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Bacteriophage 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??? Bacteriophage - Morphology • Tadpole shaped • Hexagonal head (ds DNA) • Tail: hollow core with contractile sheath • Base plate: tail fibers Composition and Structure Head/Capsid Contractile Sheath Tail Tail Fibers Base Plate Infection of Host Cells • Adsorption – LPS for T4 • Irreversible attachment • Sheath Contraction • Nucleic acid injection Bacteriophage: The Lytic Cycle 1. Attachment to cell surface receptors (chance encounter – no active movement) 2. Penetration – only genome enters 3. Biosynthesis – Production of phage DNA and proteins 4. Maturation – assembly to form intact phage 5. Release due to phage induced lysozyme production Lytic Cycle of a Bacteriophage 1 2 3 Lytic Cycle of a Bacteriophage 4 Assay for Lytic Phage Phage • Plaque assay – Method – Plaque forming unit (pfu) – Measures infectious particles Bacteria + Phage 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 Lysogenic Cycle of a Bacteriophage 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 Significance/ uses of bacteriophages 1. Phage typing: to classify bacteria 2. Epidemiological investigation to know the relatedness b/w strains of same species • S.aureus • Vi Ag typing of S.typhi • Vibrio cholerae • C.diphtheria 3. 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 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 5. 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 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
