Introduction

• Virology (study of virus) and Mycology (study of fungi) are specific branches of Microbiology
• Microbiology is one of the most applied of all biological sciences
• Microbiology is the study of microorganisms that are usually too small to be perceived by the naked eye
• An organism with a diameter of 1mm or less fall into the broad domain of microbiology
• Viruses


  Very small sized etiological agents of disease that are capable of passing through filters that retain even bacteria, increase only in the presence of living cells and give rise to new mutations
• Viruses


  Infectious nucleoproteins, the term is derived from the Latin language meaning 'poisonous liquid or poison'
• Facts Related to the Discovery of Viruses

  • Mayer (1886) showed that the juice from the infected tobacco plants could produce a disease if applied to healthy plants
  • Dimitri Ivanowski (1892) demonstrated that the causal organism of tobacco mosaic could even pass through the finest porcelain filter that withholds bacteria
  • Beijerinck (1898) showed that causal agent of tobacco mosaic could diffuse through an agar membrane and was therefore liquid in nature
  • D. Herelle (1917) discovered the bacteriophage (virus that parasites on bacteria)
  • Schelsinger (1933) was the first to determine the decomposition of virus
  • Bowden (1964) defined viruses as 'submicroscopic, infective entities that multiply only intracellularly and are potentially pathogenic'
  • Hahon (1964) said 'Viruses are bits of infectious heredity in search of a chromosome'
  • Stanley (1933) crystallized the virus causing tobacco mosaic disease
  • Hershey and Chase (1952) studied the T2 bacteriophage and demonstrated that the genetic information is carried in the phage DNA and that infection is the result of penetration of viral DNA into the cell
  • Gierrer and Schramm (1956) discovered that the nucleic acid fraction of the virus is the actual infectious agent
  • Schafferman and Morris (1963) isolated phage from blue green algae
  • Sinden (1957) discovered mycophage in mushroom (Agaricus bisporus)
• General Characteristics of Viruses

  • They do not occur free in nature but act as obligate intracellular parasite
  • They have an extremely microscopic structure which can only be seen by electron microscope
  • Their size ranges from 100-2000 millimicron
  • They can not be filtered by bacterial filter
  • Their genetic material is either DNA and RNA which occurs in the form of single molecule and can be single or double stranded
  • A single virus particle is known as virion which lacks functional autonomy
  • They lack their own enzyme system but interact with the host enzyme system and synthesize new virus particle
  • Their outer capsid is proteinaceous and harmless and provides cellular specificity to the virus
  • They are intracellular obligate parasite and can't be cultured on artificial culture media
  • All animal and plant viruses have a narrow host range while others show a broad range
  • They show replication
  • They are highly infectious and spread disease very quickly
  • They show special kind of pathogenicity (i.e. they cause disease at a particular temperature)
  • They are haploid
  • They are not affected by antibiotics
  • They show life between 5-9 pH
  • They remain active for a long time when kept in 50% glycerol solution
  • The extract of virus become inert at high pressure and high sound frequency
  • They get precipitated with ethyl alcohol and acetone
  • They can be inerted by treatment with ultraviolet rays, pyridine urea and hydrogen peroxide
  • They can be crystallized
  • They show response towards temperature, radiation and chemical substances
  • They lack cell wall, nucleus, protoplasm and cell organelles
• Difference between Virus and Bacteria

  • Viruses do not possess any cellular organization
  • Viruses do not grow on inanimate media
  • Viruses do not multiply by binary fission
  • Viruses do not possess both DNA and RNA together
  • Viruses do not possess ribosome
  • Viruses do not show any sensitivity to antibiotics
  • Viruses show sensitivity to interferon
• Virus as Living Entity

  • They show growth and multiplication (only inside the host cell)
  • They have genetic material i.e. DNA/RNA
  • They can direct protein synthesis (though they use host machinery)
  • They show mutation
  • They can be transmitted from the diseased host to the healthy ones or the ability to infect
  • They react to heat, chemicals and radiation and also show irritability, a character of only living organism
  • They possess genetic continuity and have definite races/strains
  • They have similarity between nucleoproteins with the protein and nucleic acid of living organisms
• Virus as Non-Living


  • They can be crystallized
  • They have an inert chemicals outside the host cell
  • A cell wall or cell membrane of any type is absent in viruses
  • They do not show functional autonomy
  • They do not respire or excrete or they do not show any sign of metabolism
  • They lack any energy producing enzyme system