Bacteriophages (Morphology and Uses)

  • 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.
  1. 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.
                     A Typical Structure of a T-even phage
  1. 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.

Bacteriophages (Morphology and Uses)