The world has broadly witnessed by 1900 and accepted generally that severlal of the recognized
dreadful human ailments were duly caused by various microorganisms. However, the first and foremost
evidence of viruses responsible for causing human disease came into notice in 1892 when Iwanowski
rightly demonstrated that the
cell-free extracts of the diseased tobacco leaves passed through the
bacteria-proof filters may ultimately cause disease in the
‘healthy plants’. Furthermore, such cell-free
filtrates
when cultured upon the bacterial growth media they eventually exhibited practically little
growth thereby suggesting that the said
filtrates contained the actual disease causing agents that are
other than microorganisms. Martinus Beijernick, another scientist reconfirmed the excellent epoch making
findings of Iwanowski.
Twort and d’Herrelle (1915) individually showed the
‘glassy phenomenon’ present very much
in the microorganisms when it was observed clearly and distinctly that the bacterial cells might be
adequately infected with and duly destroyed by the
filterable agents, which in turn caused various
serious diseases both affecting the
animal kingdom and the plant kingdom. Later on, these disease
producing filterable agents are known as
bacteriophages (i.e., the bacteria-eaters).
Wendell M Stanley (1935), an American Chemist, first and foremost isolated the
tobacco mosaic
virus (genus
Tobamovirus) thereby making it possible to perform the chemical as well as structural
studies
on a purified virus. Interestingly, almost within the same time, the invention of the electron
microscope
took place which eventually made it quite possible to visualize the said viruses for the
first time.
The galloping advancement and progress in the in-depth studies on the
viruses across the globe
based duly upon the latest
molecular biology techniques in the 1980s and 1990s have remarkably led
to the discovery of the new dreadful human viruses. In the year 1989, the world has duly acknowledged the
discovery of
Hepatitis C virus, and Pestivirus, which specifically causes acute pediatric diarrhoea. The
year 1993, critically observed the outbreak of a
Hantavirus infection occurring exclusively in the Southwestern
USA, which essentially possesses the potential for new infections to emerge at any time.
Hantavirus
disease refers to the acute ailment related to respirator disease and may even prove fatal.
Bacteriophages
Bacteriophages
designated the ‘last group of viruses’ which were duly recognized and best
characterized. As to date one may have the evidence for the presence of such disease producing agents
that are found to be even smaller in size than the
viruses, and termed as viriods. They usually consist of
the
nucleic acids (i.e., DNA and RNA) exclusively.
Example :
The spindle tuber disease of potatoes is a glaring example of a specific disease
invariably caused by the
viriods.
A good number of
bacteriophages infecting various microorganisms have now been duly isolated,
characterized
, and recognized. The following records the variuos bacteriophages,host(s), particle dimensions (viz., head and tail in nm), structure, and composition adequately.
Viral Species :
A viral species may be defined as ‘a group of viruses essentially sharing the
same genetic information and ecological niche
.
It is, however, pertinent to state here that the particular
epithets for viruses have not yet been
established completely, thereby logically and emphatically the
viral species are duly designated by such
common descriptive nomenclatures
as : human immunodeficiency virus (HIV), with subspecies
duly indicated by a number
(HIV-1).
Standardization of the
‘viral nomenclature’ is now in an active and progressive stage ; and as
such the following specific criteria are being adopted in the latest textbooks and literature alike,
namely :
•
New viral family
•
Genus names
•
Common species names
Growth of Bacteriophages in the Laboratory
It is practically possible to grow the
bacteriophages in two different manners, namely :
(
a) In suspensions of organisms in liquid media, and
(
b) In bacterial cultures on solid media.
Advantages of using Solid Media :
In actual practice, the use of solid media makes it feasible
and possible the
plaque method for the easy detection and rapid counting of the viruses.
Methodology (Plaque Method) :
The various steps that are involved in the ‘plaque method’
are as enumerated under :
(1) Sample of
bacteriophage is duly mixed with the host bacteria and molten agar.
(2) The resulting agar countaining the
various bacteriophages as well as the host bacteria is
then poured carefully into a
Petri-plate adequately containing a hardened layer of the agar growth
medium
.
(3) In this manner, the ensuing
mixture of virus-bacteria gets solidified into a thin top-layer
that invariably comprises of a layer of organisms nearly
one-cell thick. This specific step allows each
virus to infect a bacterium, multiplies subsequently, and helps to release several hundred altogether
new
viruses
.
(4) Nevertheless, these newly generated viruses in turn duly infect other organisms that are present
in the
immediate close vicinity ; and hence, more new crop of viruses are produced ultimately.
(5) Thus, several accomplished
virus multiplication cycles, all the organisms duly present in the
area surrounding the original virus are destroyed finally. In this way, a good number of
‘clearings’ or
plaques
are produced, which may be seen against a “lawn” of bacterial growth upon the surface of the
agar ; whereas, the plaques are observed to form
uninfected microorganisms elsewhere in the Petri
dish
(or Petri plate) undergoing rapid multiplication and giving rise to a turbid background finally.
Note : Each plaque correspond theoretically to a single virus in the initial suspension. Hence, the concentrations
of viral suspensions measured by the actual number of plaques are invariably expressed in
terms of plaque-forming units (pfu).
Bacteriophage Lambda : The Lysogenic Cycle
In a broader and precise perspective the
bacteriophage may conveniently exist in three phages,
namely :
(
a) As a free particle virion,
(
b) In a lysogenic state as a prophage, and
(
c) In the vegetative state i.e., lytic cycle.
One may, however, observe that
virion is inert in nature ; and hence, cannot reproduce.
Salient Features :
The various salient features of the bacteriophage lambda are as stated
under :
(1) In the critical
‘lysogenic state’, the DNA of the phage is duly integrated very much within the
bacterial DNA
. It usually exists in a non-infectious form known as the prophage, and adequately
replicates in synchrony with the bacterial DNA
.
(2) In the corresponding
‘lytic cycle’, the phage particle infects the susceptible host, undergoes
multiplication, and ultimately causes the lysis of the bacterial cell with the concomitant release of the
progeny virus particles.
•
Common names are expressed in regular type viz., herpes simplex virus• Genus names are now usually capitalized and italicized viz., Simplexvirus.
(3) In a situation when the
integrated phage is carefully induced to become the corresponding
vegetative phage
, the lytic cycle comes into being.
(4) Such
phages which specifically give rise to the phenomenon of ‘lysis’ are normally termed as
the
virulent phages, as opposed to such phages that may exist in a lysogenic state and are usually
called as the
‘temperate phages’.
(5) The microorganisms that particularly carry the
‘temperate phages’ are invariably termed as
the
‘lysogenic bacteria’, which are observed to be absolutely immune to the ensuing superinfectioncaused by the same phage.