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COURSE CODE: 320
Pre-requisite: 301
SEMESTER: 6
VIROLOGY/PARASITOLOGY
REFERENCE TEXT BOOK:
Howley, P.M., Roizman , B., Straus, S.E., Martin , M.A., Griffin, D.E., 2001.
Fundamental Virology, Lippincott Williams & Wilkins.
Contents
• Life cycle of bacteriophage
• Significance
• Introduction to Phylogeny
• Virus like agents (prions & viroid)
SHS.320.Lec-2..pptx
SHS.320.Lec-2..pptx
A. Lytic Cycle:
• The replication cycle of virulent phage is divided into five sequential phases :
• 1. Adsorption,
• 2. Penetration,
• 3. Synthesis of phage components,
• 4. Maturation and assembly, and
• 5. Release of progeny viruses:
1. Adsorption:
The phage particles come into contact by random collision and a phage attaches to a
specific receptor site on the host cell membrane by means of tail fibres.
• Adsorption occurs within minutes of contact.

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SHS.320.Lec-2..pptx

  • 1. COURSE CODE: 320 Pre-requisite: 301 SEMESTER: 6 VIROLOGY/PARASITOLOGY REFERENCE TEXT BOOK: Howley, P.M., Roizman , B., Straus, S.E., Martin , M.A., Griffin, D.E., 2001. Fundamental Virology, Lippincott Williams & Wilkins.
  • 2. Contents • Life cycle of bacteriophage • Significance • Introduction to Phylogeny • Virus like agents (prions & viroid)
  • 5. A. Lytic Cycle: • The replication cycle of virulent phage is divided into five sequential phases : • 1. Adsorption, • 2. Penetration, • 3. Synthesis of phage components, • 4. Maturation and assembly, and • 5. Release of progeny viruses:
  • 6. 1. Adsorption: The phage particles come into contact by random collision and a phage attaches to a specific receptor site on the host cell membrane by means of tail fibres. • Adsorption occurs within minutes of contact.
  • 7. 2. Penetration: • After adsorption of phage to bacteria: The tail sheath of phage contracts and the base plate and tail fibres are held firmly against the bacterial cell. As a result the hollow core is pushed downwards through the already weakened part of cell-wall caused by a phage muramidase present on the base plate. • The viral nucleic acid passes down the hollow tube similar to injection through a syringe. The tube does not penetrate the cell wall and the empty head (capsid) and tail remain outside as shell or ghost.
  • 8. 3. Synthesis of phage components: After the release of nucleic acid into the bacterial cell, the viral genome directs the biosynthetic machinery of host cell --------- production of viral components This is effected by synthesis of specific enzymes (called early proteins) necessary for synthesis of phage components. • Subsequently, late protein, subunits of phage head and tail appear.
  • 9. 4. Maturation and assembly: During maturation there is spontaneous assembly of phage DNA head protein and tail protein of phage. Each component of phage nucleic acid acquires a protein coat and finally the tail structures are added forming a virion (infective virus particle).
  • 10. 5. Release: The progeny phages ------------ released by the lysis of the infected bacterium. Phage enzyme (probably muramidase) weakens the cell wall during replication of phage. As a result the infected bacterium assumes a spherical shape. Muramidase concentration rises in the late stage of growth cycle, which acts on the already damaged cell-wall causing lysis of cell with release of progeny phage.
  • 11. Eclipse phase: The time interval between the entry of phage nucleic acid into the bacterial cell to the appearance of first infectious intra-cellular phage particle is called “eclipse phase” because viruses cannot be detected within the host cell during this period • latent period: The time interval between infection of host cell and sudden increase in extracellular virus is called “latent period”.
  • 12. B. Lysogenic Cycle: In lysogenic cycle, the lytic (vegetative) phage becomes integrated with the host cell chromosomes converted into prophage without lysis of bacterial cell The prophage may be converted into a virulent vegetative (lytic) phase spontaneously or by physical and chemical agents (UV rays, H202).
  • 15. Virus Interactions with Host Cells • A. Incorporation • Prophage: Viral nucleic acid incorporates onto the host chromosome • The viral DNA is replicated only when the host cell replicates
  • 17. • B. Lysogenic conversion • Prophage can confer new properties on cell • Phage DNA not completely suppressed • Genes coding for trait are expressed • Infected cells have new characteristics • Streptococcus pyogenes manufactures toxin resulting in scarlet fever
  • 18. Host Ranges of Phages Number of different bacteria that phage can infect termed host range • Usually limited to single bacterial species for a single phage • Factors limit host range • Two most important • Phage must be able to attach to host receptors • Restriction-modification system the host cell must overcome
  • 19. Host Ranges of Phages  Receptors on bacterial surface › Vary in chemical structure and location  Usually on bacterial cell wall › Sites can be altered by two mechanisms  Receptor sites can be altered by mutation  Lysogenized bacteria can alter cell surface  Results in alteration of receptor site
  • 20. Host Ranges of Phages • Restriction-modification system • Restriction enzyme that codes for endonuclease • Modification enzyme attaches methyl group to DNA recognized by restriction enzyme
  • 21. Transduction • DNA may be transferred by a bacteriophage to a bacteria in a process called transduction. • 1. Generalized Transduction: In this type any bacterial gene can be transferred • 2. Specialized Transduction: In this type only a few specific genes can be transferred
  • 22. Significance of Bacteriophage: • 1. Phage Typing: On the basis of the susceptibility of different strains of bacteria to different bacteriophages, the typing of bacteria can be done for identification and epidemiological studies. Epidemics of V. cholera, S. typhi, S. paratyphi and S. aureus can be detected by phage typing. phage typing is most useful in intra-species typing of bacteria e.g. to distinguish V. cholera (classical) from El tor biotype V. cholera.
  • 23. • 2. Phage Assay: • When phages are applied on a lawn culture of susceptible strain • clear zones appear after incubation • Clear zones are known as plaques/ colonies. • A single phage produces only one plaque. • Phage assay is an useful method in titrating the number of viable phages.
  • 24. • 3. Temperate Phage: • Prophage genes may change the properties of host bacterium e.g. toxins of C. diphtheriae and Clostridium are determined by genes carried in prophage DNA. • Bacteriophage infection in S. typhi confers a new antigenic surface structure on the host. Such acquisition of new properties by bacteria following phage infection is called “phage conversion”
  • 25. Importance • Phage therapy or viral phage therapy The therapeutic use of bacteriophages to treat pathogenic bacterial infections. Phage therapy has many potential applications in human medicine as well as dentistry, veterinary science, and agriculture. • Importance of bacteriophages in fermentation processes: • Some phages are involved in the production of enzymes and special substance • Ecological significance: • Trials are being done to remove pathogens from wastewater and soil with the help of phages
  • 26. Phylogeny Evolution of a genetically related group of organisms OR a study of relationships between collection of "things" (genes, proteins, organs..) that are derived from a common ancestor Molecular phylogenetics The branch of phylogeny that analyses hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree.
  • 28. Virus-like agents classified and studied with viruses • Viroids • No coat protein, no coding capacity • Prions • No nucleic acid
  • 30. Viroids Very small, covalently closed, circular RNA molecules capable of autonomous replication and induction of disease Sizes range from 250-450 nucleotides No coding capacity - do not program their own polymerase Use host-encoded polymerase for replication Mechanically transmitted; often seed transmitted More than 40 viroid species and many variants have been characterized “Classical” viroids have been found only in plants
  • 31. Viroid Diseases • Potato spindle tuber viroid (PSTVd) • May be limiting to potato growers • First viroid characterized • Many variants described PSTVd in tomato PSTVd in potato
  • 32. Viroid Diseases • Citrus exocortis viroid (CEVd) • Causes stunting of plants, shelling of bark • May result in little yield loss • Control by removal of infected plants, detection, clean stock Citrus exocortis viroid
  • 33. Apple crinkle fruit viroid Avocado sun blotch viroid Citrus exocortis viroid Healthy Infected
  • 34. •Stanley B. Prusiner coined the term prion from Proteinaceous infective particle and changed to prion to sound it rhythmic. •Prion diseases were caused by misfolded proteins. •They are a normal cellular protein (PrPc) that has misfolded into a pathogenic protein
  • 35. • Prions are infectious particles that are composed solely of protein (i.e., they contain no detectable nucleic acid). • They are implicated as the cause of certain “slow” diseases called transmissible spongiform encephalopathies, which include such diseases as Creutzfeldt-Jakob disease in humans and scrapie in sheep.
  • 36. Comparison of Prions and Conventional Viruses
  • 37. Prion Diseases • Kuru • Fatal Familial Insomnia (FFI) • Creutzfeldt-Jakob disease (CJD) • Scrapie • Bovine Spongiform Encephalopathy (BSE) • Chronic Wasting Disease (CWD) Human Animal
  • 38. Classification of prion diseases • Infectious/Exogenous – e.g., Kuru, BSE (mad cow disease), Scrapie – Spread by • Consumption of infected material. • Transfusion. • Sporadic • Familial/Hereditary – Due to autosomal dominant mutation of PrP.
  • 39. Differences between cellular and scrapie proteins PrPC PrPSC Solubility Soluble Non soluble Structure Alpha-helical Beta-sheeted Multimerisation state Monomeric Multimeric Infectivity Non infectious Infectious Susceptibility to Proteinase K Susceptible Resistant