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Virus

 

Introduction
Are Viruses Living or Non-Living
Structure and Chemical Composition

Nucleic Acids

Plant Virus
Animal Virus
Bacteriophage
Cyanophages

 

Introduction

The inhabitants of microcosm have a profound influence on human life. They are both friends as well as foes of man. While many have a prominent role to play in human welfare, others have caused immense harm to human life. Among the most important of microbial enemies of human beings, Viruses occupy a prominent place. Simple and tiny enough to pass through the minutest bacterial filters yet potent enough to change the destiny of human life, viruses are today the centre of attention in causing one of the deadliest of human diseases. What are these viruses? When were they discovered? What do they look like? How do they bring about the diseases? We will try to find answers to some of these in this chapter.

Discovery of Viruses:

Adolf Meyer (1886), a Dutch Agricultural Chemist observed a mottling disease in the leaves of tobacco plant and named it Mosaikkranket (mosaic). He was able to demonstrate that the mosaic disease could spread from plant to plant if the juice of the infected leaves is applied to healthy leaves. He also showed that the juice even after passing through double filter paper retained the infectivity. He further demonstrated that heating the juice to 80°C would render it ineffective. He therefore concluded that bacteria are responsible for the disease.

A further step in finding out the causative agent of tobacco mosaic disease was taken when Iwanowsky (1892) a Russian scientist showed that the juice even after passing through the fine bacterial filter (chamberland filter candles) retained the capacity of infection. This means then bacteria are not responsible for the disease. But when the filtered sap was put on the culture medium no living organism grew. The filterable agent was called a Virus. The term virus was however first introduced by Louis Pasteur to indicate the cause of canine rabies. But it was a general term used for various infectious agents.

Beijerinck (1898) a Dutch microbiologist showed that the infective agent 'Quid diffuse into agar gel like a liquid and he called it contagium vivumfluidum (contagious fluid).

Fig..1 Iwanowski's discovery of the filterable virus. Infected tobacco leaves were crushed and filtered through Chamberland's bacterial filter (b). The cell debris applied to healthy leaves without effect (c), when the clear filtrate was placed on the leaves (d) the leaves shriveled and died. This experiment showed the presence of a "filterable virus" that was smaller than any known bacteria.

Loeffler and Frosch (1898) showed that the infective agent of foot and mouth disease in cattle (like tobacco mosaic agent) passes through bacterial filter and neither it can be seen under the microscope nor it can be cultured on culture media.

Viruses attacking bacteria were first discovered by British scientist Twort and later in more detail independently by French scientist d'Herelle, who named them bacteriophage. Subsequent studies showed the existence of hundreds of other viruses in plants, animals and human beings.

Schelsinger (1933) was the first to study the chemical composition of virus. He showed that the bacteriophage consisted of only nucleic acids ant proteins. A few years later Stanley (1935) isolated tobacco mosaic virus if paracrystalline form. Later Bawden and Pirie (1938) identified the chemical nature of TMV and other viruses. Gierr and Schramm (1956) showed that the nucleic acid in viruses is the actual infective agent. Viroids and satellite viruses were discovered by Kassanis (1966) and Dicner and Raymer (1967) respectively.

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Are Viruses living or non living?

Viruses being the simplest and most primitive, it is difficult to assign to them any rank in the living kingdom. It is also not easy to define the features viruses within the accepted frame work of living beings. Hence it is better to regard them as an intermediate stage between living and non living. Some of the characters of living as well as non living exhibited by viruses are as follows:-

characters of living beings

1 .Viruses have genetic material (DNA or RNA)

2.They mutate

3.They can grow

4.They can be transmitted from one host to another

5.They are capable of multiplication within a host

6.They react to heat, radiation and chemicals

7.They show irritability

8.They bring about enzymatic changes in vitro.

9.They are able to infect and cause disease to living beings.

10. The DNA and proteins of viruses are similar in composition and structure to those of higher organisms.

Characters of non living:

1.They can be crystallized like an ordinary chemical and stored in a bottle or test tube indefinitely.

2.Outside the host viruses are inert.

3.There is no cell wall, membrane or cytoplasm.

4.There are no cell organelles and there is no metabolism.

5.Sedimentation of viruses is according to their molecular weight like

Fig 2 Diagram of virus morphology and size range (a) poxvirus (vacciniiu (b)-poxvirus (Orf). (c). Rhabdo virus, (d)paramyxovirus (Mumps virs) (e) T-even phage. (f)flexous taiuled phase.

 

6.They do not have functional autonomy i.e. they are not capable of any function unless they obtain metabolic products from others.

7.Energy producing enzyme system is absent.

Some unique characters of viruses:

The following characters are unique to viruses and are not seen-either in living or non living.

1.Presence of only D.N.A. or R.N.A

2.Capacity to reproduce from the sole nucleic acid

3.They do not show cell division.

4.They use the metabolic machinery of the host cell to replicate.

In view of the above, as many virologists believe it is better to regard "viruses are chemicals in a test tube but living beings inside the host".

Properties of Viruses.

1.Viruses are called acellular as they do no have cellular organization like other microrganisms.

2.They are ultra microscopic (invisible under ordinary microscope).

3.The genetic material in viruses is either DNA or RNA but never both

4.Viruses are obligate parasites. They cannot be cultured on inanimate media.

5.Viruses can be cultured on cell media(bacteria, cells of chick embryo etc,)

6.Viruses are filterable; they can pass through bacterial filters. They can however be filtered on molecular filters.

7.Even though viruses are made up of nucleoproteins, they lack the enzymes necessary for their synthesis. For this, they (Viruses) depend on the host enzymes.

8.Viruses do not show cell division

9.They can be crystallized.

10.Virus proteins have high molecular weight.\

11.They can be transmitted from one host to another either directly or through vectors.

12.The capsid (outer coat) of viruses is mostly made up of proteins except in some animal viruses where polysaccharides are also present.

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Structure and chemical composition

Structure and chemical composition

Viruses are not cellular hence they do not have any cell wall, membrane and cytoplasm. They are extremely small and smaller than bacteria. The technical name for a virus particle is Virion. Virions vary in size. The smallest virus (causing foot and mouth disease) is having a diameter of l0nm. Some of the large viruses may reach the size of a small bacterium. For e.g. Pox virus is about 250nm, lympho granuloma virus is 300-400nm in size.

 

fig.3 . Diagrammatic comparison of size of viruses and related individuals. The largest circle, represents the diameter ofE.coli, about 1.0μm. The other organisms are drawn approximately to the same scale.

 

Viruses vary in shape also. They may be rod shaped, bullet shaped, oval, irregular or pleomorphic. Based on shape, viruses may be categorized into two groups - polyhedral forms (Adeno virus) and helical forms (Tobacco mosaic virus).

Structurally each virion is very simple. It consists of a nucleic acid core surrounded by a protein coat (capsid) to form the nucleocapsid. The capsid is made up of many independent units called Capsomeres. The nucleocapsid may be naked or as  in some cases surrounded by a loose membranous envelop. The capsid offers protection to the nucleic acids against the action of nucleases.

Shapes of capsid The capsid  mainly exhibits three kinds of symmetry or shape. These are polyhedral, helical and complex(banal). Polyhedral and helical viruses may or may not have envelopes.

Polyhedral capsids are also called icosahedral and have tetrahedral, octahedral or Polyhedral (more than eight) corners or vertices and 20 facets or sides. The shape of each facet is like an equilateral triangle. As has already been

Fig.4 Polyhedral (icosahedral) viruses

 

mentioned, each capsid is made up of a number of capsomeres and each capsomere has a number of monomers which form polygonal rings with a central space of up to 40A°.

The capsomeres are of two types - pentameres and hexameres. The pentamer (pentagonal capsomere) is made up of five monomers while hexarner has six monomers. The monomers are held together by bonds (e.g. Turnip yellow virus).

The helical capsids have monomers arranged in the form of a helix around
a single rotational axis. The monomers curve into a helix because they are thicker
at one end than the other. E.g. TMV, Influenza virus etc.

Fig.5. A cubic virus JX174 (a) a particle consists of 12 capsomeres arranged in icosahedral symmetry, (b) six capsomeres, each made up of five submits.

The complex capsids are divided into two categories - those without identifiable capsids and those with capsids to which additional structures are attached. In vaccinia virus there is no identifiable capsid.

Bacteriophages of T even series have capsids with additional attachments. Usually the capsid has two regions - head and tail. The head capsid is a icosahedron and may be made up of up to 2000 identical subunits. The tail has helical symmetry. Thus the bacteriophage has the combined symmetry of icosahedron (in the head) and helix (in the tail). The tail is connected to the head through a collar. At the free end, the tail has a base plate with 6 corners. At each corner is a spike. In addition from each corner of the hexagonal plate a long tail fibre is given out.

Chemically the capsid is made up of proteins. Each capsomere in TMV is made up of 158 amino acids. Capsid helps the virus in - offering protection to the nucleic acid core, giving a stable shape and helping in the initial attachment of the virus to the host cell.

Structure of envelop

Some viruses have a membranous envelop external to the capsid. The envelop is about 100-150A thick and is found in some icosahedral and helical animal viruses (also in some plant viruses and bacteriophages).

Fig.6 Complex viruses

The membrane is actually a portion of the host cell membrane which the viruses acquire when they are released from the host cell after multiplication. The membrane has the typical bilayer structure with phospholipids and embedded proteins. In addition to the above, membranes may also have a significant amount of carbohydrates like galactose, mannose, glucosamine and galactosamine etc.

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Nucleic acids

Nucleic acids

The core of the virion is made up of nucleic acids. A virus has only DNA or RNA never both together. Within these parameters however considerable diversity exists. The nucleic acids may be double stranded, single stranded, linear or circular. Some have plus polarity; others may have minus polarity. Four types of nucleic acids are found in viruses with reference to the number of strands. These are:

(i)    Single stranded             DNA-(ssDNA)     E.g. Colliphage virus

(ii)   Double stranded            DNA (dsDNA)     E.g. Herpes virus

(iii) Single stranded               RNA (ssRNA) E.g. TMV

(iv) Double stranded             RNA (dsRNA)     E.g. Reovirus

Single stranded DNA is found in ØX174 virus. It was first discovered by Sinsheimer et al in 1950's. ssDNA may be linear (paroviruses) or circular Ø X174 virus). The ssDNA becomes double stranded during replication and at that time it is known as the replicative form.

Double stranded DNA is found in a number of animal viruses and bacteriophages. It has a variety of forms - linear (bacteriophages), cross linked (vaccinia virus) or closed circular duplex as in papova viruses.

Single stranded RNA is found in a variety of animal viruses and icosahedral plant viruses. The strand may be plus (infectious) as in RNA bacteriophages, togaviruses etc or minus (non infectious) as in rhabdoviruses and paramyxoviruses. Plus .MRNA acts directly as mRNA and translates proteins on the ribosomes of bacteria, whereas minus ssRNA first transcribes an mRNA and then it (mRNA) translates proteins (hence called non infectious).

Double stranded RNA is found in animal viruses like reovirus, blue tongue virus etc. The dsRNA has 10 or more segments.

The nucleic acid core of viruses may be summarised as follows

(1)Plant viruses have only RNA (ss or ds) with the exception of
cauliflower mosaic viruses (DNA virus)

(2)Animal viruses have RNA (ss or ds) and DNA (only ds no ss)

(3)Bacteriophages have DNA (ss or ds) or RNA (ss or ds). However most
phages or DNA viruses.

Replication of viruses

Like any other living being viruses also multiply but with a difference. They cannot multiply on their own. They do not reproduce by division independently because the viral nucleic acid cannot duplicate by itself. There are no raw materials to produce fresh nucleotides, neither is there an enzymatic machinery to assemble them together into a duplicate molecule.

Viruses multiply only when they are in a living cell. The DNA or RNA of the virus takes control of the host cell' metabolic machinery and new viral particles are produced utilizing the raw materials from the host cell.

Multiplication of tobacco mosaic virus (TMV) takes place inside the leaf cells of host. The virus reaches the inside of the cell either through a wound or abrasion. The protein coat of the virus is dissolved by host enzymes and the synthesis of mRNA, proteins and genome as well as the maturation of the viral particles occur in the nuclei of the host cell and not in cytoplasm. The cells do not breakdown (no lysis) and the virus particles move into other cells of the plant through plasmodesmata. Viral particles are transmitted through sap from injured  tissues.

Replication of viruses is studied in great detail in bacteriophages. Two kinds of replication cycle occur in phages. These are the virulent or lytic cycle and the temperate or lysogenic cycle. In the lytic cycle the viral particles multiply inside the bacterial cell and multiply. After the formation of new viral particles the host cell breaks down (lysis) and the viral particles are released. In the lysogenic cycle, the viral genome (after entry) gets integrated with the bacterial genome and is called prophage. The prophage multiplies along with the bacterial genome (no independent multiplication) without causing any damage to the host cell. At some stage however depending on environmental conditions, the prophage may separate from the bacterial genome and start a lytic cycle.

As the lytic cycle demonstrates the replication of viruses, we shall study it in some detail. T even bacteriophges (which infect E.coli) have been studied in great detail with reference to viral multiplication. The following account is mainly based on them. However in other viruses also multiplication is same as in T even phages.

The main stages involved in the replication of T even phages are -

(i)         Attachment of phage particle to the host

(ii)        Adsorption of virus particle.

(iii)       Separation of nucleic acid from coat

(iv)       Penetration into the host

(v)        Effect of phage attachment on host cell metabolism

(vi)       Replication of viral of genome (nucleic acid)

(vii)      Synthesis of viral protein (capsid)

(viii)      Assembly of new viral particles and

(ix)       Release of viral particles from the host cell.

 

Attachment (of phage) to the host cell.

Viral particles come into contact with host cell surface. The tail plate of the virus attaches to the surface of the host cell and anchors itself firmly with the help of the tail fibres. The attachment of tail plate (of the virus) to the host cell is a highly specific chemoreceptor regulated process. Certain protein receptors present in the tail capsid of the virus can recognise receptor sites on the host cell surface. It has been noticed that in Escherichia coli, the outer lipoprotein layer in the peptidloglycan covering has many receptors for phage attachment.

Absorption of phage

Initially the attachment of the phage to the host cell is reversible, but later it becomes irreversible (virus can not be removed). Some cat ions are known to  play a role in the absorption of the phage. The receptors of the host cell surface are complex   polysaccharides which can bind the phage on antigenic specificity.

Fig7 Mechanism of attachment and introduction ofDNA into host cell in aT-even bacteriophage. (a) the tail fibers get fixed to the surface of host cell, (b) the base
plate is brought into contact with the cell surface, (c) the sheath contracts driving the
tubular core through the cell wall of bacteria and the DNAfrom the head enters the host
cell through the core.

The adsorbed phage has its head perpendicular to the cell surface. If the host cells are capsulated an enzyme at the tail top of the phage hydrolyses the polysaccharides  of the bacterial capsule boring a tunnel through which cell wall can be reached. The receptor in phage is a mannose transport protein. The receptor in bacteria some times may be present in pili (as in males) in which case some coliphages are male specific i.e. they attach to the sex pili only.

 

Separation of nucleic acid fromcoat

            Hershey and Chase (1952) have shown that phage DNA carries the genetic information into the cell. Injection of DNA into the cell does not require any energy from the host. After the DNA is injected, the coat remains outside. The naked DNA temporarily loses the infectivity until the production of new viral particles. This temporary phase is called eclipse.

Penetration into the host

The nucleic acid together with some internal proteins is released from the capsid after the virus is irreversibly attached to the host cell. It penetrates into the cell at the sites where the inner and outer membranes are in contact with each other and remains associated with the membrane physically. The phage DNA is protected against the membrane nucleases of the host by its associated proteins and by DNA modifications. A highly specialized mechanism of injection of DNA is seen in the T-even phages. Electron microscopic studies have revealed that after the tail plate is fixed to the cell, the tail sheath protein becomes contractile and pulls the collar and the phage head towards the basal plate and pushes the tube through the cell wall. When the tube reaches the plasma membrane, DNA is ejected. The energy necessary for contraction is derived from ATP present on the phage tail. Lysozyme present in the tail may also help nNA penetration by producing the opening in the bacterial cell wall.

(Experimentally purified phage DNA has been used (after pretreatment withCa2+) to bring about infection of the bacterial cell. This process is known as transfection).

After the infection of DNA the empty head and tail of the phage remain outside the bacterial cell.

Effect of phage attachment on host metabolism

The attachment of T.phage by itself (i.e. without expression of viral genes) causes disorganization of the cell membrane. This brings in various metabolic changes including stoppage of protein synthesis of host cell. These changes sometimes lead to cell lysis even without - viral multiplicaiton.

Replication of viral nucleic acid

Viral nucleic acid molecules produce many replicas using the enzyme machinery

Fig 8 Attachment, adsorption and penetration (A-C) of a

of the host cell. Host cell nucleus is the site for the multiplication of viral DNA

Protein Synthesis of virus

In order to produce the 'capsid', fresh viral proteins are to be produced First some enzymes are synthesized. These produce proteins peculiar to phage called 'early proteins' and later the' late proteins' are produced which develop into subunits of head and tail. At this stage, bacterial metabolism including DNA RNA and protein synthesis comes to a halt.

Assembly of new viral particles

Phage DNA and capsid proteins are synthesized separately in the bacterial cell. The phage DNA is then condensed and pushed into the head and tail is added. The process of assembling the parts of a virus into a new virion is called 'maturation'.

Release of (new) viral particles

When the phage particles are being produced in the cell, the wall of the bacteria gets weakened considerably. Further, viral enzymes (produced in the bacterial cell) act on the cell wall resulting in its bursting open. As a result, phage particles are released to outside and they infect other cells repeating the cycle.

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Plant Virus

PLANT VIRUSES

The earliest viruses to be identified and described are viruses infecting higher plants and one of the most thoroughly studied is TMV. Plant viruses are generally elongate or spherical in shape. Their surface has a number of protein units which are arranged spirally in helical viruses but remain packed on the sides in spherical viruses.

Most of the plant viruses are ssRNA viruses although there are a few dsRNA viruses (rice dwarf virus) and dsDNA viruses (cauliflower mosaic virus).

Structure of TMV

This is the most thoroughly studied among plant viruses. TMV is a rod shaped helical virus. Each particle is 300A0 long with an outer diameter of 180A0. It has a hollow core 40A0 wide. Structurally there is a single stranded RNA helix with a helical protein sheath. In each viral particle the sheath consists of 129 spirals.

Electron microscopic and X-Ray crystallographic studies have shown that the protein sheath is made up of 2130 identical subunits. Each subunit has a molecular weight of 17,000. The subunits are ellipsodidal in shape and are composed of a single chain of 158 amino acids.

The nucleic acid which is in the centre is a single stranded RNA with a molecular weight of 2.4 million. The helix of RNA has a pitch of 23A° and a radius of 40A°. There are three nucleotides per particle of TMV.

Virus diseases of plants

              Viral plant diseases may be systemic or local. In systemic infections, the diseases spreads all over the plant through the vasculature while in local, the disease is restricted to certain areas. The following are some of the important symptoms of viral plant diseases.

              1. Mosaic: Mixed light yellow and green patches appear on the leaves making a mosaic pattern. This is a systemic infection. E.g. TMV, mosaic of cucurbits, mosaic of potato etc. 

                 2.Yellows: Uniform chlorosis all over the leaf. It is also called
chlorosis disease, e.g. Rice Yellow.

                 3.Enantions: Small hair like outgrowths appear on the leaves, called
enantions. These outgrowths often accompany mosaic symptoms, e.g. Bean
enantion mosaic.

                 4.Vein clearing, vein banding and vein thickening. Veins and
veinlets become light yellow and transparent while the lamina remains,
normal. It is vise versa in vein banding - the whole lamina turns yellow e a
vein clearing in Lady's finger.

5. Leaf curling or leaf rolling The infected leaves curl or roll back
e.g. leaf curl of papaya, tomato etc.,

6. Little leaf: In this, the affected leaves become small and crowded
giving a rosette appearance e.g. Little leaf of brinfal.

7. Stunting: The inflected plants have over retardation of growth e.g.
Bunchy top of banana.

8. Breaking and greening of plants: Petals of flowers change
colour, become small and green and appear variegated. Sometimes it may
enhance the beauty of the flowers, e.g. Broken tulip flowers.

9. Tumours: Some viral infections produce tumours on leaves or roots.

10.Witche's broom: Leaves get reduced and so also the internodes
resulting in crowded clusters looking like a broom.

Transmission of plant viruses

Viruses are transmitted from plant to plant in a number of ways. The chief methods of tramission of plant viruses are-

(1) Transmission by vegetative propagation

It is one of the chief methods of transmission. Whenever plants are propagated vegetatively by means of cuttings, tubers, bulbs, rhizomes etc., viruses present in the mother plant are easily carried over to the propagated plant. This method of transmission is chiefly seen in Potato, Strawberry, Raspberries etc

Fig.9 A rod of Tobacco mosaic virus, showing arrangement of spirals farming the protein sheath.

(2) Seed transmission

Transmission of viruses through seeds is not very common. It has been reported in very few cases such as; legumes, tomatoes, bean-mosaic, cucumber

mosaic etc. They seem to come from ovules of infected plants. The viruses however, do not enter the embryo.

Fig 10 A helical vims, TMV

3) Soil transmission

Soil transmission has been observed in few cases such as tobacco mosaic virus, potato mosaic virus, wheat mosaic virus, oat mosaic virus etc. These viruses live in the soil along with the plant debris after the harvest of the crop, and infect the new crop when sown in the same field.

(4) Transmission by contact

This is brought in nature, by contact of one plant with the other. This is usually brought about by strong wind, which makes the infected part to come in contact with healthy plant.

(5) Transmission by smokers

Tobacco mosaic virus can remain infective in dry tobacco leaves a long as 25-30 years. They can be easily transmitted by the fingers of the smokers and also by the unburnt pieces of cigarettes, bidies, cigars etc.

(6)        Transmission by Insects

Undoubtedly, the most common method of transmission of viruses is by the activity of insects, which are termed vectors, Some of the Insects which play an important role in the transmission of viruses are Scale Insects, white flies leaf hoppers, Aphids etc. Aphids transmit more plant viruses than any other insects. Insects with their well developed mouth parts carry viruses on their stylets (Stylet borne virus) or may accumulate them internally, and after passing through the tissues, they are introduced again through the mouth part (circulative virus). Some of these viruses may multiply forming the propagative viruses. In some cases, the vector fails to infect a healthy plant soon after it is fed upon a diseased plant, but it can infect the healthy only after some time. This period of development of infectivity for the virus within the vector is called the Incubation period. This period varies with different viruses from hours to days.

There seems to be some relationship between the plant viruses and the Insect vectors which transmit them. For eg: Stunt virus of paddy is transmitted only by a leaft hoper Nephotettix apicalis. Another type of leaf hopper namely, Circulifer tenellus transmits only the curly top virus of beet root. The exact nature of the relationship between the virus and the vector is still unknown.

(7) Fungal transmission:

Certain fungi also take part in the transmission of viruses For eg: the root infecting fungus Olipidium brassicae transmits at least 2 plant viruses namely (1) Tobacco necrosis virus and (2) Tobacco stunt virus.

 

Control

 

1. Field sanitation measures

2. Growing disease resistant varieties.

3. Diseased plants should be uprooted and burnt to avoid further infection.

4.1nsect vectors should be kept in check by spraying.

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Animal Virus

ANIMAL VIRUSES (including human)

Animal viruses bring about a number of diseases in animals and also jb human beings. Animal viruses have either DNA or RNA as the genetic material-DNA is mostly double stranded while RNA is either single stranded or double stranded. The following table gives an account of classification of various animal viruses.

Fig.11. Diagram showing acquisition of viruses by aphids A. stylet borne virus B. circulating virus

Foot and moth disease virus.

This is a pathogen of cattle. The virus is a single stranded RNA particle having about 7500 bases with a coding capacity of 2500 amino acids. The capsid is icosahedral.

Rabies Virus The pathogen is called a rhabdovirus and is bullet shaped. It is found in plants, invertebrates and vertebrates. In dogs it causes rabies. The virus is enveloped which is made up of two layers of lipids. The hereditary Material is  ssRNA..

Another example of rhabdo virus is the vesicular stomatitis virus which is a mild pathogen of cattle.

Human pathogenic viruses

These are artificially classified into the following four groups based on the organs in which disease symptoms are produced.

1.Pneutropic Viral diseases: Diseases involve respiratory tracts, e.g. fluenza, Common cold etc.

2.Dermotropic viral diseases: Diseases involve skin mainly eg: Measles

Chicken pox, Small pox, mumps etc.                                         

3.Viscerotropic viral diseases: Diseases involve blood and visceral
organs e.g. Yellow fever, dengue, hepatitis etc.                           

4.Neurotropic Viral diseases: Diseases involving the central nervous system e.g. Rabies, polio, encephalitis etc. We shall study some of these briefly.

Influenza virus The pathogen is a helical virus containing ssRNA and it is enveloped. The envelope contains a series of projections or spikes. The vim causes disease of the upper respiratory tract.

There are three types of influenza virus- Types A, B and C. Type A and B cause epidemics while type C occurs sporadically. Each type is characterized by its antigenic variation in which changes occur in the protein of the capsid. As a result, innumerable variants are produced which are unrecognizable by the antibodies produced during a previous infection. The nomenclature of influenza virus is therefore based on changing antigenic pattern.

Viral infection causes sudden chills, fatigue, headache and pain. Fever of upto l03°F or 104°F is very common. Influenza A is treated with amantadine, a synthetic drug which is known to prevent viral penetration into the host cells.

Adenoviruses: These are a group of about 35 icoscahedral particles having  dsDNA. The viral particle is about 60-90 nm in diameter having a dense central core and the capsid composed of 252 capsomeres ((240 hexons and 12 pen tons).

Fig.12 A typical myxovirus (influenza virus)

The viruses owe their name to the adenoid cells from which they were first isolated. They multiply in the nuclei of the host cells. Adenoviruses cause diseases in respiratory tracts, eyes and meninges. Type 8 adenovirus is the main agent of Kerato conjunctivitis inflammation of the eye.

Rhinoviruses: These represent a collection of about 100 icosahedral RNA viruses. RNA is single stranded and is surrounde by a capsid made up of four different polypeptide chains. Rhino viruses (Rhinonose) are involved in respitary infections causing head ache, sneezings blocked nostrils etc. Common cold is caused by about 15-29 types of rhinoviruses.

Chicken pox virus: The pathogen is a icosahedral virus having ds DNA the hereditary material. The capsid is made up of 162capsomeres. Chicken virus belongs to herpes virus group. In adults the same virus causes herpes (Virus-herpes Zoster) which is a painful disease where the spinal cord is affected with red patches on the skin

Fig.13 (a) Adeno virus (b) Herpes virus

Measles Virus: This is a common pathogen among children causing a     communicable respiratory disease the symptoms of which develop on the skin. The pathogen is called Rubeola (meaning red) owing to the appearance of red rashes on the skin.

Rubeola is a helical ssRNA virus with an envelop in addition to the capsid. An effective vaccine is available against the disease.

Mumps virus: This is a disease in children mainly affecting the parotid glands (hence the technical name - epidemic parotitis for the disease) There is swelling visible on either side of the cheek. The virus has ss RNA and is helical having an envelop in addition to 'capsid'. This is similar to measles virus except that there are no neuraminidase spikes on the envelop. A vaccine is available for the disease.

Small pox virus: Commonly called variola virus it has been eradicated the world over. The virus causing the dreadful disease has a complex architecture, brick shaped and is about 270nm in size. It has no envelop but the capsid is surrounded by a series of fibres.

Early symptoms of the disease are chilli, fever and fatigue. Pink red spots (mancles) appear after fall in temperature on the scalp, fore head and all parts of the body. The spots turn into fluid filled vesicles and then into pustules. By preventive vaccination the disease has been completely contained. The last case of small pox reported (as per WHO) was in October 1977. On October 1979 an announcement was made that small pox has been eliminated from earth.

Dengue fever virus: Four different kinds of viruses cause dengue fever. The pathogens basically are icosahedrally symmetrical with an envelope. Genetic material is RNA. Viruses are transmitted to human beings through mosquito bite. Dengue fever is a debilitating disease accompanied by sever pain in the limbs,

Rabies virus: The rabies virus not only causes disease to dogs but to human beings also with a high rate of mortality if not treated properly. Besides dogs, rabies virus also occurs in cats, horses, rats etc.

Fig.14 Poxvirus a brick-shaped virus

If timely treatment (vaccine) is given rabies can be prevented. Earlier vaccines were quite painful as they had to be taken for 14 days successively. But a new rabies vaccine- Merienx human diploid cell vaccine is shown to be more effective as only three of them are enough to protect the patient.

Poliovirus

One of the smallest among viruses the polio virus (diameter 27-30nm) has an icosahedra] capsid with a ssRNA as genetic material. The multiplication of the virus is phenomenal; within a six hour cycle it produces as many as 10,000 new particles.

The virus after gaining entry through contaminated food or water multiples in the tonsils and the lymphoid tissues. After entering the blood stream, the virus localizes in the grey matter of spinal cord. (Greek polios=grey). Three types of polio are known to exhist - Brunhidle strain - epidemics and paralysis; lansing strain occurs sporadically but causes greatest number of paralytic cases and Icon strain occurs occassionally confining itself to intestine rarely causing paralysis.

A very efficient vaccine was prepared by Jonas Salk and his associates by using formaldehyde inactivated polio viruses. Another vaccine which can be taken orally is the one prepared by Albert Sabin which consists of attenuated viruses. This is known to produce more antibodies. Polio vaccine is routinely given to children as a part of general immunization.

Fig. 15 Rabies or Rhabdovirus, a bullet-shaped virus

AIDS:

AIDS or Acquired immunity deficiency syndrome is caused by a virus belonging to Retrovirus group. It has been given the name HIV ( Human immunodeficiency virus). In this disease the viruses attack the lymphocytes (T4) and destroy them. Lymphocytes are necessary to provide immunity against any disease whenever our body gets any kind of infection Lymphocytes are produced in large numbers and they kill the infecting microbes. With the destruction of the lymphocytes the entire deface mechanism  (immunity) is disturbed and the body will not be able to combat even ordinary infections. While Aids does not kill the individual directly it renders the body weak and susceptible to all kinds of infections.

The first few AIDS cases were reported in 1981 in Atlanta USA since then it has spread to all countries including India

Transmission of AIDS : The following are the methods of viral transmission, a. Through sexual intercourse with an infected person (Homosexuals are more susceptible), b. Through Contaminated injecting needles, c. Through blood transfusion, d. Infants of infected mothers also get the disease through maternal blood.

Symptoms: All HIV infected persons need not show visible symptoms. The virus after gaining entry may remain dormant for a period ranging between 9-300 months. In blood transfusion cases this may be between 4-14 months. Persons with AIDS infecting can be classified into 3 categories.

1.    Healthy Carriers: This is the first stage there are no symptoms. But
they are capable of infecting   hers. They may develop full blown symptoms or
may remain healthy for a long period.

Prodromal Phase:This is called AIDS related complex (ARC). This group has a milder form of the disease with fever, diarrhoea, weight loss and enlarged lymph glands.   They exhibit depression of immune system.

Fig, 16 Morphology of T-even hacleriophage virus. A, External morphology; B, diagrammatic representation ofL.S. of a bacteriophage; C, various components of a bacteriophage.

3. The End Stage: Here AIDS is fully developed the patient dies of incontrollable infections of lung, exertion, chest pain fever loss of weight severe headache loss of memory, convulsions, development of skin malignancy (Kaposis sarcoma)

Identification of AIDS: Serological test of the blood by elisa or western blot will indicate the presence of antibodies against HIV in infected patients.

Cure: Till now no cure has been found for AIDS. Drugs Azidothy midine, suramin etc. may give a temporary relief. Attempts are being made to develop a vaccine against the virus. However chances of contacting AIDS may be avoided by taking the following precautions.

a. Sterilizing the needles properly before injection or use disposable syringes, b. Utmost care during blood transfusion only HIV screened blood to be used. c. This is a moral step. Avoid sexual contact with several partners.

Sexual contact to be had with only one, known partner Homosexuality should be strictly avoided.

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BACTERIOPHAGES (BACTERIAL VIRUSES)

BACTERIOPHAGES (BACTERIAL VIRUSES)

Viruses that infect bacteria are called bacteriophages. The word phage means eating, hence bacteriophages are bacteria eaters. Bacteriophages may have ssDNA, dsDNA, ssRNA or dsRNA as the genetic material. The word bacteriophage was first coined by d'Herelle (1917).

ssDNA phages are of two morphological types - icosahedral and hdical. The best known example for icosahedral in фX174. The helical phases are filamentous and are divided into three groups - (i) Ff group: Attach to the F type sex pili in bacteria, (ii) If group : Attach to I type sex pili specified by the resistance (for drug) factor and (Hi) Ike group which is physically similar to Ff group but serologically distinct.

Among the dsDNA phages the important ones are - T even phage of Escherichia coli, Njphage of cyanobacteria etc.

ssRNA phages include over thirty viruses belonging to three or four serological groups. 06 is an example of dsRNA phage.

Among the phages T-even phage has been studied in detail and we shall describe it as an example of a typical bacteriophage. T-even phage is a coliphage (attacking E.coli) and includes T2, T4 and T6 phages.

Morphologically the T even phage has a hexagonal head and a cylindrical tail. The head is made up of a protein capsid which has icosahedral symmetry enclosing a tightly packed molecule of dsDNA. The tail consists of a hollow core surrounded by a contractile sheath. At the free end of the tail is present an end plate with a number of tail fibres. The end plate also has a number of spikes. The tail connects to the head with the help of a disc or collar. The size of the head 95nm X 65nm, while the tail is 115nm long with a diameter of 17nm.

The genetic material is a dsDNA with a molecular weight of 120 X 106 daltons. Some virologists believe that the DNA has hydroxymethyl cytosine instead of the normal cytosine.

(Life cycle and multiplication of bacteriophages is discussed under virus "lultiplication.)

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CYANOPHAGES

Viruses that attack cyanobacteria (blue green algae) are called cyanophages. The first suggestion that viruses may be the cause of sudden inexplicable disappearance of blue green algal blooms was first made Krauss 0961). Earlier (1960) Lewin reported a phage attacking Spirochaeta rosea (considered to be a blue green alga by some phycologists).

The first cyanophage was isolated in 1963 by Saffermann and Morris Since then there have been several reports of cyanophages including one from India (Singh and Singh 1967).

Nomenclature:

Cyanophages are named after the hosts they attack. If they attack more than one host, the names of the hosts are abbreviated. For example cyanophage attacking Lyngbya, Phormidium and Plectonema is named LPP-1 taking the first letters of the three hosts. C-1 cyanophage attacks Cylindrospermum. So far about 14 genera of cyanobacteria are known to harbour viruses-These (hosts) are -Anacystis, Chroococcus, Microcystls, Nostoc, Lyngbya, Phormidium, Cylindrospermum etc.

Structure:

Cyanophages may be icosahedral or helical. The capsid protein has a molecular weight of about 4 X 104 daltons. The LPP-1 virus has a distinct head and a short tail similar to a bacteriophage. The capsid head is about 600nm and the tail 200nm long. The capsid is covered by an envelop. The genetic material isDNA. It has a C/G ratio of 55-57%.

The infection and multiplication of cyanophages is almost similar to bacteriophages. The viral DNA multiplies in the host cell in between photosynthetic lamellae and in the region where host DNA lies (nucleoplasm). It has been reported that LPP-1 phage breaks down half of the host DNA and incorporates it into its own DNA.

Viruses attack the host resulting in the rates of photosynthesis and respiration coming down. Lysis of the cells of the host is restricted to vegetative cells while heterocysts and spores are affected very little.

Isolation

Samples of sewage water are added to algal culture which is incubated for about a week and centrifuged. The supernatant is treated with chloroform and screened for phage activity by allowing it to grow on a test blue green alga.

Uses:

Cyanophages can be used to destroy blue green algal blooms. Goryush and Chaplinskaya (1968) have reported the destruction of blooms by using cyanophages. Daft and Stewart (1971) have also demonstrated a similar phenomenon.        

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