Exercise 10

ISOLATION OF COLIPHAGE FROM RAW SEWAGE


A virus is identified as a small, obligate, intracellular parasite made up of a protein coat enclosing a nucleic acid, which is either DNA or RNA, as its genetic material.


Outside the host, a virus cannot increase in number.


Viruses are host-specific and can be classified based on the organism they infect. (plants, animal, human, bacterial) Viruses that target bacteria are known as bacteriophages (or phages). Abundant Phage populations exist in almost any environment.


Viruses have 2 stages in their life cycle:

  • The extracellular dormant/inactive stage (stage 1)

  • The intracellular reproductive stage (stage 2)


In the second stage, viruses take over the host cell machinery to produce progeny viruses. The rate of viral multiplication varies from virus to virus. Bacteriophages often replicate rapidly, within minutes.


Viral infection steps

  1. All viruses begin infection via absorption to the host via specific receptors and injection of the nucleic acid or uptake of the total viral particle into the cell.

  2. This is followed by an eclipse period wherein no viral cells can be detected within the host because of the release and incorporation of nucleic acid.

  3. Finally, new viral particles are produced, assembled and released from the host via Lysis. In other cases release is by budding off a portion of cell membrane.


Viruses that cause Lysis are called Lytic or Virulent phages. On the other hand, lysogenic or avirulent phages which remain integrated into the host chromosome, persisting indefinitely, is mobilized to cause host lysis only after many generations of host cell division.