Title: Exercise Seven – Plaque Assay for Determination of Phage Titre
One of the most numerous microorganisms known to man is viruses and these
acellular agents can infect all types of cellular organisms. Viruses are considered
infectious elements that contain a single type of nucleic acid but require a host cell's
metabolism to replicate. They are typified by both an extracellular state and an
intracellular state. (Madigan et al, 2003; Prescott et al, 2002).
The classification of viruses can be extended further, by focusing on a specific
group known as Bacteriophages. These are viruses that infect bacterial cells and use them
as their host. Those viruses that infect Eschericha coli are known as Coliphages and are
identified by type numbers ie. T2, T4 and T7. (Prescott et al, 2002; Reddy, 2002).
Temperate bacteriophages not only infect bacteria but can also establish a
lysogenic relationship rather than immediately lysing their hosts. The DNA of the
invading phage has two possible fates: reproduction leading to generation of new phage
particles and lysis of the cell or integration into the host chromosome. Here it is
replicated passively with the host DNA – a process known as lysogeny. Some coliphages
(although not exclusively) are temperate bacteriophages. (Lehninger et al, 1992; Madigan
Despite the diversity in viral structures, most viruses complete similar stages in
the viral lifecycle. Excluding the lysogenic pathway for the moment, a typical viral
lifecycle can be categorized into five stages:
2. Penetration: by either the whole virion or just the nucleic acid
3. Synthesis of nucleic acid and proteins: where the host's metabolism is utilized
4. Assembly: of structural subunits and packaging
It should be noted that these are generalized stages and that viruses go about a
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