Classification of viruses

Classification of Viruses

Morphological Characteristics

• Viruses are mainly classified depending on their structure and shape. Commonly recognised shapes are icosahedral, rod-shaped, and complex structures.

• The presence or absence of a viral envelope, which is derived from the host cell membrane, is also used to classify viruses.

• Other morphological factors include the form of the nucleocapsid and presence of a matrix (an outer layer of proteins that surrounds the nucleocapsid in some viruses).

Genome Type

• Viruses may have RNA or DNA as their genome, which can be single-stranded (ss) or double-stranded (ds).

• The type of genome significantly influences the replication strategy of the virus and thus is a key factor in virus classification.

Baltimore Classification

• Proposed by Nobel laureate David Baltimore, this method categorises viruses into seven groups based on both their genome type and their method of transcription.

Host Range

• Viruses can also be classified according to the organisms they infect, like animals, plants, bacteria (bacteriophages), archaea, and fungi.

• Furthermore, some viruses are specific to certain tissues within an organism, such as liver cells or nerve cells. This is referred to as tropism.

Replication Strategy

• The replication cycle of a virus, including how it enters a host cell, synthesises its genetic material and new viral proteins, assembles new virus particles (virions), and exits the host cell, can also be a basis for classification.

Serological Methods

• These methods are based on antibodies’ ability to recognise and bind to specific antigens (proteins or polysaccharides) on the viral surface.

• Techniques like neutralisation, haemagglutination inhibition, and enzyme-linked immunosorbent assay (ELISA) may be used.

Remember, no single type of classification is comprehensive enough to describe all viruses. Therefore, a combination of the above methods is often used to fully characterise and classify a virus.