Viruses

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Are obligate parasites, filterable, infectious particles

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Viruses

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Size of viruses

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20nm to 14,000nm

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Example of viruses that are smaller than bacteria

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Adeno virus

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Viruses that are same size as the small bacteria

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Vaccinia virus

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Examples of vaccinia virus

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Mycoplasma, Rickettsia and Chlamydia

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How do viruses multiply inside living cells?

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By using cellular synthesis machinery

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Demonstrated that filtered sap from diseased tobacco plants was transmissible-filterable bacterium

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Ivanovski

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Provided evidence that the transmissible agent was not a bacterium

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Beijernick

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Isolated the infectious agent and called it Tobacco Mosaic Virus

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Stanley

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Meaning of "virus" in Latin

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Poison

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This enabled viruses and bacteria to be studied in detail

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Electron microscope

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An intact infectious viral particle

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Virion

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Is a single or segmented, circular or linear molecule of nucleic acid functioning as the genetic material of the virus

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Viral genome

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It codes for the synthesis of viral components and viral enzymes for replication

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Viral genome

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Is a protein shell surrounding the genome and is usually composed of protein subunits

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Capsid

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The protein subunits of capsid

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Capsomeres

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Serves to protect and introduce the genome into host cells

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Capsid

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Some viruses that consist of no more than a genome surrounded by a capsid

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Nucleocapsid or naked viruses

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2 viral structure

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1. A genome: nucleic acid core

2. A capsid

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3 viral symmetry based on the shape of capsid

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1. Helical

2. Polyhedral

3. Complex

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Capsomeres fit together as a spiral to form a rod-shaped structure

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Helical

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Capsomeres are usually arranged in equilateral triangles that fit together to form a structure resembling a geodescic dome

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Polyhedral

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It is the most common polyhedral with 20 triangular surfaces with 12 vertices and appear spherical when viewed under LP under EM

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Icosahedral

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Combination of a helical portion called tail to a polyhedral head

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Complex

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Example of a helical viral symmetry

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TMV

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4 additional structures of a combination viral symmetry

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1. Tail sheath

2. Plate

3. Pins

4. Tail fibers

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Why are animal viruses called enveloped viruses?

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Because they have an envelope surrounding a polyhedral or helical nucleocapsid

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Composition of the envelope of a virus

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Phospholipids and glycoprotein

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How are phospholipids and glycoproteins derived?

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It is derived from host cell membranes by a process called budding

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Where do the envelope may come from?

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From the host cell's nuclear membrane, vacuolar membranes or outer cytoplasmic membrane

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It function in attaching the virus to receptors on susceptible host cells

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Glycoprotein spikes or peplomers

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Function of glycoprotein spikes or peplomers

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Attaching the virus to receptors on susceptible host cells

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5 genera of viruses

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1. Enterovirus

2. Cardiovirus

3. Rhinovirus

4. Apthovirus

5. Hepatovirus

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Virus that affect alimentary tract and example

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Enterovirus (poliovirus 1, 2, & 3)

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Virus that affect neurotropic and example

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Cardiovirus (Mengovirus)

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Virus that affect nasopharyngeal region and example

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Rhinovirus (Rhinovirus 1a)

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Virus that affect cloven footed animals and example

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Apthovirus (FMDV-C)

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Virus that affect liver and example

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Hepatovirus (hepatitis A virus)

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6 basis of taxonomic classification of viruses

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1. Morphology

2. Physicochemical properties

3. Genome

4. Macromolecules

5. Antigenic properties

6. Biological properties

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Features under morphology

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Size, shape, and enveloped/unenveloped

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Features under physicochemical properties

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Molecular mass, buoyant density, pH, thermal, and ionic stability

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Features under genome

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RNA, DNA, segmented sequences, restriction map, modifications, etc.

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Features under macromolecule

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Protein composition and function

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Features under biological properties

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Host range, transmission tropism, etc.

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2 principal events involved in viral replication

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1. Adsorption/attachment

2. Penetration (virus enters the cell)

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What is absorption/attachment?

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Virus attaches to the cell surface

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How is attachment?

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Via ionic interactions which are temperature-independent

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How is penetration?

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Entry by fusing with the plasma membrane. Some enveloped viruses fuse directly with the plasma membrane

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How does entry occur in penetration of the virus?

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Via endosomes at the cell surface

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Are obligate intracellular parasites that multiply inside bacteria by making use of some or all of the host biosynthetic machinery

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Bacteriophages

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Observed that bacterial colonies sometimes dissolved and disappeared because their component cells lysed or burst

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Fredrick Twort

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What is the nucleic acid of a bacteriophage?

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May be DNA or RNA

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What are the 3 forms of bacteriophage?

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1. Icosahedral head with no tail

2. Icosahedral with tail

3. Filamentous

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Phages that infect E. coli, designated to T1 to T7

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Coliphages

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Composition of coliphages

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Composed exclusively of DNA and protein in approximately equal amounts

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2 life cycles of bacteriophages

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1. Lytic life cycle: virulent phages

2. Lysogenic life cycle: temperate phages

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Phages which can only multiply on bacteria and kill the cell by lysis at the end of the life cycle

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Lytic life cycle (virulent phages)

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3 phases of lytic life cycles

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1. Eclipse period

2. Intracellular accumulation phase

3. Lysis and release phase

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During this phase, no infectious phage particles can be found either inside or outside the bacterial cell

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Eclipse period

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In this phase, the nucleic acid and structural proteins that have been made are assembled and infectious phage particles accumulate within the cell

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Intracellular accumulation phase

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During this phase, after a while, the bacteria begin to lyse due to the accumulation of the phage lysis protein and intracellular phage are released into the medium

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Lysis and release phase

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Are those that can either multiply via the lytic cycle or enter a quiescent state in the cell

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Lysogenic life cycle (temperate phages)

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What do you call the phage DNA in lysogenic life cycle?

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Prophage

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The cell harboring a prophage

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Lysogen

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Anytime a lysogenic bacterium is exposed to adverse conditions, the lysogenic state can be terminated. What do you call this process?

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Induction

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3 conditions which favor the termination of the lysogenic state

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1. Desiccation

2. Exposure to UV or ionizing radiation

3. Exposure to mutagenic chemicals

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Lead to the production of proteases

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Adverse conditions

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1 significance of lysogeny

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Model for animal virus transformation

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Life cycle where nucleic acid has to be sufficiently uncoated so that virus replication can begin

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Uncoating

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Is the start of eclipse phase

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Uncoating

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Infection after uncoating and more viral components (nucleic acid and proteins) are manufactured by the host cell

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Synthesis or viral nucleic acid and protein

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New virus particles are assembled

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Assembly/Maturation

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Virus may be released due to cell lysis, or, if enveloped, may bud from the cell

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Release

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RNA Tumor Viruses are also known as

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Retroviruses

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Are different from DNA tumor viruses in that their genome is RNA but they are similar to many DNA tumor viruses in that the genome is integrated into host genome

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RNA tumor viruses (retroviruses)

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2 oncovirinae

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1. Rous sarcoma virus (RSV)

2. HTLV-1 (human T-cell lymphotropic virus)

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Virus that causes a slow neoplasm in chickens

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Rous sarcoma virus (RSV)

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Virus that causes adult T-cell leukemia (Sezary T-cell leukemia) which is found in some Japanese islands, the Caribbean Latin America and Africa and is sexually transmitted

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HTLV-1 (human T-cell lymphotropic virus)

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Describe the viral membrane of HIV

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This is host-derived as a result of budding from the cell surface. Some host proteins become incorporated into the viral membrane

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Describe the surface glycoprotein of HIV

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Gp160 is encoded by the envelope gene. Gp160 is leaved after translation by host enzymes in the golgi body to form Gp120 (SU) and Gp41 (TM). Gp41 is embedded in the membranen, Gp120 is not but is held to Gp41 by non-covalent interactions. It is easily shed from the virus particle.

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2 cells that are infected by HIV

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1. CD4+ (T4) helper lymphocytes

2. Follicular dendritic cells (FDCs)

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Are the most studied but monocytes/macrophages are also infected and may provide an important reservoir

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CD4+ (T4) helper lymphocytes

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Changes in the biologic function of a cell that results form regulation of the cell by viral genes and that confer on the infected cell certain characteristics of proplasia

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Transformation

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The region of the viral genome that can cause a tumor

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Oncogene

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4 examples of DNA tumor viruses

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1. Papillomaviruses

2. Polyoma viruses

3. Adenoviruses

4. Herpesviruses

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Wart-causing viruses

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Papillomaviruses

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Example of papillomaviruses

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HPV

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3 examples of polyoma viruses

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1. Simian virus 40

2. Polyoma virus

3. Human polyoma viruses

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Is a monkey polyoma virus that causes sarcomas in juvenile hamsters

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Simian virus 40 (SV 40)

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What is Simian virus 40 (SV 40)?

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Is a monkey polyoma virus that causes sarcomas in juvenile hamsters

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Where was SV 40 isolated?

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From normal monkey kidney cells

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It causes a wide range of tumors in a number of animal species

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Polyoma virus

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Where was polyoma virus isolated?

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From AK mice and is fully permissive for replication in mouse cells

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What disease is caused by polyoma virus?

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Leukemias in mice and hamsters

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2 human polyoma isolates

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1. BK

2. JC

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What does human polyoma virus causes?

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It causes progressive multifocal leukoencephalopathy, a disease associated with immunosuppression

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These viruses are highly oncogenic in animals and only a portion of the virus is integrated into the host genome

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Adenoviruses

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Viruses where mechanism of oncogenesis, perhaps by causing chromosomal breakage or other damage

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Herpesviruses

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Example of herpesviruses

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Epstein-Barr virus

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Herpes virus that is most strongly associated with cancer

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Epstein-Barr virus

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4 associated with Epstein-Barr virus

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1. Burkitt's lymphoma in Africa

2. Nasopharyngeal cancer in other areas (common in China and SE Asia)

3. B cell lymphomas in immune suppressed individuals (such as in organ transplantation or HIV)

4. Hodgkin's lymphoma

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He developed the first rabies virus vaccine grew the virus in rabbits

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Louis Pasteur

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Was first successfully grown in the lab in ferrets

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Influenza

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First grown in monkeys

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Polio virus

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4 animal viruses of medical importance

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1. Oncogenic viruses

2. Animal hosts

3. Embryonated eggs

4. Animal cell culture

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Fertile eggs are incubated for 6-10 days before inoculation

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Embryonated eggs

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This technique began in the early parts of this century but did not become "routine" until after WWII when antibiotics became available

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Animal cell culture

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2 basic types of cultured animal cells

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1. Primary cell culture

2. Continuous cell lines

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For this, some tissue is obtained from an animal cell, the cells are dispersed by an enzyme and the resulting cells are grown

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Primary cell culture

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These cells can be maintained and subcultured almost indefinitely

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Continuous cell lines

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Where do continuous cell lines originate?

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Either from normal cells that became transformed of from a malignant tumor

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Extremely small, circular pieces of RNA that are infectious and pathogenic in plants

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Viroids

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4 characteristics of viroids

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1. Extremely small, circular pieces of RNA that are infectious and pathogenic in plants

2. Similar to RNA virus, but lack capsid

3. May appear linear due to H bonding

4. Causes Cadang-cadang disease in coconut; potato spindle tuber disease

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4 characteristics of prions

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1. Proteinaceous infectious agents

2. Cellular PrP protein

3. Prion PrP

4. Prion PrP converts cellular PrP into prion PrP inducing conformational change

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3 prion diseases

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1. Fatal neurological degeneration, fibril deposits in brain, and loss of brain matter

2. Large vacuoles form in brain, characteristic spongy appearance

3. Spongiform encephalopathies - Bovine Spongiform Encelopathy (madcow disease), Scrapie, Creutzfeldt-Jakob Disease, Kuru

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Influenza virus are also known as

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Orthomyxovirus

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Is an acute infectious disease caused by a member of the orthomyxovirus family

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True influenza

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Any febrile respiratory illness with systemic symptoms which may be caused by a myriad of bacterial or viral agents as well as influenza

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Flu

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Is involved in attachment and membrane fusion in the Endosome of the infected cell

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HA (hemagglutinin) protein

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Is involved in penetration of mucus layer and in facilitating release from infected cell

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NA (neuraminidase) protein

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1 cultivation of viruses

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Bacterial hosts

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1 bacterial hosts

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Plaque count

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A sample of bacteriophage is mixed with thr bacteria and melted agar

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Plaque count

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How is plaque count done?

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A sample of bacteriophage is mixed with the bacteria and melted agar. The agar containing bacteriophage and host bacteria is then poured into a petri dish containing a hardened layer of agar growth medium