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

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

6. 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

7. Lytic Cycle of a Bacteriophage1 2 3

8. Lytic Cycle of a Bacteriophage4

9. Assay for Lytic 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
Vibrio cholerae
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