Viruses and Prions

General Characteristics

Ø   Extremely small (filterable):  20-1,000 nm

Ø   Obligatory intracellular parasites

Ø   Distinctions from bacteria

l    Single type of nucleic acid (contain DNA or RNA)

l    Contains a protein coat (capsid) and sometimes enclosed by an envelope

l    Uses cell machinery

l    Transfer the viral nucleic acid to other cells with special equipment

l    Few or no enzymes of their own (no ribosomes)

l    Not affected by antibiotics, but are affected by interferon

Viruses

Host Range

Ø   Species specific and cell specific

l   Infect only specific types of cells in one host

l    Bacteriophages (phages)

•   Infect only bacteria

l    Specificity depends on receptors on cell surface

• Specific host attachment sites and cellular factors

Ø   General categories of viruses based on host

l    Bacteria virus (bacteriophage)

•    Virus detects and attaches to cell wall, fimbrae or flagella

l    Animal viruses

•   Virus detects and attached to receptors in plasma membrane

Viral Structure

Ø   Size

l    Electron microscopy

l    20-14,00 nm

Ø   Virion

l    Extracellular form

•    The vehicle for transmission (host to host travel)

•   Virus cannot replicate or undergo metabolic functions unless it is inside cells

l    All virions have

•   Nucleic acid

l   Possibilities

•    Single strand or double strand

•    RNA or DNA

•    Linear or circular or segmented

•   Capsid

l   Protein coat composed of capsomeres

l   Single protein or many types of protein

l   Give virus its shape and helps it to attach to host cells

Viral Structure

Ø  Some virions have an envelope

l    Found outside of capsid

l    Made of lipid mostly

•    Sometimes have spikes

l   Proteins and carbohydrates attached to lipid

l   Assistance in attachment

l   Used to diagnose viruses involved in viral infections

l    Example of animal virions with envelopes

•   Influenza or herpes simplex

MORPHOLOGY

Ø   Helical

l    Long rigid or flexible rods

l    Rabies or Ebola

Ø   Polyhedral

l    Icosahedron:  20 faces (equilateral triangles) and 12 corners

l    Many-sided

l    Polio or adenovirus

Ø   Enveloped

l    Can be helical or polyhedral (but spherical with envelope)

Ø   Complex

l    Mostly bacteriophage (head, sheath, tail, base, pin)

Helical Viruses

Polyhedral Viruses

Enveloped Viruses

Complex Viruses

VIRAL TAXONOMY

Ø    Past methods based on symptoms

Ø    Now based on

l     Nucleic acid

l     Morphology

l     Replication strategy

Ø    Classified into families, orders, genera, species, and subspecies

l     Family names end in –viridae.

l     Genus names end in –virus.

l     Common names are used for species.

l     Subspecies are designated by a number.

Ø    Table 13.2 has families of viruses that affect humans

Ø    Viral species

l     A group of viruses sharing the same genetic information and ecological niche (host).

Viral Taxonomy

Ø  Herpesviridae

Ø  Herpesvirus

Ø  Human herpes virus HHV-1, HHV-2, HHV-3

Ø  Retroviridae

Ø  Lentivirus

Ø  Human immunodeficiency virus HIV-1, HIV-2

Growing Bacteriophage

Ø   Viruses must be grown in living cells

Ø   Liquid or solid media with appropriate bacterial cells

Ø   Cannot visualize them in liquid culture

Ø   On solid media can be visualized as plaques

l    Lawn of bacterial cells

l    Virus plated on top of bacteria

l    Viruses lyse bacteria and produce clearing zones called plaques

Growing Bacteriophage

Growing Animal Viruses

Ø   Live animals

l    To study infection

Ø   Embryonic eggs

l    For maintaining growth

Ø   Cell cultures (preferred)

l    Animal cells treated with enzymes and nutrients form monolayer

l    Cytopathic effect of infected monolayer

•   Tranformed cells are infected

l   No monolayeràtumor forms

Growing Animal Viruses

Ø  Animal and plants viruses may be grown in cell culture.

l    Continuous cell lines may be maintained indefinitely.

Bacteriophage Multiplication

Ø  Lytic Cycle

l    Ends with Cell lysis

l    Death of host cell

Ø  Lysogenic Cycle

l    Viral Genome integrates into host chromosome

•   Prophage viral DNA

Ø  Result of lysogeny

l    Phage immunity

l    Phage conversion (C. diphtheriae)

Lytic Cycle:  Bacteriophage

Ø    APBMD

l     Attachment

•    To host PM through cell wall

l     Penetration

•    DNA is injected into cell

l     Biosynthesis

•    Replicate DNA

•    Make viral proteins

l     Maturation

•    Assemble new virus particles

l     Dissemination

•    Release by lysing

•    Lysozyme expressed during maturation.

A Viral One-Step Growth Curve

Lysogenic Cycle:  Bacteriophage

Ø   AàP

Ø   Integration

l    Viral DNA becomes prophage

Ø   Cell divides like normal replicating prophage DNA while it reproduces

Ø   Induction

l    Stimulated by environmental factors

Ø   BàMàD

Animal Virus Multiplication
 

Ø     Attachment

l     Receptor sites on animal cell plasma membrane (glycoproteins or proteins)

l     Receptor sites vary per individual host (susceptibility)

Ø     Penetration

l     Endocytosis to form vesicles or enveloped virus by fusion of envelope to plasma membrane

Ø     Uncoating

l     Lysosomal enzymes of host, enzymes encoded by virus, or enzymes in host cytoplasm

Ø     Biosynthesis: Differs for DNA and RNA viruses

l     DNA virus (Herpesviridae or herpes virus) example

l     RNA virus (Retroviridae or HIV) example

Ø     Maturation

l     Putting the virus particles together

Ø     Dissemination of new progeny

l     Enveloped versus naked

l     Lytic versus non-lytic

Attachment, Penetration, and Uncoating

Ø  Pinocytosis

Attachment, Penetration, and Uncoating

Ø  Fusion

Release of an Enveloped Virus by Budding

Multiplication of Herpes Virus

•            AttachmentàPenetrationàuncoating

•            Early transcription and translation

àTo express DNA synthesis genes

•            DNA replication starts

•            Late transcription/translation starts slightly later

àTo express DNA that codes for capsid and lysozyme

•            Maturation

•            Dissemination

àNon-enveloped viruses are released from cell by lysis

Multiplication of DNA Virus

Multiplication of Retrovirus

•            AttachmentàPenetrationàuncoating

•            Reverse transcription

àRNA template is used to make a complimentary DNA strand

•            DNA strand is replicated to get double stranded DNA

•            Integration

àProvirus form of virus (latent disease)

àViral proteins (leading to virulence) can be expressed here

•            Biosynthesis

àDNA transcribed to make RNA for new virus

àCapsid proteins are made

•            Maturation

•            Dissemination

àViruses are released from cell by budding

Multiplication of a Retrovirus

CANCER

Ø   Cancer caused by viruses often goes unrecognized

Ø   Oncogenes

l    Portion of a genome that, when activated by cancer-causing alterations in DNA, causes malignant tumors

Ø   Oncogenic viruses

l    10% of cancers are virus induced

l     Virus genetic material integrates in host’s chromosome and replicates causing host cells to go through a transformation

l    Transformed cells usually have chromosomal abnormalities, increased growth and express T antigen on their surface

Cancer Today

·         At least six viruses are thought to contribute to cancers :

o    Hepatitis B virus

o    Hepatitis C virus

o    Human Papillomavirus

o    Epstein-Barr virus

o    Kaposi’s sarcoma-associated herpesvirus

o    Human T-lymphotropic virus types I and II

·         80% of viral-associated cancers

o    Cervical cancer (HPV)

o    Liver cancer (HBV and HCV)

Human Papillomaviruses (HPV)

·         HPV infections most common among sexually active adults and adolescents.

·         Detected through Pap smears

o    Reduction in cervical cancer mortalities

·         There are over 100 different types of HPV’s (low, medium and high-risk).

·         Low-risk types cause warts or papillomas (e.g. genital warts).

·         High-risk types cause cervical, vulva, vagina, anus and penis cancers (e.g. types 16 and 18).

Papillomavirus Structure and Genome

·         Small (52-55 nm in diameter)

·         Nonenveloped

·         Icosahedral-shaped

·         Circular dsDNA genome (~8000 bp in length)

Human Papillomaviruses (HPV)

·         Infect stratifying basal epithelial cells

o    Metabolically active

o    Support viral replication

·         2nd vaccine available to prevent a cancer.

·         Merck GARDASIL—licensed by FDA in 2006

·         Quadrivalent vaccine

·         Prepared from VLP’s (virus-like particle) of major capsid (L1) protein of HPV types 6, 11, 16, and 18

·         Saccharomyces cerevisiae

·         Cervarix

PRIONS

Ø  Proteinaceous Infectious particles (proteins)

^Ø       A gene exists in host cell normally as PrP^c

Ø  When host is infected with PrP^Sc (abnormal prion protein) it transforms PrP^c to PrP^Sc

Ø  PrP^Sc aggregates in nerve cells and eventually impairs function and later kills those cells

Ø  Requires dry heat to decontaminate

Prions

PRIONS

·         Inherited and transmissible by ingestion, transplant, and surgical instruments

Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease

Ø   Diseases usually transmitted via contact with nerve tissue

Ø   Examples of diseases causes by PrP^Sc

l    Scrapie in sheep

•   Exact mechanism of transmission among sheep is unknown

l    Bovine spongiform encephalitis in cows (BSE)

•   Possibly transmitted to cows from feed that was fortified with bone marrow from sheep

l    Diseases in Humans

•   Kuru in old New Guinea tribes

l   Cannibalistic rituals—contact with brain matter

•   Creutzfeldt-Jakob disease (CJD)

l   Known cases include

•    Contamination during corneal transplants (CJD)

•    Contamination from eating beef from cows with BSE (vCJD)