Viral Biology

Virology is the discipline of microbiology that is concerned with the study of viruses. Knowledge of the basics of viral biology, viral reproduction (viral replication), and the ability to identify potential virus-related pathologies are increasingly important skills for some forensic scientists. There are a number of different viruses that challenge the human immune system and that may produce disease in humans. Although virologists are the scientists most directly concerned with viral biology, with the rise of terrorism and global health issues such as the evolving H5N1 influenza (commonly called bird flu), forensic scientists now find that their work overlaps interests in epidemiology and/or national security.

Viruses are essentially nonliving repositories of nucleic acid that require the presence of a living prokaryotic cell (where DNA is present in the cytoplasm) or eukaryotic cell (where DNA is present within the nucleus) for the replication of the nucleic acid. They 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.

Viral replication refers to the means by which virus particles make new copies of themselves. 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 Zaire.

In the viral classification scheme, all families end in the suffix viridae, for example Picornaviridae. Genera have the suffix virus. 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 is the Herpesviruses. Other viruses only have a single strand of DNA such as the Parvoviruses, which can replicate their DNA in the host's nucleus. The replication involves the formation of what is termed as a negative-sense strand of DNA, 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. 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 contains the code for 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 contains the code for an RNA intermediate. An example is the Hepadnaviruses.

One 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. 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.

Any particular virus will have an attached 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 pleomorphic (irregular) 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.

Viruses cannot replicate by themselves. They require the participation of the replication equipment of the host cell that they infect in order to replicate. The molecular means by which this replication takes place varies, depending upon the type of virus. Viral replication can be divided into three phases: initiation, replication, and release.

The initiation phase occurs when the virus particle attaches to the surface of the host cell, penetrates into the cell, and undergoes a process known as uncoating, where the viral genetic material is released from the virus into the host cell's cytoplasm. The attachment typically involves the recognition of some host surface molecules by a corresponding molecule on the surface of the virus. These two molecules can associate tightly with one another, binding the virus particle to the surface. A well-studied example is the haemagglutinin receptor of the influenzae virus. The receptors of many other viruses have also been characterized.

A virus particle may have more than one receptor molecule, to permit the recognition of different host molecules, or of different regions of a single host molecule. The molecules on the host surface that are recognized tend to be those that are known as glycoproteins. For example, the human immunodeficiency virus recognizes a host glycoprotein called CD4. Cells lacking CD4 cannot, for example, bind the HIV particle.

In the replication, or synthetic, phase the viral genetic material is converted to deoxyribonucleic acid (DNA) if the material originally present in the viral particle is ribonucleic acid (RNA). This so-called reverse transcription process needs to occur in retroviruses, such as HIV. The DNA is imported into the host nucleus where the production of new DNA, RNA, and protein can occur. The replication phase varies greatly from virus type to virus type. However, in general, proteins are manufactured to ensure that: the cell's replication machinery is harnessed to permit replication of the viral genetic material; the replication of the genetic material does indeed occur; and the newly made material is properly packaged into new virus particles.

Replication of the viral material can be a complicated process, with different stretches of the genetic material being transcribed simultaneously with some of these gene products required for the transcription of other viral genes. Also, replication can occur along a straight stretch of DNA, or when the DNA is circular (the so-called "rolling circle" form). RNA-containing viruses must also undergo a reverse transcription from DNA to RNA prior to packaging of the genetic material into the new virus particles.

In the final stage, the viral particles are assembled and exit the host cell. The assembly process can involve helper proteins, made by the virus or the host.

Release of viruses can occur by a process called budding. A membrane "bleb" containing the virus particle is formed at the surface of the cell and is pinched off. For herpes virus this is in fact how the viral membrane is acquired. In other words, the viral membrane is a host-derived membrane. Other viruses, such as bacteriophage, may burst the host cell, spewing out the many progeny virus particles. But many viruses do not adopt such a host destructive process, as it limits the time of an infection due to destruction of the host cells needed for future replication.

Although precise mechanisms vary, viruses cause disease by infecting a host cell and commandeering the host cell's synthetic capabilities to produce more viruses. The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the host. Because viruses invade cells, drug therapies have not yet been designed to kill viruses, although some have been developed to inhibit their growth. The human immune system is the main defense against a viral disease.

Bacterial viruses, called bacteriophages, infect a variety of bacteria, such as Escherichia coli, a bacteria commonly found in the human digestive tract. Animal viruses cause a variety of fatal diseases. Acquired immune deficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV); hepatitis and rabies are viral diseases; and hemorrhagic fevers, which are characterized by severe internal bleeding, are caused by filoviruses. Other animal viruses cause some of the most common human diseases. Often, these diseases strike in childhood. Measles, mumps, and chickenpox are viral diseases. The common cold and influenza are also caused by viruses. Finally, some viruses can cause cancer and tumors. One such virus, human T-cell leukemia virus (HTLV), was only recently discovered and its role in the development of a particular kind of leukemia is still being clarified.

Edward Jenner (1749–1823) is credited with developing the first successful vaccine against a viral disease, with his vaccine for smallpox. A vaccine works by eliciting an immune response. During this immune response, specific immune cells, called memory cells, are produced that remain in the body long after the foreign microbe present in a vaccine has been destroyed. When the body again encounters the same kind of microbe, the memory cells quickly destroy the microbe. Vaccines contain either a live, altered version of a virus or bacteria, or they contain only parts of a virus or bacteria, enough to elicit an immune response.

In 1797, Jenner developed his smallpox vaccine by taking infected material from a cowpox lesion on the hand of a milkmaid. Cowpox was a common disease of the era, transmitted through contact with an infected cow. Unlike smallpox, however, cowpox is a much milder disease. Using the cowpox pus, he inoculated an eight-year-old boy. Jenner continued his vaccination efforts through his lifetime. Until 1976, children were routinely vaccinated with the smallpox vaccine, called vaccinia. Reactions to the introduction of the vaccine ranged from a mild fever to severe complications, including (although very rarely) death. In 1977, when the last naturally occurring case of smallpox appeared and the global eradication of smallpox was complete, vaccinia vaccinations for children were discontinued, although vaccinia continues to be used as a carrier for recombinant DNA techniques. In these techniques, foreign DNA is inserted in cells. Efforts to produce a vaccine for HIV, for instance, have used vaccinia as the vehicle that carries specific parts of HIV.

SEE ALSO Bacterial biology; Careers in forensic science; Ebola virus; Pathogens; Vaccines; Variola virus.