Talaro, Talaro / Foundations in Microbiology, 4e 1
Lecture-7 / Chapter-6
An Introduction to the Viruses
Lecture Outline
I. The Search for the Elusive Viruses
- Existence of living entities smaller than bacteria first postulated by Louis Pasteur
- Pasteur proposed the term virus (from Latin for “poison”) for these infectious agents
- Pasteur developed first rabies vaccine in 1884
- Filterable virus – early researchers showed fluids or tissue extracts passed through ceramic
filters designed to remove all cellular material could remain infectious.
II. The Position of Viruses in the Biological Spectrum
- Infectious particles – large complex molecules w/ genetic material
- Obligate intracellular parasites – must invade the cells of a host in order to propagate
- Active or inactive – terms to describe infectivity – instead of alive or dead
III. The General Structure of Viruses
- Size Range
- most less that 0.2 microns – need electron microscope to observe details
- Animal viruses – smallest parvoviruses: 0.02 microns, largest poxviruses: 0.4 microns
- see table 6.2 on p 161 in text
- Unique viral constituents: capsids, nucleocapsids, and envelopes
- Capsid (protein) + nucleic acid core (DNA or RNA) = nucleocapsid
- Enveloped vs. non-enveloped (“naked” nucleocapsid)
- Structure of viral capsid
- Capsomeres – repeating subunits made of protein
- Helical Capsids – from rod-shaped viral particles with nucleic acid coiled inside
- Icosahedron – 20-sided polyhedron with triangular facets and 12 corners
- each facet may contain dozens to hundreds of capsomeres
- Structure of viral envelopes
- Envelopes made form cell membranes – lipid bilayer with proteins
- Host membrane proteins partially to completely repklaced with viral proteins
- Peplomeres – protruding “spikes” of glycoprotein
- Functions of the Viral Capsid/Envelope: role in infection and host defense
- protection of nucleic acid core
- adhesion and entrance to host cells
- Atypical or Complex Viruses
- Poxviruses – several layers of lipoproteins and coarse fibrils
- Bacteriophages – polyhedral “head”, helical “tail” with fibers protruding from end
- Nucleic Acids: At the Core of a Virus
- DNA or RNA – not both
- Single or double-stranded
- Viruses are genetic parasites
- Other Substances in the Virus Particle
- Preformed enzymes – e.g.; polymerases
- some may contain ribosomes taken form host
- some may take tRNA form host cell
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IV. How Viruses Are Classified and Named
- DNA vs. RNA
- Enveloped or non-enveloped
- Animal, plant, or bacterial
- Family name ends in -viridae
- Characteristics used to classify and name include: capsid type, nucleic acid strand,
envelope, viral size, host cell specificity, and shape or size
- Examples:
- Rhabdovirus: bullet-shaped
- Togavirus: Cloak-like envelope
- Adenovirus: found in adenoids
- Lentivirus: slow, chronic infection
- Picornaviruses: small, RNA
- Reoviruses: respiratory, enteric, orphan viruses
- Common names generally based on disease are still most widely used.
- Study table 6.3 in text
V. Modes of Viral Multiplication
- All viruses have some means of infecting a host cell with the viral genetic material and
then forcing the cell’s synthetic machinery to produce more viral particles.
- General stages of viral multiplication cycle (analogous to “life cycle”)
1. Adsorption: Attachment onto the host cell surface
2. Penetration: Entry of the nucleic acid into cell
3. Replication: Copying and expression of viral genome
4. Assembly and Maturation: Making components and assembling into complete
particles
5. Release: escape of viral particles out of cell – usually by cell lysis or by exocytosis
- The Multiplication Cycle in Bacteriophages or “Phages”
- Study pages 169 – 170 in text
- Lysogeny -- incorporation of viral genome into host genome yielding a latent
infection
- Temperate phages
- Prophage
- Lysogeny compared with the lytic cycles
- Multiplication Cycles in Animal Viruses
- Adsorption – attachment to specific receptors on cell membrane
- Host Range – determined by viral affinity for specific receptors
- Receptors are usually glycopoteins on cell surface
- Penetration
- Endocytosis vs. fusion
- “Uncoating”of animal viruses by lysosomal enzymes – release genome into
cytoplasm
- The free nucleic acid uses the host cell’s machinery and molecules
- Replication and Maturation
- DNA viral genomes enter host cell nucleus to replicate (except pox viruses)
- RNA viral genomes replicate in the cytoplasm (except retroviruses)
- Positive sense – viral RNA encodes directly for the viral proteins
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- Negative sense – complementary RNA encodes for viral proteins
- Release of Mature Viruses – budding or exocytosis (enveloped) vs. cell lysis (non-
enveloped)
- Damage to the Host Cell and Persistent Infections
- Cytopathic effects – virus-induced changes in microscopic appearance of cell
- Inclusion bodies – compact masses of viruses and cellular organelles
- Persistent infection – cell not lysed or killed – continues to produce viral particles
- Chronic latent state – infected cells not expressing viral proteins
- Oncogenic viruses, oncoviruses – cause cancerous changes in cells
(transformation)
VI. Techniques in Cultivating and Identifying Animal Viruses
- Purpose: Meidcal identification, production of vaccines, scientific study
- Using live animal inoculation
- Using Bird Embryos: pocks
- Using cell (tissue) culture techniques
1. Primary cell culture – first derivation of cells from a tissue
2. Cell lines – repeated passage of cells into fresh media passage
3. Plaques – areas of cell lysis noted in tissue culture
VII. Medical Importance of Viruses
- Most common infections
-Colds and flu, childhood diseases
- Other non-cellular infectious agents
- Prions
- Creutzfeldt-Jakob disease
- Bovine spongiform encephalopathy
- Satellite viruses – co-dependent infection
- Adeno-associated virus
- Viroids – infections in plants – naked RNA
- Several commercially important plant diseases
- Detection and control of viral infections
- Clinical signs / syndromes
- Serology and immunologic tests
- PCR to amplify viral genomes
- Isolation in culture – microscopic identification
- Vaccination is chief means of control
- Treatment of Viral Infections
- Traditional antibiotics for bacteria do not work
- AZT Azidothymidine
- Acyclovir
- Interferon