the phase nucleic acid into the host cell after adsorption.
Uses of bacteriophages:
- They are convenient model for the study of virus-host interaction.
- They play an important role in transmission of genetic information by the process of transduction.
- They are used as cloning vectors for genetic manipulation.
- Engineering of DNA into phage M13 (phage with single-stranded DNA) has provided single strands that are valuable sources of DNA analysis and manipulation.
- They are used for phage typing.
- Phage typing is a pattern method in which a set of standard phages is employed for the intra-species typing of certain bacteria.