1. Viruses and Acellular Agents
MICROBIOLOGY
LECTURES
Viruses and Acellular Agents
Sofronio Agustin
Professor
11/17/2018

2. Lesson Objectives:
At the end of the lesson, the students should be able to:
Describe the structures and components of viruses, viroids and prions.
Describe the mechanism of infection and replication of bacteriophages and animal viruses.
Understand association between viruses and cancer and the phenomenon of viral latency and persistent infections.
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3. Basic Terminology
Viruses are obligate intracellular parasites and require living host cell genetic and metabolic machinery for reproduction.
Viral genomes contain either DNA or RNA, not both.
Nucleic acid enclosed in a protein coat called Capsid.
Some are surrounded by membrane layer called Envelope, which may contain integral proteins such as Spikes.
Tropism – property of viruses to infect specific host cells; determined by specific host receptors and viral glycoproteins.
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4. Viruses and Bacteria Compared
Rickettsias/
Chlamydias
Viruses
Intracellular
Parasite No
Yes
Genome
Both DNA & RNA
DNA or RNA
Passage through bacteriologic filters
May pass through
Sensitive to Antibiotics
Sensitive to Interferon
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5. Virus Sizes and Shapes
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6. Helical Viruses
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7. Enveloped Helical Virus
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8. An ICOSAHEDRON is a polygon with 12 triangular facets and 12 corners
Polyhedral Viruses
An ICOSAHEDRON is a polygon with 12 triangular facets and 12 corners
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9. Enveloped Icosahedral Virus
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10. Complex Viruses
Bacteriophages have icosahedral heads, sheathed tail and tail fibers for host cell attachment.
Morphology akin to NASA’s Lunar Lander.
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11. Viral Taxonomy ICTV = International Committee on Taxonomy of Viruses
Family names end in –viridae e.g. Herpesviridae
Genus names end in -virus e.g. Papillomavirus
Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common English names are used for species e.g. Measles, Mumps, Rabies.
Subspecies are designated by a number e.g. HSV-1
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12. Viral Taxonomy Retroviridae Herpesviridae Lentivirus Herpesvirus
Human herpes virus 1, HHV 2, HHV 3
Retroviridae
Lentivirus
Human Immunodeficiency Virus 1, HIV 2
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13. Electron Microscopy
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14. Bacteriophage Culture
Viruses must be grown in living cells.
Bacteriophages form plaques on a lawn of bacteria.
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15. Tissue Culture Animal and plants viruses may be grown in cell culture.
Continuous cell lines may be maintained indefinitely.
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16. Cytopathogenic Effects
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17. Embryonated Egg Cultures
Animal viruses may be grown in living animals or in embryonated eggs.
Usually used in conjunction with vaccine production e.g. Flu vaccine
Some people who are allergic to eggs show sensitivity to these vaccines.
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18. Virus Identification
Cytopathogenic effects (CPE) as seen in Tissue Culture.
Fluorescent Antibody Techniques (DFA or IFA)
Serological tests – to detect antibodies against viruses in a patient (4-fold rise in titers)
Antigen tests – rapid screening e.g. ELISA and Latex Agglutination tests.
Nucleic Acid Analysis – e.g. Western blot, Gene Probes,
PCR – Polymerase Chain Reaction.
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19. Stages of Replication in Lytic Phages
Attachment - phage attaches by tail fibers to host cell
Penetration - phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell
Biosynthesis - production of phage DNA and proteins
Maturation - assembly of phage particles
Release - phage lysozyme breaks cell wall
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20. Replication Cycles in Phages
Lytic cycle - Phage causes lysis and death of host cell
Lysogenic cycle - Prophage DNA incorporated in host DNA; when activated becomes lytic.
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21. Lytic Cycle of T-even Phages
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22. Lytic Cycle of T-even Phages
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23. One Step Growth Curve
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24. Lysogenic Phages
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25. Specialized Transduction
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26. Replication of Animal Viruses
Attachment - Viruses attaches to cell membrane
Penetration - By endocytosis or fusion
Uncoating - By viral or host enzymes
Biosynthesis - Production of viral nucleic acid and proteins
Maturation - Nucleic acid and capsid proteins assemble
Release - By budding (enveloped viruses) or rupture (non-enveloped viruses)
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27. Attachment , Penetration and Uncoating
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28. Replication of DNA Viruses
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29. Replication of RNA Viruses
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30. Replication of Retroviruses
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31. Release of Enveloped Viruses
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33. Viruses and Cancer
Activated oncogenes transform normal cells into cancerous cells.
Transformed cells have increased growth, loss of contact inhibition, tumor specific transplant and T antigens.
The genetic material of oncogenic viruses becomes integrated into the host cell's DNA.
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34. Oncogenic Viruses Oncogenic DNA Viruses Adenoviridae Heresviridae
Poxviridae
Papovaviridae
Hepadnaviridae
Oncogenic RNA viruses
Retroviridae
Viral RNA is transcribed to DNA which can integrate into host DNA
HTLV 1
HTLV 2
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35. Latent and Persistent Viral Infections
Latent Viral Infections
Virus remains in asymptomatic host cell for long periods
Cold sores, shingles
Persistent Viral Infections
Disease processes occurs over a long period, generally fatal
Subacute sclerosing panencephalitis (measles virus)
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36. Prions Infectious proteins
Inherited and transmissible by ingestion, transplant, & surgical instruments
Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease
PrPC, normal cellular prion protein, on cell surface
PrPSc, scrapie protein, accumulate in brain cells forming plaques
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37. Prion Infection Model
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38. Plant Viruses and Viroids
Plant viruses enter through wounds or via insects
Viroids are infectious RNA; potato spindle tuber disease
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43. Human Viruses Single-stranded DNA, nonenveloped viruses
Human parvovirus
Fifth disease
Anemia in immunocompromised patients
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44. Human Viruses Respiratory infections in humans Tumors in animals
Mastadenovirus
Respiratory infections in humans
Tumors in animals
Double Stranded DNA Nonenveloped Viruses
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45. Human Viruses Papillomavirus (human wart virus) Polyomavirus
Cause tumors, some cause cancer
Double Stranded DNA Nonenveloped Viruses
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46. Double Stranded DNA Nonenveloped Viruses
Human Viruses
Orthopoxvirus (vaccinia and smallpox viruses)
Molluscipoxvirus
Smallpox, molluscum contagiosum, cowpox
Double Stranded DNA Nonenveloped Viruses
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