Virology essentially refers to — ‘the study of viruses and viral diseases’.
Preamble : Towards the later part of the nineteenth century Pasteur and his co-workers were
vigorously attempting to unfold the precise and exact mechanism of the phenomenon of disease development
by examining meticulously a good number of infectious fluids (drawn from patients) for the
possible presence of specific disease producing agent(s) by allowing them to pass through filters with a
view to retain the bacterial cells. An affirmative conclusion could be reached easily in the event when
the filtrates (obtained above) failed to produce any infection, and the presence of the disease producing
bacterial agent in the original (infectious) fluid.
The following researchers determined the presence of ‘virus’ in pathological fluids in the following
chronological order :
Chamberland (1884) : First and foremost developed the specially designed ‘porcelain filters’
that exclusively permitted the passage of fluid but not the microorganisms ; and, therefore, could be
used gainfully for the sterilization of liquids. Besides, the application of such devices may also suggest
and ascertain if at all ‘infective agents’ smaller in dimensions than the bacteria could exit actually.
Iwanowski (1892) : Repeated the similar sort of test but employed an extract meticulously
obtained from the infected tobacco plants, with ‘mosaic* disease’. Iwanowski observed that the clear
filtrate was found to be extremely infectious to the healthy tobacco plants.
Beijerinck (1898) : He confirmed Iwanowski’s findings and baptised the contents of the clear
filtrate as ‘virus’ (i.e., infectious poisonous agent). He further affirmed that the virus could be propogated
strategically within the living host.
Loeffler and Frosch (1998) : They first and foremost demonstrated that the clear filtrate happened
to be the main culprit, virus, which had the capability of being transmitted from one infected
animal to another. Later on they amply proved that the lymph** obtained from infected animals suffering
from ‘foot and mouth disease’, whether it was either filtered or unfiltered, both caused infection in
healthy animals almost to the same extent. From the above critical studies one may infer that since
animals infected with the filtered lymph served as a source of inoculum*** for the infection of healthy
animals thereby suggesting overwhelmingly that the infective filterable agent never was a toxin****,
but an agent capable of undergoing multiplication.
FW Twort (1915) : Twort inoculated nutrient agar with smallpox vaccine fluid with a possible
expectation that a virulent variant of vaccinia virus could grow up eventually into colonies. In fact, the
only colonies which actually showed up on the agar plates were nothing but bacteria that proved to be
contaminants in the vaccine lymph. However, these bacterial colonies had undergone a transformation
that turned into a ‘glassy watery transparent substance’, which could not be subcultured anymore.
Salient Features of ‘Glassy-Watery Transparent Substance : The various salient features of
the glassy-watery transparent substance are as given under
* Genetic mutation wherein the tissues of an organism are of different genetic kinds even though
they were derived from the same cell.
** An alkaline fluid found in the lymphatic vessels and the cisterna chyli.
*** A substance introduced by innoculation.
**** A poisonous substance of animal or plant origin
(1) When a ‘normal bacterial colony’ was contacted even with a trace of the ‘glassy-watery
transparent substance’, the normal colony would in turn be transformed right from the
point of contact.
(2) Even when the ‘glassy-watery transparent substance’ subjected to a million-fold dilution
it affords transformation as well as gets across the porcelain bacteria-proof filters.
(3) By successive passages from glossy to normal colonies it could be feasible to transmit the
disease for an indefinite number of times ; however, the specific agent of the disease would
neither grow of its own on any medium, nor would it cause the glassy transformation of heat
killed microorganisms.
(4) The specific agent may also be stored for more than 6 months at a stretch without any loss in
activity whatsoever ; however, it would certainly be deprived of its activity when heated to
60°C for 1 hour.
Twort, in 1915, put forward three logical and possible explanations based on his original discoveries,
namely :
(1) The bacterial disease may represent a stage of life-cycle of the bacterium, wherein the bacterial
cells would be small enough to pass via the porcelain bacteria proof filters, and are
also unable to grow on media which actually support the growth of normal microorganisms.
(2) The causative organism (agent) could be a bacterial enzyme that invariably leads to its own
production and destruction, and
(3) The organism (agent) could be a virus that ultimately grows and infects the microorganisms.
It is, however, pertinent to state here that the later two probabilities (i.e., ‘1’ and ‘2’ above)
gained tremendous recognition and turned out to be the hottest topic of various vigorous investigations
inspite of the brief forceful and unavoidable interruptions caused by the World War 1.
F. d’Herelle (1917) : For almost two years the splendid research and observations of Twort
remained unnoticed until the investigations of d’ Herelle-an entomologist who incidentally encountered
during that period a particular transmissible disease of bacteria while investigating the organisms causing
diarrohea in locust. While experimenting with the coccobacilli* d’Herelle observed that the cell-free filtrates
could give rise to ‘glassy’ transformation. Besides, he watched carefully that in the absence of
cocobacilli the agent i.e., ‘glassy-watery transparent substance’ failed to grow in any culture media.
Interestingly, d’Herelle carried out his research absolutely in an independent manner without the least
knowledge about Twort’s findings. His work prominently and emphatically attracted immense and widespread
attention which ultimately paved the way towards the dawn of a relatively more clear picture of
bacterial viruses.
In addition, d’Herelle helped in the discovery of certain earlier preliminary methodologies for
the assay** of bacteriophages.*** It has been duly observed that the lysates displayed practically little
effect upon the inactivated organisms (bacteria), which fact was further looked into and adequately
established that the bacteriophages are nothing but definitive self-producing viruses that are essentially
parasitic on microorganisms.
* Bacilli that are short, thick, and somewhat ovoid.
** The analysis of a substance or mixture to determine its constituents, and the relative proportion of each.
*** A virus that infects bacteria
A. Lwoff (1921) : Lwoff further ascertained and proved the fact that bacteria invariably carry
bacteriophages without undergoing ‘any sort of clearance’, and it was termed as ‘lysogeny
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