Essay on Virus: For Students, Teachers and Researchers

In this essay we will discuss about Virus. This essay is especially written for students, teachers and researchers. After reading this essay you will learn about:- 1. History of Virus 2. Definition of Virus 3. Structure of Virus 4. Classification of Viruses 5. Chemical Composition of Virus 6. Effect of Physical and Chemical Agents of Virus 7. Design and Construction of Virus 8. Viral Entry 9. Targeted Organs by Virus 10. Effect of Virus on Cell 11. Host Resistance to Viral Infection and Other Details.

Contents:

1. Essay on the History of Virus
2. Essay on the Definition of Virus
3. Essay on the Structure of Virus
4. Essay on the Classification of Viruses
5. Essay on the Chemical Composition of Virus
6. Essay on the Effect of Physical and Chemical Agents of Virus
7. Essay on the Design and Construction of Virus
8. Essay on the Viral Entry
9. Essay on the Targeted Organs by Virus
10. Essay on the Effect of Virus on Cell
11. Essay on the Host Resistance to Viral Infection
12. Essay on the Cultivation of Virus
13. Essay on the Temperate and Virulent Phages
14. Essay on the Arthropod Borne Virus (ARBO) Viruses

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Essay # 1. History of Virus:

Many common contagious diseases of animals and man were clinically known since ancient time for which no bacterial aetiology has been assigned. From pre-Christian times Small pox was considered as a deadly infection and Hippocrates (C. 430 BC) described swollen neck in mumps.

In 1884, Pasteur believed that Rabies of dogs is caused by a “microorganism infinitesimally small” as he could not detect bacteria in the infective material of rabid dogs. In 1892, Iwanowsky showed that the mosaic disease of tobacco plants was caused by a minute agent which was so small that it was ultramicroscopic and could pass through pores of bacteria stopping filters.

In 1898, Loeffler and Frosch have shown that foot and mouth disease of cattle was caused by a filterable infectious agent. In 1901, Walter Reed et al proved that yellow fever was caused by a mosquito borne virus. Viruses were originally described as ultramicroscopic and filter passing.

Later, it was shown in animals such as foot and mouth disease and rinderpest in cattle, distemper and rabies in dogs and fowl pest in poultry. Rous in 1911 demonstrated that certain sarcomata of fowls could be transmitted with cell free filtrates of the tumours.

Since that time, it was established that virus can cause tumour in animal and they are called “Oncogenic viruses“. Fibromas, papilloma’s, and related tumours in rabbit and other mammals; mammary carcinomata, leukemia’s and parotid tumours (polyoma virus in mice and leucosis in poultry were caused by oncogenic viruses).

Insects are vectors of viruses which attack man, animals and plants. Nematodes, parasitic to pigs, spread swine influenza virus as in case of plant nematodes. Twort in 1915 and d’Herelle in 1917 observed independently that viruses can infect and cause lysis of bacteria (clear zone of lysis-plaque), these viruses are called as Bacteriophage— a name now abbreviated as “phage.”

This phenomenon of lysis was first noted in Shigella shigae—causative agent of bacillary dysentery. Because of this, knowledge of mechanism of phage infection and reproduction is much advanced than knowledge of the corresponding mechanism of animal viruses.

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Essay # 2. Definition of Virus:

Virus (Vim, poison) were earlier defined as an ultramicroscopic agent which can be seen only under electron microscope, and can pass through the bacteria stopping fillers, reproduce by replication, can grow only in living cells, resist the action of antibiotics and contain Ribonucleic acid (RNA) or Deoxyribonucleic acid (DNA) covered by a protein coat.

Luria, in 1967, produced a composite definition incorporating all the accepted essential features of viruses.

A virus is one of a group of minute infections agents characterised by a lack of independent metabolism and by the ability to replicate only within living host cells.

They range from 15-300 run in size and are morphologically, heterogeneous (Dorland’s Medical Dictionary).

“Viruses are entities whose genome is an element of nucleic acid, either DNA or RNA, which reproduces machinery to direct the synthesis of specialised particles, the virions, which contain the viral genome and transfer it to other cells.”

Following are two cardinal features of viruses:

1. They possess the genetic material which, when it is within the host cell, behaves as a part of the cell;

2. They also exist in an intracellular form, which is the product of the host cell under the control of the virus itself. The extracellular form—the virion—serves as a vehicle to carry the viral genome to other cells.

Viruses can reproduce only when they are within their host cells and are of necessity intracellular parasites. They have no ribosomes, no enzymes to generate high energy bonds, no mitochondria. They lack a rigid cell wall.

Thus, there is no muramic acid in their outer coatings, hence they are unaffected by antimicrobial agents. Indeed, viruses have no real cell structures and it is difficult to regard them as microorganisms. True microorganisms multiply by binary fission.

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Essay # 3. Structure of Virus:

A complete infective virus particle is called virion and the genome is in the core and consists of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) but not both; the outer shell of protein covering the nucleic acid is known as capsid.

The protein and nucleic acids are closely integrated to form a nucleoprotein of distinct symmetry. When a virion enters the host cell, its protein capsid is stripped off and its nucleic acid is liberated within the host cell.

At this stage, it ceases to exist as a particle although its nucleic acid is still present and intact. Component parts are still detectable. This stage in viral multiplication is known as the eclipse phase. All true viruses have an eclipse phase in their reproductive cycle (Fig. 48.1).

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Essay # 4. Classification of Viruses:

Animal viruses may be arranged into groups—the names of these groups and in the suffix—Virus, e.g. adeno virus, herpes virus and pox virus. Members of these groups share similar features and constitute families ending with the suffix—idae, i.e. the family Herpes viridae would have as its type genus Herpes virus which is a group that contains the Herpes simplex, varicella zooster.

Following are the important distinguishing features of animal viruses:

1. Type of nucleic acid;

2. Chemical composition;

3. Susceptible to chemical and physical agents;

4. Size and measurement;

5. Design and construction of virus;

6. Antigenic characters.

Deoxyribo viruses at present are placed in five groups (Pox virus, Herpes virus, Adeno virus, Papova virus, Parvo virus) and ribo viruses in nine groups (Orthomyxo virus, Paramyxo virus, Rhabdovirus, Togavirus, Arenavirus, Reovirus, Picornavirus, Leukovirus, Coronavirus). There are many viruses that do not fit neatly into these groups. They are known as Arbo viruses, because they are transmitted by the bite of arthropods.

Types of Nucleic Acid:

DNA or RNA carries in their nucleic acids the genome and infectivity of viruses. Most deoxyribo viruses have linear double stranded DNA; whereas, in the parvo viruses, the molecule is single stranded RNA. When the protective capsids of the viruses are removed, some fragile viral nucleic acid retains their infectivity for some time, they can be seen and measured under electron microscope.

Most of them are linear but some are circular.

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Essay # 5. Chemical Composition of Virus:

Proteins are the main components of all viruses. The most important function of these proteins is to provide the shield for the underlying delicate nucleic acid molecules. The surface proteins of the virion have special affinities for specific receptor sites on the host cell and provide a means of attachment of the particle to a position where it can enter the host cell and initiate the process of infection.

Viral strength, which are protein in nature, stimulate the host immune response during infection. They are responsible for the distinctive serological characteristics of the viruses. The capsomeres of most pathogenic viruses contain several different polypeptides. These polypeptides make up the surface configuration.

They constitute the haemagglutinin prickles of ortho and paramyxo viruses and form a part of the mushroom shaped projections of neuraminidase subunits.

Viral Enzymes:

There are many enzymes coded in the nucleic acids.

Lipids:

When some mature viruses are liberated from nuclear and cytoplasmic membrane, they acquire a surface layer of lipoprotein derived in part from the host cell. The lipids may cement the proteins in the envelopes of the viruses. These viruses are inactivated by lipid solvents (ether, chloroform, bile salt).

Carbohydrates:

Some viruses contain minimal amounts of carbohydrates.

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Essay # 6. Effect of Physical and Chemical Agents of Virus:

Heat and Cold:

The viruses causing the diseases in man and animals are readily inactivated by moderate heat (56°C to 60°C for 30 minutes) except serum hepatitis and poliomyelitis viruses. Viruses are resistant to extremes of cold; freezing at – 35°C or – 70°C is satisfactory common method for their preservation in the laboratory.

The majority of viruses can be well preserved by freezing drying method, pH variation. Viruses remain viable within a range of pH 5 to 9, but they are destroyed by extreme acidity or alkalinity.

Tricidal Agents:

The most efficient disinfectants for use against viruses are oxidising agents (hydrogen peroxide, potassium permanganate and organic iodine derivatives). Chromaldehyde (a reducing agent) is slower in action but is valuable in defined concentration in the preparation of inactivated poliomyelitis vaccines.

Glutaraldehyde has an important role in the disinfection of apparatus used in renal dialysis units. Phenol and certain cresol disinfectants (i.e. lysol) are active against only a few viruses and are not to be recommended for material contaminated with the poliomyelitis or small pox viruses.

Acyclovir, amantadine, idoxuridine, trifluridine, vidarabine, ribavirin and azidothymidine are currently licensed antiviral agents. However, ideal antiviral agents remain to be developed.

Ether:

Viruses containing lipids in their envelopes are sensitive to ether. Naked viruses (Polio, echo, coxsackie, adeno, papova and parvo viruses and some pox viruses) are resistant to ether, whereas herpes, orthomyxo and paramyxo, rhabdo, corona and leuko viruses are inactivated by ether.

Vital Dyes:

Acridine dyes, neutral red and toluidine blue permeate the nucleic acid and render virions vulnerable to ultraviolet rays. Herpes simplex and vaccinia viruses are more severely damaged then polio viruses. Double stranded DNA viruses stained with acridine dyes fluoresce with a yellow colour when seen under electron microscope while single stranded DNA viruses fluoresce red.

Antibiotics and chemotherapeutic agents have no effect on viruses, but the agents of psittacosis lymphogranuloma group are susceptible to these drugs.

The unit measurement of whole virus particle is the Nano-metre (µm) or millimicron (mµ) or one thousandth part of a micron (µ) or micro-metre (µm) or 0.000001 mm or it is 10^-9 metre; whereas the component parts or the internal structure (capsomere) of the virus is expressed as Angstrom (A°) which is one tenth of a millimicron (0.000001 mm).

The Angstrom is the unit used to measure the wave length of light in microscope.

Virions vary in diameter from 300 to 18 nm. The largest ones are nearly half the size of small bacteria and can just be seen under an optical microscope. Most viruses can be seen only under electron microscope. Largest virus is vaccinia virus (330 x 100 nm), smallest one is picorna virus and parvo virus (40-20 nm).

It is customary to include in a suspension of purified viruses some latex particles of known size (e.g. 88 nm) in electron microscope, so in electron micrographs, the size and shape can be determined accurately; vaccinia virus is brick shaped; influenza virus is round or filamentous; bacteriophage is sperm-like with head and tail.

Formerly, the size of viruses was measured by their capacity to pass through the filters or by their rate of sedimentation in a suspended fluid.

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Essay # 7. Design and Construction of Virus:

A virion (the old term which is still in use) in its simplest form is a single molecule of nucleic acid enclosed within a protein capsid. It was known as “elementary body”. The shape taken by the capsid is that of rigid spherical cage or of a tube; mainly the capsid protects the nucleic acid. Viral capsids are made up of large number of “morphological units” or capsomeres” that are attached to each other by their bonds.

Cubical Capsids:

Viruses with cubical symmetry take the form of an icosahedron. An icosahedron has 20 facets, each an equilateral triangle and 12 vertices or corners.

Virus Haemagglutinin:

Many different viruses agglutinate red blood cells when viruses and erythrocytes are in a suspension, they collide and adhere to each other, the cells become speckled with attached virions and they are bound to each other by viral bridges.

Inclusion Bodies:

During the course of multiplication, the viruses are associated with the appearance of large distinctive structures known as “inclusion bodies”. They may be situated either in the cytoplasm (intracytoplasmic inclusion bodies) or in the nucleus (Intra-nuclear inclusion bodies)—as in the case of measles—in both.

The inclusion bodies are usually acidophilic and appear as pink masses in smears stained with Giemsa stain or Mann’s eosin methylene blue stain; basophilic inclusion bodies are characteristic of psittacosis- lymphogranuloma group. They vary in size from 1-30 p in diameter.

The presence of large intracytoplasmic acidophilic inclusion bodies in the nerve cells of hippocampus of the brain is the presumptive diagnosis of Rabies.

Acidophilic Intra-nuclear inclusion bodies are of two types:

Cowdry Type A is granular in appearance and of variable size (herpes simplex, zoster varicella, yellow fever). Type B is more round and multiple (Poliomyelitis, adenovirus).

Virus Tropism:

Viruses are classified according to their particular tissue predilection as dermotropic, neurotropic, pneumotropic, enterotropic and viscerotropic.

Viral Pathogenicity:

When the virus multiplies in the cell, it may spread from cell to cell producing a small focal lesion (Wart or molluscum contagiosum). When the viruses are deposited on the mucosa membrane of respiratory tract, they infect, multiply inside the mucosa and spread over a large area producing common cold; when trachea and bronchi are infected, influenza ensues.

Pathogenesis of viral infections depends upon the viral interactions and host factors:

1. Entry of virus into host cell

2. Multiplication of virus and spread

3. Effect of virus on cell functions

4. Host immune response

5. Viral establishment and infection

6. Viral shedding.

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Essay # 8. Viral Entry:

Viruses enter the host through skin, respiratory, gastrointestinal, urogenital route or conjunctiva. Mostly they enter through respiratory mucosa (e.g. chicken pox) or gastrointestinal mucosa (e.g. Polio virus).

Virus may be introduced into the blood stream by needles (Hepatitis B; HIV) or by insect bite (Arbo virus) Congenital infection of Rubella and Cytomegalo virus may lead to maldevelopment or severe neonatal disease. Mammary tumour viruses of mouse are transmitted through breast milk.

Milk viruses (small pox, chicken pox, entero virus) produce lesions in organs distant from the site of entry. After primary multiplication at the site of entry these viruses pass along the lymphatic’s to local lymph nodes. After further multiplication in lymph nodes, they enter the blood stream causing primary viraemia (short-lasting), then they spread along blood to “central foci” (liver, spleen).

They reenter blood stream after extensive multiplication in central foci, thus resulting into secondary viraemia with onset of clinical signs. The viruses are spread into target organs during secondary viraemia. The distinctive lesions are produced in these organs after their further multiplication in these target organs.

These changes are variable from one species of virus to another. The incubation period is the time taken by the viruses to enter the body and to reach the target organs. Rabies virus spread by neural route.

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Essay # 9. Targeted Organs by Virus:

Main target organs are skin and central nervous system; other organs are also involved. Generalized viral infections with skin rash.

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Essay # 10. Effect of Virus on Cell:

Viruses have cell and organ specificity. They act on target cell surface receptors (cell tropism).

As a result of virus and host cell interaction, there may be:

(a) cytotoxic or cytolytic growth;

(b) non-cytocidal productive growth;

(c) abortive infection and

(d) cell transformation.

(a) In cytotoxic or cytolytic growth, viruses replicate inside host cells, release progeny viruses which ultimately infect new cells, produce the disease after destroying target cells by their cytopathic effect (CPE) in the form of inclusion bodies they produce and cell necrosis. This is referred to as cytolytic cycle and the susceptible host cells are termed permissive cells.

(b) In non-cytocidal productive growth, the viruses infect the host cells and do not destroy or kill the cells during replication without affecting the host cell metabolism. The infection is productive but non-cytolytic (e.g. this type of co-existence of virus and host cells can be demonstrated by Influenza virus growth in monkey kidney cell culture).

This co-existence may result in persistent infection which may be latent, in-apparent, subclinical or may progress to chronic infection.

(c) In abortive infection, the viruses infect cells but do not replicate and there is no loss of cell function.

(d) Transformation virus infects the cell and its nucleic acid gets integrated in the host cell DNA leading to cell transformation. The virus is not generally released from the infected cell.

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Essay # 11. Host Resistance to Viral Infection (Viral Immunity):

The host can resist the viral infection by

(A) Immunological, or

(B) non-immunological response (non-specific) response.

(A) Immunological Response:

After primary infection or immediately, an immune response develops in immunized host against viral infection. It may be antibody and cell mediated immune response. Antibody plays an important role as the principal immuno-defence for some diseases (entero and arbo virus infections), while in others (HSV, CMV), cell mediated immunity (CMI) plays a more important role.

(I) Circulating Antibody:

In an immunized individual, IgA may limit viral attack on mucosal surface and serum Ig G may act on lower respiratory tract. Ig M and Ig G cannot dislodge the virus from the site of primary entry in an immunized host. Both prevent the spread of some virus (polio virus) to target organ and also help in recovery from viral disease.

The degree of protection is related to the level of neutralizing antibody in the blood.

Ig M and Ig G antibodies may act in vivo in the following ways:

(a) Neutralisation of virus may prevent attachment, penetration and subsequent events.

(b) Immune opsonisation of virus for phagocytosis and destruction of virus by macrophages and neutrophils.

(c) After interaction with virus on cell membranes, the antibodies render the infected cells susceptible to complement dependent destruction by killer lymphocytes (K cells) or by phagocytosis.

(II) Cell Mediated Immunity:

This immunity plays an important role at a later stage of infection. It prevents infection of target organs and promotes recovery from the disease. Infected cell surface contains virus-specific antigens. Activated T-cells release lymphokines. Recruited activated macrophages destroy some viruses better than macrophages of infected persons. Cytotoxic T-cells also destroy infected target cells.

(B) Non-Specific Defence against Virus:

This non-specific defence also plays an important role in adults than in young animals, which involve three factors: humoral, cellular, and environmental factors which may be present before infection or induced by the virus:

(a) Humoral factors may be (i) humoral inhibitors, or (ii) Interferon.

(i) Humoral inhibitors contain non-specific inhibitors (lipid and non-lipid) that inhibit attachment of virus to cell receptors. When viruses attach to cells, cell nuclease may remain free in the cytoplasm which may destroy the viral nucleic acid.

(ii) Interferon virus phagocyte interaction leads to production of interferon and endogenous pyrogen which mediates fever and sometimes affects replication of virus.

Three human interferon’s are recognised: a produced by leucocytes; β-by ‘fibroblasts; and y by T-cells. α and β interferon’s are potent antiviral agents is an immuno-modulator. If IFN α and β induce several biochemical reactions with the help of three enzymes (synthetase, RNase, protein kinase), each of them inhibit synthesis of viral protein, but do not affect host protein synthesis.

(b) Cellular Factors:

(i) Polymorphonuclear Leucocytes (PMN):

Migrate to the site of viral infection, adsorb to the virus and ingest some viruses. They help the body defence by inhibiting viral growth or by destroying them, though viruses inhibit chemo taxis.

(ii) Mononuclear Phagocytes (MN):

Both blood and tissue macrophages ingest and destroy some viruses; some may resist ingestion. Sometimes ingested viruses may resist killing and replicate intracellularly.

(iii) Environmental Factors:

Certain viruses inactivated non-specifically by lymphocytes and certain virus infected cells are killed by natural killer (NK) lymphocytes without the help of antibody. Thus they act non-specifically.

Transmission of Virus Infection:

Inclusion conjunctivitis is transmitted by the direct contact between persons during intercourse from one person to another. Later it reaches the eyes of the newborn at it passes through the infected birth canal. Polio viruses and other entero viruses may be transmitted after the ingestion of food or drink.

Interferon:

Among animal viruses, interference of one virus with the reproduction of another may be mediated by a protein called “interferon” which is produced by the cell after infection with the first virus. Interferon is a protein with a molecular weight of 30,000 Daltons. Because of its size, the interferon is released from cells and can diffuse in the body throughout the extracellular fluids.

Types of Interferon:

There are three antigenically and chemically distinct types of interferon:

(1) α-interferon (IFN-α)—produced by monocytes and B lymphocytes.

(2) β-interferon (IFN-β)—produced by fibroblasts and epithelial cells.

(3) y-interferon (IFN-y)—a lymphokine produced by T-cells.

IFN-α and IFN-β are produced only by cells in response to presence of viruses and certain intracellular bacteria; but IFN- is produced from antigen activated T-lymphocytes.

Mechanism of Action:

Interferon’s inhibit viral replication and are also able to modify immune response. They also act as anti-tumour agents.

Interferon binds to specific cell surface receptors. Both IFN-α and IFN-β share a common receptor; while IFN- binds to its own specific receptor.

Properties:

1. Species Specific:

Interferon produced by one species can protect only cells of the same species against viral infection.

2. Non-Specificity:

Interferon activity is not virus specific. Interferon induced by non- virus renders cell resistant to infection by the same broad spectrum of viruses.

3. Virus Susceptibility:

Though all viruses are inhibited by interferon, they vary in their susceptibility to interferon e.g. RNA viruses are more susceptible to interferon than DNA viruses.

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Essay # 12. Cultivation of Virus:

Viruses can be grown in

(1) The animal;

(2) Chick embryo (Fig. 48.2),

(3) Tissue culture.

 

1. Animal:

Many animals and man are susceptible to rabies. Certain species (skunks, opossums, fowls) are resistant. Mice are the animals of choice for the experiment at Rabies infection.

Animal can be inoculated by any route, intra-cerebral intoculation may cause encephalitis and the animal dies in 5-30 days.

Viruses freshly isolated from natural infection, which have not undergone any modification in the laboratory, is known as “Street” virus. They cause fatal encephalitis in the laboratory animals (with an incubation period of 21-60 days in dogs). They also produce intracytoplasmic inclusion bodies (Negri bodies) in brain.

After several serial intra-cerebral passage in rabbits some strains of rabies virus are adapted to laboratory animals. They are called “fixed virus“. These fixed viruses can cause fatal encephalitis if inoculated intra-cerebrally with a short incubation period of 4-6 days.

As they are Neutro-neurotropic, they are less effective by other routes and do not produce Negri bodies in most of the cases. White mice (infant mice) are animal of choice for cultivation of virus. Toga and coxsackie viruses grow only in suckling infant mice after inoculation by intra-cerebral or intranasal routes.

Animal Inoculation is still carried out for:

(a) Primary isolation of certain viruses and for studies of

(b) The pathogenesis of viral diseases and of

(c) Viral oncogenesis.

Other Animals:

Monkeys can be used for selective viruses, e.g. viral hepatitis. Animals can be inoculated by intra-peritoneal, subcutaneous intra-cerebral or intranasal routes. After signs of disease or death, the animals should be sacrificed and their tissue are tested for the presence of virus by microscopic examination of stained smear for inclusion bodies and by neutralisation and haemagglutination inhibition (NI) tests.

2. Chick Embryo (Embryonated Egg):

After incubation in the incubator, the hen’s egg embryos of 7 to 12 days are used for cultivation of viruses.

They can be Inoculated by:

(a) Chorioallantoic membrane (CAM);

(b) Allantoic cavity,

(c) Amniotic cavity route, incubated and examined daily for virus growth.

(a) Chorioallantoic Membrane (CAM):

Pox viruses can grow on CAM, produce visible lesions (pocks) which indicate replication of single virus. The morphology of pocks is variable according to different viruses.

(b) Allantoic Cavity:

Influenza virus, yellow fever (17 D strain) and rabies (flurry strain) vaccines were produced by inoculating allantoic cavity. Duck’s egg yields more viruses than hen’s egg.

(c) Amniotic Sac:

Primary isolation of influenza virus is done in this amniotic sac.

(d) Yolk Sac:

Some viruses, some bacteria (chlamydiae and rickettsiae) are inoculated in yolk sac.

3. Tissue Culture:

Small pieces of organs (organ culture), fragments of minced tissue (explant culture) from man and animal are inoculated into tissue culture growth medium. This medium is balanced salt solution and contains 13 essential amino acids, glucose, salts, buffering system, protein supplement (lactalbumin hydrolysate), calf serum (5%), antibiotics (penicillin, streptomycin) and phenol red (indicator).

Cell Culture:

Tissue is trypsinised and the dissociated cells are washed, counted and suspended in a growth medium. These cells undergo a very slow cell division and are distributed in tubes, bottles, Petri dishes. Fibroblastic cells adhere and grow on glass surface of test-tubes, bottles. They grow on incubation and spread out on the glass surface to form a confluent monolayer sheet of cells in a week period.

Types of Cell Culture:

On the basis of their origin, chromosomal characters and the number of generations through which they can be maintained, cell cultures are classified into three types:

(a) Primary Cell Culture:

Normal fresh cells are cultured and get attached to the glass surface and form a confluent monolayer of cells by their mitotic division. These primary cell cultures are commonly used for primary isolation of viruses and in preparation of vaccine. They are monkey kidney cell and human amnion cell culture.

(b) Semi-Continuous Cell (Diploid Cell Strain) Culture:

These fibroblasts, having same number of chromosomes as in parent cells, are diploid, grow rapidly and can be sub-cultured for a limited time, they undergo “senescence”—hence they are lost. These fibroblasts are derived from embryo tissue (human embryo lung cell), are susceptible to large number of human viruses, and are also used for primary isolation of some viruses and production of viral vaccine.

(c)Continuous Cell Culture:

These cells of single type are able to grow infinitely in vitro, are derived from cancer cells, grow faster and their chromosomes are haploid. They can be cultivated indefinitely, they are called “continuous cell lines”. They are derived from cancer cells e.g. HeLa cells. These cells are from cervical cancer of a lady, HeLa; hence the name HeLa.

Other cells are Hep 2; KB cells. These continuous cells can be stored in deep freeze at – 70°C or sub-cultured and used for primary viral isolation but not for preparation of viral vaccines because they are considered unsafe for human use as they are cancer cells. Rhesus monkey kidney cell culture are very commonly used as they support the growth of many viruses.

Recognition of Virus Growth in Cell Culture:

Viral growth can be detected by:

(1) Cytopathic Effect (CPE):

Microscopical examination of culture will reveal cell degeneration or death caused by viruses. This CPE can help to identify the viral isolates:

(a) Syncytium Formation:

This syncytium (multi-nucleated giant cells) due to fusion of monolayer cells can be noticed in measles virus.

(b) Cell necrosis and Lysis:

It can be caused by entero viruses.

(c) Cellular Clumping:

This resembles clusters of grapes (but no fusion) which is produced by adeno viruses.

(d) Inclusion Bodies:

These are virus-specific intracellular globular masses produced during viral replication. They are visible under light microscope after staining as acidophilic or basophilic inclusion.

(e) Discrete Focal Degeneration:

It is characteristic of herpes virus infection.

(f) Transformation:

Tumour or oncogenic virus can transform cells into “micro-tumours“.

(2) Haemadsorption:

When guinea pig erythrocytes are added to tissue culture infected by haem-agglutinating viruses (orthomyxo, paramyxo and toga viruses), these erythrocytes will adhere to the infected cells. This haemadsorption can be Blocked by the addition of specific antiserum against these haem-agglutinating viruses.

(3) Immunofluorescence (Direct or Indirect):

It can detect viruses in infected cells by antiviral serum labelled with a fluorescent dye.

(4) Interference:

Rubella virus multiplies within the host cells and it produces direct cytopathic changes. These host cells are not susceptible to super-infection with CPE producing virus due to viral interference.

(5) Neutralisation Test:

Known antiviral serum is allowed to react with unknown virus patients. Specific antiserum neutralizes virus activity and the consequent mixture fails to produce CPE. This test can be used to type the haemadsorption viruses. Microscopical examination of culture will reveal cell degeneration or death caused by viruses. This CPE can help to identify the viral isolates.

Bacteriophage: (Fig. 48.3):

The structure of bacteriophage is complex and sophisticated than that of virus. The basic design is sperm shaped with a polyhedral head and a cylindrical tail.

The head of T₂ phage of E. coli is bi-pyramidal hexagonal prism and consists of an outer shell of about 1,000 subunits of a protein with a molecular weight of 30,000 and an inner central mass of DNA. The tail is about 100 mµ long and 25 mµ wide and is composed of contractile sheath surrounding a central hollow core.

At the distal extremity of the tail, there is a hexagonal plate to which six fibres are attached. The sheath of the tail contracts less than half of the original length. The central core extrudes through the bacterial wall to enable the transference of DNA to the host cells. The epidemiology of the disease can be studied by bacteriophage typing.

The cholera phage typing method has obvious epidemiological uses in tracing the epidemic spread of cholera.

The genome of most bacteriophages contain double stranded DNA (T-even phages of E. coli, T₂, T₄, T₆, T₃, T₅, T₁₇) and lambda phages of E. coli; P₂₂ of Salmonella). Some contain single stranded DNA (S_J3, M₁₃ phages of E. coli); some others contain only single stranded RNA (mu, M₅₂ of E. coli).

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Essay # 13. Temperate and Virulent Phages:

There are two types of bacterial phages—virulent or temperate phage. Infection of a bacterium with a virulent phage results in replication of virus and subsequent release of virus particles causing death and lysis of the host cell.

On the contrary, temperate phage persists indefinitely in a quiescent state (pro-phage state) without phage production. When in the latent state, the phage genome is called pro-phage. Since phage DNA is integrated in the host cell chromosome, the pro-phage multiplies synchronously with bacterial DNA as an inheritable entity.

The pro-phage acts as an additional element of chromosome and expresses new characters. The state of indefinite persistence of phage DNA in host cell without phage production is called lysogeny and bacteria carrying pro-phage are called lysogenic; only double stranded DNA phage can be integrated in host cell chromosome as pro-phage.

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Essay # 14. Arthropod Borne Virus (ARBO) Viruses:

The term arbo virus is used to denote a large number of viruses transmitted by arthropod. These viruses are taken in by the arthropods, while sucking blood of man or animal. These viruses, multiplying within the body of the arthropods without causing damage to the tissues of the arthropod, produce the disease in man.

Slow and Oncogenic Viruses:

They have in common the long continued association with the host. In general, the associated diseases occur sporadically and they exceptionally assume epidemic features (slow virus infections, Scrapie, kuru, Animal virus tumours).

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