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 Tail Tail Fibers Base Plate Head/Capsid Contractile Sheath

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

Bacteriophage: The Lytic Cycle Attachment to 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 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 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 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 Vibrio cholerae C.diphtheria

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 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