Virus: Classification, Structure and Cultivation

The basic structure of a virus is made up of a genetic molecule (DNA, RNA) and a protein layer that protects that genetic material. When a single virus is in its complete form and become able to infect target cells, it is known as a virion. The viral structure is mainly having following parts:

• Viral Genome,

• Capsid

• Envelop

Viral Structure

The viral structure is mainly having following parts: Viral Genome, Capsid, Envelop

Viral Genomes: The genomes of viruses can be comprised of RNA or single or double stranded DNA. The core of the virus is made up of nucleic acids, which contains the genetic information in the form of RNA or DNA.

• DNA: Double Stranded - linear or circular, Single Stranded - linear or circular

• RNA: Double Stranded – linear (in rotavirus), Single Stranded – linear.

Capsid

Viral genomes are surrounded by protein shells known as capsid. The protein layer that surrounds and protects the nucleic acids is called the capsid. Capsid is almost always made up of repeating structural subunits (also called protomers) of single polypeptide and double polypeptide

Viral Envelope

In some animal viruses, the nucleocapsid is surrounded by a membrane, also called an envelope. This envelope is made up of a lipid bilayer, and is comprised of host-cell lipids. It also contains virally encoded glycoproteins, Glycoproteins sometimes are trans-membrane proteins. These viral proteins serve many purposes, such as binding to receptors on the host cell, playing a role in membrane fusion and cell entry, etc

Types of Virus Structures

Icosahedral:

These viruses appear spherical in shape, but a closer look actually reveals they are icosahedral. The icosahedron is made up of equilateral triangles fused together in a spherical shape. This is the most optimal way of forming a closed shell using identical protein sub-units.

The genetic material is fully enclosed inside of the capsid.

An icosahedron is defined as being made up of 20 equilateral triangular faces arranged around the surface of a sphere Examples are the poliovirus, rhinovirus, and adenovirus

Enveloped:

This virus structure is a conventional icosahedral or helical structure that is surrounded by a lipid bilayer membrane, meaning the virus is encased or enveloped.

The envelope of the virus is formed when the virus is exiting the cell via budding, and the infectivity of these viruses is mostly dependent on the envelope. Examples are the influenza virus, Hepatitis C and HIV

Complex or Head & Tail type:

These virus structures have a combination of icosahedral and helical shape and may have a complex outer wall or head-tail morphology The head-tail morphology structure is unique to viruses that only infect bacteria and are known as bacteriophages. The head of the virus has an icosahedral shape with a helical shaped tail. The bacteriophage uses its tail to attach to the bacterium, creates a hole in the cell wall, and then inserts its DNA into the cell using the tail as a channel.

Helical:

This virus structure has a capsid with a central cavity or hollow tube that is made by proteins arranged in a circular fashion, creating a disc like shape.

The disc shapes are attached helically (like a toy slinky) creating a tube with room for the nucleic acid in the middle.

All filamentous viruses are helical in shape. They are usually 15-19nm wide and range in length from 300 to 500nm depending on the genome size. Example of a this type of virus is the tobacco mosaic virus

Classification of Viruses

Baltimore Classification of Viruses:

In addition to this formal taxonomy, David Baltimore proposed that viruses be classified according to the nature of their genome and the relationship between the genome and the viral mRNA. The classes that he proposed are the following:

Viral Culture / Cultivation

Viruses are obligate intracellular parasites so they depend on host for their survival.

They cannot be grown in non-living culture media or on agar plates alone, they must require living cells to support their replication.

Viruses can be cultured mainly by three methods

• Animal Inoculation

• Inoculation into embryonated egg

• Cell Culture

Animal Inoculation Method

Viruses sometime are cultivated in laboratory animals such as mice, guinea pig, hamster, rabbits and primates. The selected animals should be healthy and free from any communicable diseases.

which are inoculated by intracerebral and intranasal route. Viruses can also be inoculated by intraperitoneal and subcutaneous route.

Suckling mice(less than 48 hours old) are most commonly used for cultivation of togavirus and coxsackie virues,

After inoculation, virus multiply in host and develops disease. The animals are observed for symptoms of disease and death.

Then the virus is isolated and purified from the tissue of these animals.

Inoculation into Embryonated Egg

The process of cultivation of viruses in embryonated eggs depends on the type of egg which is used. Viruses are inoculated into chick embryo of 7-12 days old.

For inoculation, eggs are first prepared for cultivation, the shell surface is first disinfected with iodine and penetrated with a small sterile drill.

After inoculation, the opening is sealed with gelatin or paraffin and incubated at 36°c for 2-3 days.

After incubation, the egg is broken and virus is isolated from tissue of egg.

Viral growth and multiplication in the egg embryo is indicated by the death of the embryo, by embryo cell damage, or by the formation of typical pocks or lesions on the egg membranes Viruses can be cultivated in various parts of egg like chorioallantoic membrane, allantoic cavity, amniotic sac and yolk sac.

Cell Culture (Tissue Culture)

In this method the viral particles are introduced in the cells isolated from Various tissues.

The cells are freshly isolated from the tissue and gets separated from each Other (single cell condition) by using enzymes trypsin and collagenase.

Then these cells are placed in petri dishes or culture flasks with suitable Culture media containing amino acids, sugar, proteins, salts, calf serum, buffer, and cells are allowed to settle.

On incubation the cell divide and spread out on the glass surface to form a confluent monolayer.

Then the viral particles are inoculated over these monolayer. The viral particles infected the cells, grows and destroyed them, and form a plaque over the place where they grow.

Primary cell culture:

These are normal cells derived from animal or human cells. They are able to grow only for limited time and cannot be maintained in serial culture. They are used for the primary isolation of viruses Examples: Monkey kidney cell culture, Human amnion cell culture

Diploid cell culture (Semi-continuous cell lines):

They are diploid cells and contain the same number of chromosomes as the parent cells. They can be sub-cultured up to 50 times by serial transfer They are used for the isolation of some fastidious viruses and production of viral vaccines. Examples: Human embryonic lung strain, Rhesus embryo cell strain.

Heteroploid cultures (Continuous cell lines):

They are derived from cancer cells. They can be serially cultured indefinitely so named as continuous cell lines. Examples: HeLa (Human Carcinoma of cervix cell line), HEP-2 (Human Epithelioma of larynx cell line), Vero (Vervet monkey) kidney cell lines

Cultivation of Plant Viruses

There are some methods of Cultivation of plant viruses such as plant tissue cultures, cultures of separated cells, or cultures of protoplasts, etc. viruses can be grown in whole plants.

Mixture of viruses are mechanically inoculated by rubbing on plant tissue.

When the cell wall is broken by the abrasion, the viruses directly enter in the cytoplasm through the plasma membrane and infect the exposed host cells.

Some plant viruses can be transmitted only if a diseased part is grafted onto a healthy plant.