Virology, Viral Classification, Types of Viruses

Virology is the discipline of microbiology that is concerned with the study of viruses. Viruses are essentially nonliving repositories of nucleic acid that require the presence of a living prokaryotic or eukaryotic cell for the replication of the nucleic acid.

Scientists who make virology their field of study are known as virologists. Not all virologists study the same things, as viruses can exist in a variety of hosts. Viruses can infect animals (including humans), plants, fungi, birds, aquatic organisms, protozoa, bacteria, and insects. Some viruses are able to infect several of these hosts, while other viruses are exclusive to one host.

All viruses share the need for a host in order to replicate their deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The virus commandeers the host's existing molecules for the nucleic acid replication process. There are a number of different viruses. The differences include the disease symptoms they cause, their antigenic composition, type of nucleic acid residing in the virus particle, the way the nucleic acid is arranged, the shape of the virus, and the fate of the replicated DNA. These differences are used to classify the viruses and have often been the basis on which the various types of viruses were named.

The classification of viruses operates by use of the same structure that governs the classification of bacteria. The International Committee on Taxonomy of Viruses established the viral classification scheme in 1966. From the broadest to the narrowest level of classification, the viral scheme is: Order, Family, Subfamily, Genus, Species, and Strain/type. To use an example, the virus that was responsible for an outbreak of Ebola hemorrhagic fever in a region of Africa called Kikwit is classified as Order Mononegavirales, Family Filoviridae, Genus Filovirus, and Species Ebola virus Zaire.

In the viral classification scheme, all families end in the suffix viridae, for example Picornaviridae. Genera have the suffix virus. For example, in the family Picornaviridae there are five genera: enterovirus, cardiovirus, rhinovirus, apthovirus, and hepatovirus. The names of the genera typically derive from the preferred location of the virus in the body (for those viral genera that infect humans). As examples, rhinovirus is localized in the nasal and throat passages, and hepatovirus is localized in the liver. Finally, within each genera there can be several species.

As noted above, there are a number of criteria by which members of one grouping of viruses can be distinguished from those in another group. For the purposes of classification, however, three criteria are paramount. These criteria are the host organism or organisms that the virus utilizes, the shape of the virus particle, and the type and arrangement of the viral nucleic acid.

An important means of classifying viruses concerns the type and arrangement of nucleic acid in the virus particle. Some viruses have two strands of DNA, analogous to the double helix of DNA that is present in prokaryotes such as bacteria and in eukaryotic cells. Some viruses, such as the Adenoviruses, replicate in the nucleus of the host using the replication machinery of the host. Other viruses, such as the poxviruses, do not integrate in the host genome, but replicate in the cytoplasm of the host. Another example of a double-stranded DNA virus are the Herpesviruses.

Other viruses only have a single strand of DNA. An example is the Parvoviruses. Viruses such as the Parvoviruses replicate their DNA in the host's nucleus. The replication involves the formation of what is termed a negative-sense strand of DNA, which is a blueprint for the subsequent formation of the RNA and DNA used to manufacture the new virus particles.

The genome of other viruses, such as Reoviruses and Birnaviruses, is comprised of double-stranded RNA. Portions of the RNA function independently in the production of a number of so-called messenger RNAs, each of which produces a protein that is used in the production of new viruses.

Still other viruses contain a single strand of RNA. In some of the single-stranded RNA viruses, such as Picornaviruses, Togaviruses, and the Hepatitis A virus, the RNA is read in a direction that is termed "+ sense." The sense strand is used to make the protein products that form the new virus particles. Other single-stranded RNA viruses contain what is termed a negative-sense strand. Examples are the Orthomyxoviruses and the Rhabdoviruses. The negative strand is the blueprint for the formation of the messenger RNAs that are required for production of the various viral proteins.

Still another group of viruses have + sense RNA that is used to make a DNA intermediate. The intermediate is used to manufacture the RNA that is eventually packaged into the new virus particles. The main example is the Retroviruses (the Human Immunodeficiency Viruses belong here). Finally, a group of viruses consist of double-stranded DNA that is used to produce a RNA intermediate. An example is the Hepadnaviruses.

An aspect of virology is the identification of viruses. Often, the diagnosis of a viral illness relies, at least initially, on the visual detection of the virus. For this analysis, samples are prepared for electron microscopy using a technique called negative staining, which highlights surface detail of the virus particles. For this analysis, the shape of the virus is an important feature.

A particular virus will have a particular shape. For example, viruses that specifically infect bacteria, the so-called bacteriophages, look similar to the Apollo lunar landing spacecraft. A head region containing the nucleic acid is supported on a number of spider-like legs. Upon encountering a suitable bacterial surface, the virus acts like a syringe, to introduce the nucleic acid into the cytoplasm of the bacterium.

Other viruses have different shapes. These include spheres, ovals, worm-like forms, and even irregular (pleomorphic) arrangements. Some viruses, such as the influenza virus, have projections sticking out from the surface of the virus. These are crucial to the infectious process.

As new species of eukaryotic and prokaryotic organisms are discovered, no doubt the list of viral species will continue to grow.

See also Viral genetics; Virus replication