How can we classify Viruses?

Virus classification is the process of naming viruses and placing them into a taxonomic system related to the classification systems used for cellular organisms. Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The proper taxonomic classification of viruses is the responsibility of the International Committee on Taxonomy of Viruses (ICTV) system. Still, the Baltimore classification system can be used to place viruses into one of seven groups based on their manner of mRNA synthesis. Specific naming conventions and more classification guidelines are set out by the ICTV.

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Definition of Virus:

Viruses are non-cellular, microscopic infectious agents that can only replicate or duplicate inside a host cell. From a biological prospect, viruses cannot be classified as either living organisms or non-living. A virus can be an infectious or virulent agent which only replicates within a host organism. This is due to the fact that they retain certain defining characteristic features of living organisms and non-living entities or individuals. To the point, a virus is a non-cellular, infectious entity made up of genetic material and protein. Thus, these can infect and reproduce only within the living cells of bacteria, plants and animals.

For instance, a virus cannot replicate or duplicate itself outside the host cell. This is because viruses lack the necessary cellular machinery. Thus, it enters and attaches itself to a specific host cell, injects its genetic material and , reproduces by using host genetic material. Finally, the host cell rupture, releasing new viruses. No other living organisms crystallized but Viruses can do. It is these factors that lead to viruses being classified in the grey area (an area or part of something existing between two extremes and having mixed characteristics of both)– between the living and non-living.

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Viruses are teeny and smaller in its size, ranging between 30-50 nm. Viruses do not have cells and usually lack a cell wall but are surrounded by a protective protein coating called the capsid. It can be seen as a genetic element and is characterized by the mixed evolution of the virus and the host. They consist of either RNA or DNA as the genetic material.

Viruses mainly depend on a host to deliver the complicated metabolic machinery of prokaryotic or eukaryotic cells for propagation. The main function of the virus is to carry its DNA or RNA genome to the host cell, which then can be transcribed by the host cell. The viral genome structure is arranged in a capsulated symmetric protein. The protein associated with nucleic acid called nucleoprotein produces the nucleocapsid with the genome.

Viruses can be classified usually on their phenotypic characteristics, core content, chemical composition, capsid structure, size, shape, modes of replication, and many other viral genome structures. The Baltimore classification is more often used for studying the system of virus classification. In the 1970s, this system was developed by an American biologist David Baltimore, for which he was awarded the Nobel Prize.

 The below virus information describes the classification of viruses on the basis of their different criteria like-

• On the basis of the presence of nucleic acid
• On the basis of structure or symmetry
• On the basis of replication properties and the site of replication
• On the basis of the host range
• On the basis of the mode of transmission

Viruses Classification on the basis of the presence of nucleic acid:

DNA virus:

The virus has DNA as its genetic material. There are two different types of DNA viruses, which are as follows-

• Single-stranded (ss) DNA viruses, for example, Picornaviruses, Parvovirus, etc.
• Double-stranded (ds) DNA viruses, for example, Adenovirus, Herpes virus, etc.

RNA virus:

The virus has RNA as its genetic material. There are two different types of RNA viruses, are as follows-

• Double-stranded (ds) RNA virus, for example, Reovirus, etc.
• Single-stranded (ss) RNA virus: It is again classified into two Positive sense RNA (+RNA) and negative sense RNA (-RNA).
• Examples of single-stranded RNA viruses are single Poliovirus, Hepatitis A, Rabies virus, and Influenza virus.

Viruses Classification on the basis of structure or symmetry:

Viruses come in different types of shapes, from basic helical and icosahedral shapes to more intricate or complex ones. The classification on the basis of different shapes and symmetry of viruses are as follows:

• Complex viruses, for example, Poxvirus.
• Radial symmetry virus, example: Bacteriophage
• Cubical or icosahedral-shaped virus, Examples are Reovirus, Picornavirus
• Rod or Spiral shaped or helical-shaped virus, for Example Paramyxovirus, orthomyxovirus

Viruses Classification on the basis of properties of replication and site of replication:

Some of the viruses invade or enter the host cell, where it replicates and associates within the cell organelles.

• Replication inside the cytoplasm of the host cell.
  Example: All RNA viruses except the Influenza virus.
• Replication inside the nucleus and the cytoplasm of the host cell. Examples: Influenza virus, Poxvirus, etc.
• Replication inside the nucleus of the host cell. Example: All DNA viruses excluding Pox virus.
• Replication of the virus through the double-stranded DNA ds (DNA) intermediate. Example: All DNA viruses, Retrovirus and some tumour-causing RNA viruses.
• Replication of the virus through a single-stranded RNA or ss(RNA) intermediate. Example: All RNA viruses excluding Reovirus and tumour-causing RNA viruses.

Viruses Classification on the basis of host range (spectrum):

On the basis of types of the host, there are mainly four different types of viruses are:

Animal viruses:

These viruses infect by invading or destroying the cells of animals, including humans. Prominent examples of animal viruses consist of the influenza virus, mumps virus, rabies virus, poliovirus, Herpes virus, etc.

Plant viruses:

These viruses infect plants by destroying the plant cells. Replication of plant viruses is obligate and does not take place without a host. Well-known examples of plant viruses consist of the potato virus, tobacco mosaic virus (TMV), beet yellow virus, turnip yellow virus, cauliflower mosaic virus, etc.

Bacteriophage:

Bacteriophages are those viruses which infect bacterial cells. There are many varieties or variations of bacteriophages like DNA viruses,  MV-11, RNA viruses, λ-page, etc.

Insect virus:

Insect viruses are those viruses which infect insects, also known as the viral pathogen of insects. Present Powerful biocontrol agents in the landscape of present-day agriculture are considered due to the virus. The best example of insect viruses are Ascovirus virions and  Entomopox virus.

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Viruses Classification on the basis of mode of transmission:

• Airborne infections – Transmission of the virus through the air into the respiratory tract, like Swine flu, and Rhinovirus.
• Fecal oral route  – Transmission of the virus through contaminated water or food, like, Hepatitis A virus, Poliovirus, and Rotavirus.
• Sexually transmitted diseases – Transmission of the virus through sexual contact with the infected person, like, Retrovirus, human papillomavirus, etc.
• Transfusion-transmitted infections– Transmission of the virus through blood transfusion, like, Hepatitis B virus, Human Immunodeficiency Virus, etc.
• Zoonoses -Transmission of the virus through the biting of infected animals, birds, and insects to humans, like Rabies virus, Alphavirus, Flavivirus, Ebola virus, etc.

Virus classification is most important for viruses. It is also an extremely difficult task for many virus families. Generally, virus classification relied on properties such as virion morphology, genome organization, replication mechanism, serology, natural host range, mode of transmission, and pathogenicity. Thus far, viruses sharing the above properties can reveal tremendous differences at the genome level.

Keep Reading! 

Team MBD

Further reading- Virus Classification – an overview | ScienceDirect Topics