Chapter 6: Acellular Pathogens

Characteristics of Viruses

• Infectious, acellular pathogens

• Obligate intracellular parasites with host and cell-type specificity

• DNA or RNA genome (never both)

• Genome is surrounded by a protein capsid and, in some cases, a phospholipid membrane studded with viral glycoproteins.

• Lack genes for many products needed for successful reproduction, requiring exploitation of host-cell genomes to reproduce.

• Viruses infect all types of organisms, including microorganisms.

Parvoviruses

All DNA viruses are ds except for

Reoviruses

All RNA viruses are single-stranded except for dsRNA

Viral Structure

Virus Particle

• Covering

  • Capsid

  • Envelope (not found in all viruses)

• Central Core

  • Nucleic Acid Molecule(s) (DNA or RNA)

  • Matrix proteins enzymes (not found in all viruses)

Naked Nucleocapsid Virus

virus composed of a nucleic acid core, either DNA or RNA, surrounded by a capsid.

• More resistant than enveloped virus.

Capsids

(Protein Coat)

• Nucleocapsid

  • Capsid + Nucleic Acid

• Capsomers/Capsomeres

  • Protein Subunits of the Capsid

• Enveloped Virus

• Nonenveloped virus/Naked virus

• Spikes; protein extensions.

Helical

cylindrical or rod shaped

Icosahedral/Polyhedral

three-dimensional, 20-sided structure with 12 vertices

Complex Virus

virus shape that often includes intricate characteristics not seen in the other categories of capsid.

• Atypical viruses

  • Poxviruses

  • Bacteriophages

Poxvirus

Lack a typical capsid and are covered by a dense layer of lipoproteins.

Viral Envelope

• Mostly animal viruses

• Acquired when the virus leaves the host cell

• Exposed proteins on the outside of the envelope; spikes.

• Sensitive to extreme pH, heat, dryness, and simple disinfectant.

Viral Genome

Either DNA or RNA (never both)

DNA Viruses

• usually double stranded (ds)

• Circular or linear

• Eg. Herpes, smallpox

RNA viruses

• Usually single stranded (ss)

• Eg. Influenza, HIV

Virus Classified

• Morphology

• Size of virion

• Genome composition

  • ssDNA, dsDNA, ssRNA, or dsRNA

• Type of host

Virion

inert particle that is the reproductive form of a virus

• Complete infective form of a virus outside a host cell.

Bacteriophages Infection

• Attach with tail fibers attach to cell wall proteins

• Viral DNA injected into host cell

• Uncoating is not required

• Biosynthesis in cytoplasm

• Lysogeny during release

• Host cell becomes lysed.

Animal Viruses

• Attachment sites are plasma membrane proteins

• Capsid enters by endocytosis or fusion

• Uncoating occurs during enzymatic removal of capsid proteins

• Biosynthesis occurs in the nucleus (DNA viruses) or cytoplasm (RNA viruses)

• Lysogeny: Latency; slow viral infection; cancer

• Enveloped viruses bud out; nonenveloped viruses rupture plasma membrane.

Attachment

Many viruses are host specific: attach specifically to receptor sites on the host cell membrane.

Host Range

Spectrum of cells a virus can infect

• Hepatitis B

• Poliovirus

• Rabies

Tissue Tropism

Specific to certain tissues

• Eg. Flu → Respiratory tract only

DNA Virus

Replicate in the Nucleus

RNA Virus

Replicate in the cytoplasm

Genome

The nature of the ____ determines how the ___ is replicated and expressed as viral proteins

dsDNA

Normal flow of genetic information

• DNA → RNA → Protein

ssDNA

Produce dsDNA using host enzymes to process

dsRNA

Translated into protein

+ssRNA

Can be translated directly to make viral proteins

-ssRNA

Must be converted into +ssRNA to proceed

Budding

unequal reproductive division in which a smaller cell detaches from the parent cell

• exocytosis

Lysis

Nonenveloped and complex viruses release when cell dies and ruptures

Persistent infections

Virus present continuously at usually low level over an extended period after the acute infection and disease have ended.

Latent Infections

Maintain viral genome in the host cell without replicating (Latency)

• Herpes simple virus

• Herpes zoster virus

• Epstein-Barr virus

Chronic Infections

Shed the virus continuously for a long period of time.

• Hepatitis C Virus

• HIV → Interfere with immune function

In vitro

Outside living organism in a test tube or artificial environment.

In vivo

In living organism

Cytopathic Effects

Distinct observable cell abnormalities due to viral infection.

• Changes in size and shape

• Cytoplasmic inclusion bodies

• Cells fuse to form multinucleated cells

• Cell lysis

• Alter DNA

• Transform cells into cancerous cells

Transformation Cells

Animal viruses enter the host cell and permanently alter its genetic material resulting in cancer - _______ of the cell

• ______ ____ have an increased rate of growth, alterations in chromosomes, and the capacity to divide for indefinite time periods resulting in tumors.

• Oncovirus

  • Papillomavirus (HPV)

  • Epstein-Barr Virus

Oncovirus

Mammalian viruses capable of initiating tumors

Importance of Viruses

• Cause many diseases in animals, plants, and humans to balance nature.

• Used a a vector in biotechnology (to transfer genes)

• Development of vaccines, phage therapy, etc.

• Major participants in earth’s ecosystem.

Viroid’s, Virusoids, and Prions

Nonliving disease agents quite different from viruses.

• Particles consisting of only RNA or only Protein.

Viroid

An infectious RNA particle, smaller than a virus, lacking a capsid that causes various plant diseases.

Virusoid

(Satellite nucleic acids/viruses)

• Same as viroid; small ssRNA molecule lacking a capsid

• Lacks genes required for the replication; therefore it requires a helper (satellite) virus to replicate

• Causes various plant diseases.

Prions

• Misfolded proteins; no nucleic acid

• Extremely resistant to usual sterilization techniques

• Cause transmissible spongiform encephalopathies

  • Fatal neurodegenerative diseases

Prions Disease

• Scrapie

  • Sheep and goats

• Bovine Spongiform Encephalopathies (BSE)

  • Mad Cow Disease

• Wasting Disease

  • Elk, Moose, Deer

• Creutzfeldt-Jakob Syndrome (CJS) & Kuru

  • Munching on humans