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Exploring Bacteriophages: Morphology, Cycles, and Medical Significance

This article delves into the world of bacteriophages, viruses that infect bacteria, discussing their morphology, lytic and lysogenic cycles, and their role in gene transfer. Learn about phage typing, phage therapy, and the significance of lysogeny in bacterial transformation. Discover how bacteriophages can be used in medical applications, including treatment of bacterial infections and gene transfer. Explore their ability to alter antigenic properties of bacteria and code for toxins such as diphtheria and cholera toxins. Uncover the potential of bacteriophages in recombinant DNA technology and transduction for gene transfer.

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Exploring Bacteriophages: Morphology, Cycles, and Medical Significance

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  1. Bacteriophage

  2. Bacteriophage (Phage) • Definition : viruses that infect bacteria (twort & d’Herelle 1917) • Significance • Models for animal cell viruses • Gene transfer in bacteria • Medical applications • Identification of bacteria - phage typing • Treatment and prophylaxsis???

  3. Bacteriophage - Morphology • Tadpole shaped • Hexagonal head (ds DNA) • Tail: hollow core with contractile sheath • Base plate: tail fibers

  4. Head/Capsid Contractile Sheath Tail Tail Fibers Base Plate Composition and Structure

  5. Infection of Host Cells • Irreversible attachment • Adsorption • LPS for T4 • Sheath Contraction • Nucleic acid injection

  6. Bacteriophage:The Lytic Cycle • Attachmentto cell surface receptors (chance encounter – no active movement) • Penetration– only genome enters • Biosynthesis–Production of phage DNA and proteins • Maturation– assembly to form intact phage • Releasedue to phage induced lysozyme production

  7. Lytic Cycle of a Bacteriophage 1 2 3

  8. Lytic Cycle of a Bacteriophage 4

  9. Assay for Lytic Phage Phage • Plaque assay • Method • Plaque forming unit (pfu) • Measures infectious particles Bacteria + Phage

  10. Lysogenic cycle • Phage DNA integrates into bacterial chromosome • Integrated genome: PROPHAGE • Bacterium carrying the prophage: LYSOGENIC bacterium • Super infection immunity: a lysogenic bacterium is resistant to re-infection by the same or related phages • Lysogenic phage can become lytic

  11. Lysogenic Cycle of a Bacteriophage

  12. Significance of Lysogeny • Model for animal virus transformation • Lysogenic or phage conversion • Definition: A change in the phenotype of a bacterial cell as a consequence of lysogeny • Modification of Salmonella O antigen • Toxin production by Corynebacterium diphtheriae

  13. Significance/ uses of bacteriophages • Phage typing: to classify bacteria • Epidemiological investigation to know the relatedness b/w strains of same species • S.aureus • Vi Ag typing of S.typhi • Vibriocholerae • C.diphtheria

  14. Phage assay • When virulent phages are spread over the lawn culture of susceptible bacteria areas of clearing/ lysis called PLAQUES are seen around growth of each phage

  15. 4. Phage therapy • Lytic phages can kill bacteria: treatment of bacterial infections: burn & wound infections • Used in diagnosis: identification of M.tuberculosis • Used as vectors for cloning in recombinant DNA technology • TRANSDUCTION: Temperate phages vehicles for transferring genes from one bacterium to another: antibiotic resistance genes • Eg: In S.aureus plasmids encoding B-lactamases are transferred by transduction

  16. 6. CODE FOR TOXINS: phage genome codes for the following bacterial toxins: • Diphtheria toxin • Cholera toxin • Verocytotoxin of EHEC • Botulinum toxin C & D 7. Alter antigenic properties of bacteria

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