Viruses and Virology

• Viruses are a unique group of biological entities known to infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals.
• Viruses, being much smaller than bacteria, fungi, and protozoa, had to be indirectly studied until the 20th century, when they were finally seen with an electron microscope.
• Scientists don’t agree about whether viruses are living or not. They are obligate intracellular parasites.
• Virology is a field of microbiology concerned with the study of viruses and other infectious particles such as prions.
• Complete virus particles are called virions.
• Some viruses, called oncogenic viruses or oncoviruses, cause specific types of cancer.
• A typical virion consists of a genome of either DNA or RNA, surrounded by a capsid (protein coat), which is composed of protein units called capsomeres.
• Some viruses (enveloped viruses) have an outer envelope composed of lipids and polysaccharides.
• Ultramicroscopic size, ranging from 20 nm up to 450 nm (diameter)
• Viruses are not cellular in nature; structure is very compact and economical
• Viruses are inactive macromolecules outside the host cell and active only inside host cells.
• Viruses are infectious particles and not cells.
• They lack organelles and locomotion of any kind.
• Viruses contain large, complex molecules and they can be crystalline in form.
• A single virus particle is called a virion and it is composed of:

  1. A nucleic acid core (DNA or RNA, not both)
  2. A capsid (a geometric protein shell surrounding the nucleic acid) The combination is called a nucleocapsid; a capsid can be helical or icosahedral in configuration
  3. Many are covered by a membranous envelope containing viral protein spikes.
  4. Complex viruses have additional external and internal structures.
• Enveloped virus is enclosed within a lipid membrane; sensitive to heat. Whereas, non-enveloped virus lacks a bilayer lipid membrane; and is heat-resistant.
• Viruses come in many shapes and forms (icosahedral, helical, spherical, or cylindrical).
• Smallest infectious forms range from the largest poxvirus (0.45 mm or 450 nm) to the smallest viruses (0.02 mm or 20 nm).
• Viruses lack enzymes for processing food or generating energy; are tied entirely to the host cell for all needs.
• Components of a typical variation of viruses with helical nucleocapsids:

  1. Hemagglutinin spike
  2. Neuraminidase spike
  3. Matrix protein
  4. Lipid bilayer
  5. Envelope
  6. Nucleocapsid
• Sizes of Viruses:

  1. Megavirus - 800 nm
  2. Pox virus - 400 nm
  3. Herpes Simplex virus - 150 nm
  4. Rabies - 125 nm
  5. HIV - 110 nm
  6. Influenza - 100 nm
  7. Adenovirus - 75 nm
  8. T2 Bacteriophage - 65 nm
  9. Poliomyelitis - 30 nm
  10. Yellow fever - 22 nm
• A = HIV

  B = Hepatitis B

  C = Ebola Virus

  D = Adenovirus

  E = Influenza

  F = Rabies

  G = Bacteriophage

  H = Papillomavirus

  I =Rotavirus

  J = Herpes virus

  
• The life cycle steps of an animal virus are adsorption, penetration, synthesis and assembly, and release from the host cell.
• Bacteriophages are viruses that attack bacteria. They penetrate by injecting their nucleic acid and are released as virulent phages upon lysis of the cell.
• Some viruses go into a latent, or lysogenic, phase in which they integrate into the DNA of the host cell and later may be active and produce a lytic infection.
• The need for an intracellular habitat makes it necessary to grow viruses in living cells, either in isolated cultures of host cells (cell culture), in bird embryos, or in the intact host animal.
• Viruses are identified by means of cytopathic effects (CPE) in host cells, direct examination of viruses or their components in samples, genetic analysis to detect virus nucleic acid, and growing viruses in culture.
• Cell culture lines are started by treating a slice of animal tissue with enzymes that separate the individual cells. These cells are suspended in a solution that provides the osmotic pressure, nutrients, and growth factors needed for the cells to grow.
• Commonly used commercial media:

  1. Dulbecco’s Modified Eagle Medium (DMEM)
  2. Roswell Park Memorial Institute-1640 (RPMI)
  3. Ham’s F12 Nutrient Mixture (F12)
• Culturing animal viruses in a developing bird embryo

  -The shell is perforated using sterile techniques, and a virus preparation is injected into a site selected to grow the viruses.
• In using bird embryo to cultivate bacteria, targets include:

  1. Allantoic cavity – a sac for embryonic waste removal
  2. Amniotic cavity – cushions and protects the embryo
  3. Chorioallantoic membrane for embryonic gas exchange
  4. Yolk sac for the nourishment of the embryo
  5. Embryo itself
• In embryonated eggs, viral growth is signaled by:

  1. The death of the embryo
  2. Embryo cell damage
  3. By the formation of typical pocks or lesions on the egg membranes
• Some animal viruses can be cultured only in living animals, such as mice, rabbits, and guinea pigs.
• Animal inoculation is used in distinguishing between viruses, which produce similar lesion such as foot and mouth disease and vesicular stomatitis of cattle. In such cases calves and horses are inoculated.
• Latent virus infections are viral infections in which the virus is able to hide from a host's immune system by entering cells and remaining dormant.
• Herpes viral infections are latent.
• Antibiotics are not effective against viral infections.
• Antiviral agents are drugs that are used to treat viral infections.
• Examples of ONCOGENIC VIRUSES or ONCOVIRUSES:

  1. Epstein-Barr Virus
  2. human papillomaviruses
  3. human T-lymphotrophic virus type 1
• Human immunodeficiency virus causes acquired immunodeficiency syndrome (AIDS).