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  1. 1. Bacteriophage Dr. Pendru Raghunath Reddy Assistant Professor Dept of MicrobiologyDr. VRK Women’s Medical College
  2. 2. BacteriophageDefinition:Bacteriophages are viruses that infect and parasitise bacteriad’Herelle named the bacteriophage (Greek phagein, to eat)Phages occur widely in the environment such as sewage, faeces, soiland other natural sources of mixed bacterial growth
  3. 3. MorphologyBacteriophages that infect E. coli, called the T-even phages (T2, T4, T6),have been extensively studiedT-even phages are tadpole-shaped, possess a head and a tail1. Head Hexagonal in shape and consists of a tightly packed core of nucleic acid (double stranded DNA) enclosed by a protein coat, called a capsid The size of the head varies in different phages from 28 nm to 100 nm
  4. 4. 2. TailIt is composed of a hollow core surrounded by a contractile sheath, anda terminal base plate which has attached to prongs, tail fibres or both Head/Capsid Contractile Tail Sheath Tail Fibers Base Plate
  5. 5. Life cyclePhages exhibit two different types of life cycle1. Virulent or lytic cycle2. Temperate or lysogenic cycleLytic cycleReplication of a virulent phage can be divided into five stagesa) Adsorptionb) Penetrationc) Synthesis of phage componentsd) Maturatione) Release
  6. 6. Adsorption Phage attaches to a specific receptor site by means of tail fibres Adsorption is a specific process and depends on the presence of complementary chemical groups on the receptor sites and on the terminal base plate of the phage Infection cannot occur in the absence of adsorption The infection of a bacterium by the naked phage nucleic acid is known as ‘transfection’
  7. 7. Penetration
  8. 8. Virulent or lytic cycle of bacteriophageEclipse phaseThe interval between the entry of the phage nucleic acid into the bacterialcell and the appearance of first infectious intracellular phage particle
  9. 9. Lysogenic cycle In lysogenic cycle, the bacteriophage nucleic acid becomes inserted into the bacterial chromosome. The integrated phage genome in this state is known as the ‘prophage’ The prophage behaves like a segment of the host chromosome and replicates synchronously with it. This phenomenon is called ‘lysogeny’ The bacterium that carries a prophage is known as a ‘lysogenic bacterium’ The prophage confers certain new properties on the lysogenic bacterium. This is known as ‘lysogenic conversion or phage conversion’ Bacteriophage that parasitise a bacterium without lysing it is known as ‘temperate phage’
  10. 10. Bacteriophages: Lysogenic Conversion Virulence factors coded by prophage Gene Medical Bacterium Phage Product importance Vibrio cholerae CTX phage cholerae toxin cholera lambda hemorrhagic Escherichia coli shigalike toxin phage diarrhea CEβ and botulinum botulism (foodClostridium botulinum DEβ toxin poisoning) Corynebacterium diphtheria beta phage diphtheria diphtheriae toxin Streptococcus erythrogenic T12 scarlet fever pyogenes toxins
  11. 11. Events Leading to Lysogeny  Circularization of the phage chromosome  Cohesive ends Cohesive Ends LygaseLinear Double Stranded Opened Circle Closed Circle
  12. 12. Events Leading to Lysogeny Site-specific recombination Phage coded enzyme gal bio Repression of the phage genome Repressor protein gal bio Immunity to superinfection gal bio
  13. 13. Termination of Lysogeny Occasionally integrated prophage may become gal bio excised from bacterial DNA and initiates lytic cycle and gal bio the daughter phage particles are released, which infect other bacteria and render them lysogenic gal bio This is known as ‘spontaneous induction of prophage’ gal bio
  14. 14. Significance of phages1. Virulent phagea) Phage typingb) Phage therapyc) Phage display2. Temperate phagea) Toxin production b) Antigenic propertyc) Transduction d) Cloning vector
  15. 15. Phage display