Virus News and Research

A virus is a microscopic infectious agent that can reproduce only inside a host cell. Viruses infect all types of organisms: from animals and plants, to bacteria and archaea. Since the initial discovery of tobacco mosaic virus by Martinus Beijerinck in 1898, more than 5,000 types of virus have been described in detail, although most types of virus remain undiscovered. Viruses are ubiquitous, as they are found in almost every ecosystem on Earth, and are the most abundant type of biological entity on the planet. The study of viruses is known as virology, and is a branch of microbiology.

What is a Virus?

By Dr Ananya Mandal, MD

Viruses are tiny organisms that may lead to mild to severe illnesses in humans, animals and plants. This may include flu or a cold to something more life threatening like HIV/AIDS.

How big are viruses?

The virus particles are 100 times smaller than a single bacteria cell. The bacterial cell alone is more than 10 times smaller than a human cell and a human cell is 10 times smaller than the diameter of a single human hair.

Are viruses alive?

Viruses by themselves are not alive. They cannot grow or multiply on their own and need to enter a human or animal cell and take over the cell to help them multiply. These viruses may also infect bacterial cells.

The virus particle or the virions attack the cell and take over its machinery to carry out their own life processes of multiplication and growth. An infected cell will produce viral particles instead of its usual products.

Structure of a virus

A virion (virus particle) has three main parts:

·         Nucleic acid – this is the core of the virus with the DNA or RNA (deoxyribonucleic acid and ribonucleic acid respectively). The DNA or RNA holds all of the information for the virus and that makes it unique and helps it multiply.

·         Protein Coat (capsid) – This is covering over the nucleic acid that protects it.

Lipid membrane (envelope) – this covers the capsid. Many viruses do not have this envelope and are called naked viruses.

Receptors

Viruses are not simply taken into cells. They must first attach to a receptor on the cell surface. Each virus has its specific receptor, usually a vital component of the cell surface. It is the distribution of these receptor molecules on host cells that determines the cell-preference of viruses. For example, the cold and flu virus prefers the mucus lining cells of the lungs and the airways.

How do viruses infect?

Viruses do not have the chemical machinery needed to survive on their own. They, thus seek out host cells in which they can multiply. These viruses enter the body from the environment or other individuals from soil to water to air via nose, mouth, or any breaks in the skin and seek a cell to infect.

A cold or flu virus for example will target cells that line the respiratory (i.e. the lungs) or digestive (i.e. the stomach) tracts. The HIV (human immunodeficiency virus) that causes AIDS attacks the T-cells (a type of white blood cell that fights infection and disease) of the immune system.

Life cycle of a basic virus

There are a few basic steps that all infecting viruses follow and these are called the lytic cycle. These include:

A virus particle attaches to a host cell. This is called the process of adsorption

The particle injects its DNA or RNA into the host cell called entry.

The invading DNA or RNA takes over the cell and recruits the host’s enzymes

The cellular enzymes start making new virus particles called replication

The particles of the virus created by the cell come together to form new viruses. This is called assembly

The newly formed viruses kill the cell so that they may break free and search for a new host cell. This is called release.

·        Virus History

Viruses have existed as long as life has been on earth.

Early references to viruses

Early references to viral infections include Homer’s mention of “rabid dogs”. Rabies is caused by a virus affecting dogs. This was also known in Mesopotamia.

Polio is also caused by a virus. It leads to paralysis of the lower limbs. Polio may also be witnessed in drawings from ancient Egypt.

In addition, small pox caused by a virus that is now eradicated from the world also has a significant role in history of S. and Central America.

Virology – the study of viruses

The study of viruses is called virology. Experiments on virology began with the experiments of Jenner in 1798. Jenner did not know the cause but found that that individuals exposed to cow pox did not suffer from small pox.

He began the first known form of vaccination with cow pox infection that prevented small pox infection in individuals. He had not yet found the causative organism or the cause of the immunity as yet for either cow pox or small pox.

Koch and Henle

Koch and Henle founded their postulates on microbiology of disease. This included that:

·         the organism must regularly be found in the lesions of the disease

·         it must be isolated from diseased host and grown in pure culture 

·         inoculation of such a pure organism into a host should initiate the disease and should be recovered from the secondarily infected organism as well

Viruses do not confer to all of these postulates.

Louis Pasteur

In 1881-1885 Louis Pasteur first used animals as model for growing and studying viruses. He found that the rabies virus could be cultured in rabbit brains and discovered the rabies vaccine. However, Pasteur did not try to identify the infectious agent.

The discovery of viruses

1886-1903 – This period was the discovery period where the viruses were actually found. Ivanowski observed/looked for bacteria like substance and in 1898, Beijerink demonstrated filterable characteristic of the virus and found that the virus is an obligate parasite. This means that the virus is unable to live on its own.

Charles Chamberland and filterable agents

In 1884, the French microbiologist Charles Chamberland invented a filter with pores smaller than bacteria. Chamberland filter-candles of unglazed porcelain or made of diatomaceous earth (clay)-kieselguhr had been invented for water purification. These filters retained bacterium, and had a pore size of 0.1-0.5 micron. Viruses were filtered through these and called “filterable” organisms. Loeffler and Frosch (1898) reported that the infectious agent of foot and mouth diseases virus was a filterable agent.

In 1900 first human disease shown to be caused by a filterable agent was Yellow Feverby Walter Reed. He found the yellow fever virus present in blood of patients during the fever phase. He also found that the virus spread via mosquitoes. In 1853 there was an epidemic in New Orleans and the rate of mortality from this infection was as high as 28%. Infectivity was controlled by destroying mosquito populations

Trapping viruses

In the 1930's Elford developed collodion membranes that could trap the viruses and found that viruses had a size of 1 nano meter. In 1908, Ellerman and Bang demonstrated that certain types of tumors (leukemia of chicken) were caused by viruses. In 1911 Peyton Rous discovered that non-cellular agents like viruses could spread solid tumors. This was termed Rous Sarcoma virus (RSV).

Bacteriophages

The most important discovery was that of the Bacteriophage era. In 1915 Twort was working with vaccinia virus and found that the viruses grew in cultures of bacteria. He called then bacteriophage. Twort abandoned this work after World War I. In 1917, D'Herelle, a Canadian, also found similar bacteriophages.

Images of viruses

In 1931 the German engineers Ernst Ruska and Max Knoll found electron microscopy that enabled the first images of viruses. In 1935, American biochemist and virologist Wendell Stanley examined the tobacco mosaic virus and found it to be mostly made from protein. A short time later, this virus was separated into protein and RNA parts. Tobacco mosaic virus was the first one to be crystallised and whose structure could therefore be elucidated in detail.

Molecular biology

Between 1938 and 1970 virology developed by leaps and bounds into Molecular biology. The 1940's and 1950's was the era of the Bacteriophage and the animal virus.

Delbruck considered father of modern molecular biology. He developed the concepts of virology in the science. In 1952 Hershey and Chase showed that it was the nucleic acid portion that was responsible for the infectivity and carried the genetic material.

In 1954 Watson and Crick found the exact structure of DNA. Lwoff in 1949 found that virus could behave like a bacterial gene on the chromosome and also found the operon model for gene induction and repression. Lwoff in 1957 defined viruses as potentially pathogenic entities with an infectious phase and having only one type of nucleic acid, multiplying with their genetic material and unable to undergo binary fission.

In 1931, American pathologist Ernest William Goodpasture grew influenza and several other viruses in fertilised chickens' eggs. In 1949, John F. Enders, Thomas Weller, and Frederick Robbins grew polio virus in cultured human embryo cells, the first virus to be grown without using solid animal tissue or eggs. This enabled Jonas Salk to make an effective polio vaccine.

Era of polio research was next and was very important as in 1953 the Salk vaccine was introduced and by 1955 poliovirus had been crystallized. Later Sabin introduced attenuated polio vaccine.

In the 1980’s cloning of viral genes developed, sequencing of the viral genomes was successful and production of hybridomas was a reality. The AIDS virus HIV came next in the 1980’s. Further uses of viruses in gene therapy developed over the next two decades. 

·        Virus Origins

Viruses have been referred to since ancient times. However, the exact origin of these tiny organisms that carry only the genetic information in a protein coat is still unknown.

The main problem is no fossils of viruses have ever been detected. So the exact origins are difficult to speculate. These particles are too small and too fragile for the process of fossilisation or even for preservation of nucleic acid sequences in leaf tissues or insects in amber.

Thus viral origin studies rely upon viruses that are isolated in the present, or from material that is at most a few decades old.

Virus molecular systematics

The new branch of virus molecular systematics helps in understanding the distant relationships of and origins of many important groups of viruses. The researchers have now sequenced all or part of the DNA and/or RNA of the known varieties of viruses, including the largest (pox- and herpesviruses) and the smallest (gemini- and other ssDNA viruses). The cellular sequences help in understanding the evolution of viruses over centuries.

For example, Geminiviruses are a diverse group of viruses and each of the subtypes have different genes and genome components. The differences however may be traced back to a common origin when considering geographical diversity, and genetic divergence of the vehicles or hosts that carry the viruses.

Similarly Potyviruses are an ancient family of viruses and the genomes vary among the subtypes and are not shared by all members. These are transmitted by aphids while rymo- and triticiviruses are mite-transmitted, and ipomoviruses are whitefly-transmitted. These have been found to have descended from a fungal virus.

A single ancestor?

Tracing back evolution the descent of the viruses could be speculated to be from a single ancestor containing RNA functions or from cellular organisms (containing DNA in cases of DNA viruses). Retroviruses like the HIV virus, as well as pararetroviruses, retrotransposons and retroposons share a common origin of the reverse transcription function. This means these viruses have the enzyme that switches the RNA-based genetics to DNA-based heredity.

In addition some animal viruses - like picornaviruses and alphaviruses - have origins in plant viruses which do not have same structure, genome components, organisation or number of genes. The small spherical picornaviruses (ssRNA, 1 genome component, infects animals) has relations with comoviruses (small spherical, 2 genome components, infects plants) and Potyviridae (filamentous, 1 or two genome components, infects plants).

From the evolutionary studies it is apparent that there can have been no single origin of viruses as organisms. Thus, there can be no simple "family tree" for viruses. Their only common feature is their role as an obligate parasite that needs a host to propagate.

Evolution of viruses

Evolution may have begun from the beginning of life in water, as well as the timeline of colonisation of dry land by organisms. Viruses of nearly all the major classes of organisms - animals, plants, fungi and bacteria/archaea - probably evolved with their hosts in the seas and the viruses emerged from the waters with their different hosts.

Most viruses of land plants are probably evolved from those in the green algae that emerged +/- 1000 Million years ago.

Where Did Viruses Come From?

There are three main hypotheses regarding the origins of viruses:

The progressive, or escape, hypothesis states that viruses arose from genetic elements that gained the ability to move between cells;

The regressive, or reduction, hypothesis asserts that viruses are remnants of cellular organisms;

The virus-first hypothesis states that viruses coevolved with their current cellular hosts.

The Progressive Hypothesis

According to this hypothesis viruses originated through a progressive process. The mobile of movable pieces of genetic material capable of moving within a genome, gained the ability to exit one cell and enter another.

The Regressive Hypothesis

Some virologists feel viruses may have originated via a regressive, or reductive, process. Certain bacteria that are obligate intracellular parasites, like Chlamydia and Rickettsia species, evolved from free-living ancestors. Viruses thus could have evolved from more complex, possibly free-living organisms that lost genetic information over time as these became parasitic in their replication. Viruses of nucleocytoplasmic large DNA viruses (NCLDVs) illustrate this hypothesis.

The Virus-First Hypothesis

This hypothesis suggests that viruses existed before cells. Koonin and Martin (2005) hypothesized that viruses existed in a pre-cellular world as self-replicating units.

Which Hypothesis to choose?

None of the hypothesis may be correct. To date, no clear explanation for the origin(s) of viruses exists. And so viruses could have arisen from mobile genetic elements that gained the ability to move between cells or they may have descended from previously free-living organisms that adapted a parasitic replication strategy or may have existed before, and led to the evolution of, cellular life.

·       Virus Microbiology

Viruses have existed since the existence of life forms on earth. They are tiny organisms with just a genetic code or nucleic acid and a protein cover. Their small size makes them visible only under the electron microscope.

Are viruses living organisms?

There is controversy regarding whether viruses should really be considered as living organisms. Viruses depend entirely on a host cell for their multiplication and functioning.

Types of virus

These pathogens are one of the most widespread of all organisms and are capable of infecting every species of animal from mammals down to insects, protozoa, and even bacteria and plants. In fact there are more species of virus than of all other creatures put together.

Some are harmless while some are extremely dangerous and like HIV causing AIDS or Ebola and Marburg virus etc.

Origin of viruses

Researchers have found resemblance of the genomes of viruses with genes of higher animals that can ‘jump’ from one chromosome to another. Some believes these viruses may have originated from bacterial plasmids, which are little packets of genes lying outside the bacterial chromosomes and capable of being transferred to another bacterium.

Viruses may also have originated from degenerate bacteria that have become obligate parasites.

Viral structure, replication, and function

Viruses vary greatly in size as well as complexity. But some of their features are common among all species of viruses. There are three basic parts:

·       nucleic acid

·       protein coat

·       lipid membrane

Viral nucleic acid

The nucleic acid is the core of the virus. It is either DNA or RNA (deoxyribonucleic acid and ribonucleic acid respectively). The DNA or RNA holds all of the information for the virus and that makes it unique and helps it multiply.

There may be either RNA or DNA and never both and this determines the way in which viruses replicate themselves. Both types of genome however function to make viral proteins and more copies of the viral coat and DNA/RNA.

DNA viruses are the simplest. They use the host cell’s RNA polymerase can make mRNA that translate on the host ribosomes to make viral proteins.

RNA viruses on the other hand need to provide their own RNA polymerase to make mRNA.

In addition, those that are negative-sense RNA viruses need yet another step to make positive-sense RNA.

Retroviruses need another enzyme that they carry with them called reverse transcriptase. This converts RNA to DNA that is inserted into the host genome. Then the synthesis of viral proteins, and their assembly into new viral particles may take place in the host cell’s nucleus (in influenza virus, measles virus) or in the cytoplasm (e.g. rabies, herpes).

Protein Coat (capsid)

This is a covering over the nucleic acid that protects it. This has a symmetrical structure and is built of one or more subunits packed like a chemical crystal.

Lipid membrane (envelope)

This covers the capsid. Many viruses do not have this envelope and are called naked viruses. This membrane is usually acquired by the virus from the host cell in the process of leaving the cell. This coat enables the virus to survive outside the cell sufficiently long to spread elsewhere via the blood.

Viral structure

The capsid and entire virus structure can be of four main types:

1. Helical

There is a capsomer coiled around a central axis to form a helical structure. This is a common structure seen in single stranded RNA viruses. Tobacco mosaic virus is a helical virus.

2. Icosahedral

These are near-spherical and this shape is adopted because the coat forms a closed shell. Rota virus has twelve capsomers and appear spherical.

3. Envelope

The virus is covered with a lipid membrane in a modified form of one of the cell membranes. The outer membrane is from the infected host cell and internal membranes from nuclear membrane or endoplasmic reticulum forming a lipid bilayer known as a viral envelope. This membrane is studded with proteins or receptors.

4. Complex

There is a capsid that is neither purely helical, nor purely icosahedral. There may be extra features like protein tails or a complex outer wall. Bacteriophages are examples of this type of viral structure.

Viral receptors

The virus has to enter the cell in order to infect it. Viruses are not taken up by the cell directly. They must attach to a receptor on the cell surface first in order to gain entry. If the receptor is not a necessary one, the cell once infected with the virus, may go on to remove the receptor altogether.

Each virus has its specific receptor and it is a vital component of the cell surface so that the cell cannot get rid of it to avoid the infection. The selectivity of the viruses determines the cell-preference.

For example, rhinoviruses have a preference for cells lining the nose, airways and the lungs, HIV with CD4, CCR5, CXCR4 viruses, Epstein–Barr virus (EBV) with Rabies virus with CR2, Acetylcholine receptor, Influenza virus with Neuraminic acid on red blood cells etc.

HIV infects mainly T lymphocytes and macrophages because only they carry a surface molecule known as CD4 receptor and EBV infects B lymphocytes carrying the complement receptor CR2.

Virus Classification

By Dr Ananya Mandal, MD

Initially after viruses were discovered there was no system for classifying viruses. Consequently viruses were named haphazardly. Most of the vertebrate viruses have been named according to:

·         the associated diseases (poliovirus, rabies)

·         the type of disease caused (murine leukemia virus),

·         the sites in the body affected or from which the virus was first isolated (rhinovirus, adenovirus)

·         the places from where they were first isolated (Sendai virus, Coxsackievirus)

·         the scientists who discovered them (Epstein-Barr virus), or

·         due to common cultural perceptions e.g. influenza ‘influence’ of bad air or dengue  ‘evil spirit’

When did the classification of viruses begin?

The actual classification of viruses began in the 1960’s when new viruses were being discovered and studied by electron microscopy. When structure was clarified the need for a new system of classification was felt.

Lwoff, Horne, and Tournier suggested a comprehensive scheme for classifying all viruses in 1962. Their proposal used the classical Linnaean hierarchical system of phylum, class, order, family, genus and species. Although the full scheme could not be adopted for viruses but animal viruses were soon classified by family, genus, and species.

Characteristics used to classify viruses

According to the classification, viruses are grouped according to their properties, not the cells they infect. The main criteria were the type of nucleic acid – DNA or RNA.

Four characteristics were to be used for the classification of all viruses:

1. Type of the nucleic acid including size of the genome, strandedness (single or double), linear or circular, positive or negative (sense), segments (number and size), sequence and G+C content etc.
2. Symmetry of the protein shell
3. Presence or absence of a lipid membrane
4. Dimensions or the size of the virion and capsid

Other properties include the physicochemical properties including molecular mass, pH, thermal stability, susceptibility to chemicals and physical extremes and to ether and detergents.

ICTV classification

Naming convention primarily depends on the genome and nucleic acid material of the viruses with the development of nucleic acid sequencing technologies in the 1970s. Naming is performed by the International Committee on the Taxonomy of Viruses (ICTV). A complete catalog of known viruses is maintained by the ICTV at ICTVdb.

The order is as follows;

·         Order – virales

·         Family –viridae

·         Subfamily –virinae

·         Genus –virus

·         Species –virus

In the 2011 ICTV classification there are six orders – Caudovirales, Herpoesvirales, Mononegavirales, Nidovirales, Picornavirales and Tymovirales. The seventh Ligamenvirales has been proposed.

The Baltimore classification

This classifies according to the viral mRNA synthesis. This came from Nobel prize winner David Baltimore.

ICTV and Baltimore classifications used together

At present both ICTV and Baltimore classification are used together. Group I for example possesses double stranded DNA and group II single stranded DNA, Group III with double stranded RNA and Group IV with positive single stranded RNA and Group V with negative sense single stranded RNA. Group VI further has single stranded RNA with reverse transcriptase that converts RNA to DNA like HIV virus and Group VII has double stranded DNA with reverse transcriptase and this includes Hepatitis B virus.

Human Diseases Caused by Viruses

By Dr Ananya Mandal, MD

When a cell is infected with a virus several effects may be seen. Many viruses cause no harm or disease whatsoever. However, some viruses may attack certain cells and multiply within them.

Once mature the daughter viruses break the cell and spread elsewhere. This is called a lytic infection. Eventually, if host immunity operates effectively, the virus-infected cell may be killed by the host, leading to interruption of the virus cycle and cure of the infection. However, this is not true for all viral infections.

The viruses may persist in the cell without damaging it and make the cell a carrier. The patient may appear to be cured but the infection persists and can spread to others. In addition, the infection may reappear later after this period of lull or latency.

Spread of viruses

Viruses cannot exist on their own and for survival they need to spread to another host. This is because the original host may either die or eliminate the infection. Some important routes of viral transfer include:

Route	Examples
Skin contact	HPV (warts)
Respiratory	Cold virusues, influenza, measles, mumps, rubella
Faecal-oral	Polio, echo, Coxsackie, Hepatitis A, Rotavirus
Milk	HIV, HTLV-1, CMV
Transplacental	Rubella, CMV, HIV
Sexually	Herpes 1 and 2, HIV, HPV, Hepatitis B
Insect vector	Yellow fever, Dengue fever
Animla bite	Rabies
CMV - cytomegalovirus, HPV - Human Papilloma Virus, HTLV - Human T-Lymphotropic Virus

In addition, in order to spread the viruses also need to withstand the immune system. A special category of viruses is those that cause disease only when the immune system is deficient in some way; these are called opportunists, and  opportunistic infection is one of the main problems in patients with, for example, AIDS.

Where do viruses reside?

There are several viruses that have an animal or plant reservoir from where they affect humans. Some of the common reservoirs of viruses include;

Virus	Animal reservoir
Influenza	Birds, pigs, horses
Rabies	Bats, dogs, foxes
Lassa and Hanta viruses	Rodents
Ebola and marburg viruses	Monkeys
HIV-1 and -2	Chimpanzees, monkeys
Newcastle disease	Poultry
West Nile virus	Birds

Host defence to viral infections

The body's first line of defence against viruses is the innate immune system. This is made up of cells and other mechanisms that defend the host from infection. This provides a temporary protection against the viral onslaught.

Once within the adaptive immunity faces the virus and remembers it. This is a more permanent form of immunity that may last a life time against the particular strain of virus. Specific antibodies are produced against the virus. This is called humoral immunity.

Two types of antibodies are important. The first called IgM is highly effective at neutralizing viruses but is only produced by the cells of the immune system for a few weeks. The one that lasts a life time is the IgG antibodies.

The second line of defence is called cell-mediated immunity and involves immune cells known as T cells. T cell recognises a suspicious viral fragment there and the killer T cells destroy the virus.

Virus spread control

Viral diseases can be prevented from spreading by vaccinations and the most successful of these is the small pox vaccine that has completely eradicated the disease in 1980. It is hoped that several other viruses, such as polio and measles, will follow.

Epidemics and pandemics of viral infections

Spread or outbreak of a viral infection in a community is termed an epidemic. Apandemic occurs when there is a worldwide epidemic.

The 1918 flu pandemic, commonly referred to as the Spanish flu was such a pandemic. It was caused by an unusually severe and deadly influenza A virus. The victims were often healthy young adults in contrast from weakened and elderly who are usual victims. It killed around 100 million people or at least 5% of the world's population in 1918.

HIV is now considered a pandemic with an estimated 38.6 million people now living with the disease worldwide.

Viruses and cancer

Some viruses may incorporate their DNA (or DNA copied from viral RNA) into host DNA, with effects on the control of cell growth. This may sometimes lead to transformation, in other words a tumour.

However, integration does not always lead to transformation and is not mandatory for transformation. The association of viruses with tumours in animals was first suspected 90 years ago but only in the 1960s was a virus (EBV) shown convincingly to be associated with a human tumour (Burkitt’s lymphoma).

Now the role of oncogenes that are activated for causing cancer is being better understood to know why all viruses and all infections do not cause cancer in all individuals.

Treatment of viral infections

Several antiviral drugs that are used to treat viral infections have been developed over the past two decades. Many of these are focussed against HIV. These do not cure HIV infection but stop the virus from multiplying and prevent the progress of the disease. Another notable antiviral drug is Ribavarin against hepatitis C.

Viruses in general are notoriously difficult drug targets as they modify and adapt themselves rapidly to build up a resistance against the drug. Case in point is Oseltamivir(trade name - Tamiflu) used in influenza.

Virus Infection in Other Species

By Dr Ananya Mandal, MD

Viruses may infect all cells and each cellular organism has its own specific range of viruses that often infect only that species. Some viruses in addition can replicate only in cells that have been taken over by other viruses. These are called satellites or parasites of other viruses.

Viruses affecting animals and livestock

Viruses are important pathogens of livestock. Common infections include Foot and Mouth Disease and bluetongue etc.

Pets like cats, dogs, and horses are also susceptible to serious viral infections. For example, dogs may be affected by rabies, canine parvovirus infections (fatal to puppies) etc.

Honey bees used in agriculture may also be susceptible to many viral infections.

Birds may also be affected by viral infections and notable among these that may be transmitted to humans as well includes the bird flu or avian flu. Flu virus may also be transmitted from pigs to humans and is termed swine flu.

Other infections in animals and livestock include:

·         influenza

·         viral pneumonia in pigs

·         infectious atrophic rhinitis or rhinotrachitis in pigs

·         myxovirus parainfluenza of cows

·         keratoconjunctivitis (viral eye infections) of cows

·         polio-encephalomyelitis virus infections or gastrointestinal infections of pigs

·         transmissible gastroenteritis of pigs

·         Beran’s swine enterovirus infections

·         Foot-and-mouth disease

·         rinderpest

Horses may be affected by the Hendra virus that is highly contagious and fatal.

Viruses affecting plants and crops

Plant viruses are harmless to humans and other animals because they can reproduce only in living plant cells. Most plants have resistance genes that protect them against viruses. Each R gene confers resistance to a particular virus. The gene triggers death of the cells around the affected area. This stops the infection from spreading.

When they are infected, plants often produce natural disinfectants that kill viruses. This includes nitric oxide, salicylic acids and reactive oxygen molecules.

Viruses affecting bacteria

Viruses affecting bacteria are the Bacteriophages. These are the most common and the most diverse group as well as the most abundant form of biological entity in water environments. There are up to ten times more of these viruses in the oceans than there are bacteria.

Bacteriophages infect specific bacteria by binding to surface receptor molecules and then entering the cell. The bacterial polymerase enzyme then starts to translate the virus RNA into protein. These proteins go on to become either new virions within the cell that helps in assembling the new virions, or proteins involved in cell lysis.

The cell is broken down within twenty minutes after injection releasing over 300 new bacteriophages.

Viruses affecting arachea

Some viruses replicate within archaea. These are usually double stranded DNA viruses. They may have unique shapes

Virus Uses

By Dr Ananya Mandal, MD

With advent of virology, researchers have found several uses for these unique organisms. They have been used extensively in medicine and in genetic engineering. Some of the uses of viruses are outlined as follows.

Viruses in biological studies

Viruses have been used extensively in molecular and cellular biology studies. These viruses provide the advantage of being simple systems that can be used to manipulate and investigate the functions of cells.

Viruses have been used extensively in genetics research and understanding of thegenes and DNA replication, transcription, RNA formation, translation, protein formation and basics of immunology.

Viruses in medicine

Viruses are being used as vectors or carriers that take the required material for treatment of a disease to various target cells. They have been studied extensively in management of inherited diseases and genetic engineering as well as cancers.

Viruses in bacteriophage therapy

These are highly specific viruses that can target, infect, and (if correctly selected) destroy pathogenic bacteria. Bacteriophages are believed to be the most numerous type of viruses accounting for the majority of the viruses present on Earth. These are basic tools in molecular biology. They have been researched for their use in therapy.

Viruses in nanotechnology

Nanotechnology deals with microscopic particles. These have various uses in biology and medicine and nanotechnology has been used in genetic engineering. Viruses can be used as carriers for genetically modified sequences of genomes to the host cells.

Viruses in weapons and biological warfare

Viruses may be tiny but have the capacity to cause death and devastation to large populations in epidemics and pandemics. This has led to the concern that viruses could be used for biological warfare.

Viruses in agriculture

Modification and genetic engineering methods can be used to make modified genomes that can be carried into plants and animals by viruses acting as vectors or vehicles. This method can lead to more productive transgenic animals and plants.

Viruses in cancer prevention and control

Similar modifications (as plants and animals in agriculture) of humans have not been attempted for technical and ethical reasons. But the modification of genes of cells of individuals has been under investigation for many years. This is known as gene therapy.

The key element of gene therapy is the introduction of functioning genes into the cells of a human patient. This new gene shows desired functions and corrects defective or non-operational genes within those cells.

The most common target has been cancers, accounting for almost two-thirds of all clinical trials to date. Adenoviruses are widely used as vectors, and can be engineered both to enhance specificity and to minimize unwanted effects.

Viruses and vaccines

Viruses have been used since the time of Edward Jenner in vaccines. Jenner used cow pox viruses to inoculate people against small pox infection.

Vaccines against polio, measles, chicken pox etc. use live and weakened viruses causing the disease or dead virus particles. These, when introduced into an healthy individual, help the immune system to recognise and mount an immunity against the virus. The body remembers the organism and attacks it in case of a later infection thus preventing the disease.

Vaccines for cancer prevention

Vaccines for hepatitis B and those for human papillomavirus protect against liver andcervical cancer respectively. Both use selected proteins of the virus (subunit vaccines).

Virus-directed enzyme prodrug therapy (VDEPT)

This is a therapy when the target cells are inserted with an enzyme that can activate an inactive a precursor or inactive form of a cytotoxic drug that is administered systemically. Thus, the active, cytotoxic form of the drug is only produced where the relevant enzyme is present and active.

For example, an adenovirus expressing the thymidine kinase (TK) enzyme of herpes simplex virus can be combined with systemic administration of ganciclovir, which is converted by the TK to its active form only in cells where this enzyme is present. This is used in HIV treatment.

Viruses and biological pest control

Viruses can also be used to control damaging pests. Traditionally this has been used in agriculture, but applications exist in the control of agents important to human health as well.

The types of agents used for this purpose may prey on the target species, may be parasites on the target pests, be pathogens or cause disease in the target species or may be competing species.

Viruses used for pest control are commonly pathogens causing disease of the target species. Although they account for a small amount of total pesticide use, viruses are used for the control of multiple species of insects and also for rabbits.

Biological agents can produce long-lasting effects and in some cases are able to spread among the target population. They have also been recognized as inherently less toxic than conventional pesticides by the US Environmental Protection Agency.

Their disadvantages include limited range of action, slow effects compared to chemical agents, high costs of initial treatment, low environmental stability, particularly in sunlight etc.