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Viruses are microscopic, non-cellular parasites. They lack the capacity to thrive and reproduce outside of a host body.

In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle, virus, smaller than any bacteria.

Most viruses (Figure 1) have either RNA or DNA as their genetic material. They may be single or double-stranded. The entire infectious virus particle, called a virion, consists of nucleic acid and an outer shell of protein called capsid.

A virus that infects bacteria is known as bacteriophage or phage. Although they may seem like living organisms because of their sophisticated reproductive abilities, viruses are considered non-living organisms.

This is because without a host cell, viruses cannot carry out their life-sustaining functions or reproduce.

Figure 1. Red blood cells and viruses
Figure 1. Red blood cells and viruses

Viroids are disease-causing organisms that contain only nucleic acid and have no structural proteins.

Prions are virus like particles composed primarily of a protein tightly integrated with a small nucleic acid molecule. They are very resistant to inactivation and appear to cause degenerative brain disease.

Viruses cannot synthesize proteins, because they lack ribosomes. They also derive energy from the host cell.

This course covers structure, classification, life cycle of a virus, list of most common disease-causing virus in humans and plants.

Structure of a Virus

The infective form of virus, virion, exists outside the host organism. They consist of a genetic material, and an outer protective layer called capsid, collectively known as nucleocapsid.

Small viruses, e.g., polio and tobacco mosaic virus, can also be crystallized. Some virus families have an additional covering, called envelope, that is derived from the modified host cell membranes.

Genetic Material (DNA/RNA)

Most viruses carry single-stranded RNA as their genetic material. RNA viruses comprise of 70% of all viruses.

DsRNA viruses, e.g., members of the reovirus family, contain separate genome segments coding for enzymes involved in RNA replication, major capsid proteins, and several smaller structural proteins.

Most DNA viruses contain a single genome of linear dsDNA. Papovaviruses, such as polyomaviruses and papillomaviruses, however, have circular DNA genomes (Figure 2).

Viruses containing different genetic materials are shown below:

  • dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
  • ssDNA viruses (e.g. Parvoviruses)
  • dsRNA viruses (e.g. Reoviruses)
  • ssRNA viruses (e.g. Picornaviruses, Togaviruses, Orthomyxoviruses, Rhabdoviruses)
  • ssRNA (e.g. Retroviruses)
Figure 2. Medical Infographic set with icons of viruses, RNA and DNA
Figure 2. Medical Infographic set with icons of viruses, RNA and DNA

Capsid

Capsid is a protein layer made up of subunits called capsomeres, which are formed by smaller subunits called protomers. Capsid proteins are coded for by the virus genome.

Multiple protein copies self-assemble to form continuous three-dimensional capsid structure. Main functions of capsid are as follows:

  • Capsid protects viral genome from nucleases
  • It attaches to specific receptors exposed on the prospective host cell
  • It provides proteins that enables virion to penetrate through the surface of cell membrane

Envelope

It resembles the lipoprotein bilayer, which is formed by host cell-derived lipid layer. This lipid layer closely surrounds the shell of virus-encoded membrane-associated proteins.

Enveloped viruses often exhibit a fringe of glycoprotein spikes or knobs, also called peplomers. These peplomers aid in the attachment of the viruses to the specific host surfaces.

Outer capsid and envelope proteins of viruses are glycosylated and are important in determining the host range and antigenic composition of the virion (Figure 3).

Figure 3Figure 3. A bacteriophage is a virus that infects and replicates within a bacterium
Figure 3. A bacteriophage is a virus that infects and replicates within a bacterium

The structure of a virus can be one of the following types: icosahedral, enveloped, complex, and helical (Figure 4).

Icosahedral

Mostly the capsids of sphere-like viruses have symmetry of an icosahedron and are composed of coat proteins. An icosahedral is a polygon with 12 vertices (corner), 20 facet (sides) and 30 edges.

It is the most stable form and are found in human pathogenic virus e.g. polio virus (Figure 5), adenovirus, papovavirus, herpes virus (Figure 6), etc.

Enveloped

Some species of virus envelop themselves in a modified form of cell membranes. They differ from non-enveloped/naked forms.

Complex

They are composed of several separate capsomere with varied shapes and symmetries. Poxvirus belongs to this group. 

Prolate/ bacilliform

This is an icosahedron form elongated along one axis. E.g. Bacteriophages

Helical

Helical capsid are shaped like hollow tubes with protein walls. These viruses are composed of a single type of capsomere.

Capsomeres are stacked around a central axis to form a helical structure with nucleic acid. E.g. Tobacco mosaic virus (TMV), Orthomyxoviridae (causes influenza).

Figure 4. Various shapes and sizes of viruses
Figure 4. Various shapes and sizes of viruses

Table showing difference between  Non-enveloped and Enveloped Viruses

Features

Non-enveloped Virus

Enveloped Virus
CapsidOutermost covering is capsid and is made of proteinsOutermost envelop is made of phospholipids, proteins, and glycolipids which surrounds the capsid
VirulenceVirulent and causes host cell lysis. Infectious even after dryingLess virulent and are released by budding; rarely causing host cell lysis. Loses infectivity after drying
SensitivityResistant to heat, acids, and drying. The virus is more powerful and proliferates rapidly in an acidic environmentSensitive to heat, acids, and drying
SurvivalSurvive inside gastrointestinal tract. Hence, usually, intestinal infections are caused by these virusesCannot survive in gastrointestinal tract. Bile salts in the GI tract tend to show detergent-like activity, which can destroy these viruses
SterilizationDifficult to sterilize, as they can easily adjust to changes in temperatureDo not show much resistance to desiccation and heat treatment, hence they are easier to sterilize
Immune systemInduces antibody productionInduces cell mediated immune response
InfectionsUsually do not cause recurrent infectionPotent at attacking the immune system
TransmissionThrough feces or mouth, and dustThrough blood and organ transplant
ExamplesNorovirus, Rotavirus, Poliovirus (Figure 5)Chickenpox virus, Herpes simplex virus (Figure 6), Ebola virus
Figure 5. Structure of a poliovirus
Figure 5. Structure of a poliovirus
Figure 6. Structure of a herpes virus
Figure 6. Structure of a herpes virus

Classification of Viruses

Viruses are classified based on their morphology, chemical composition, and mode of replication. Their morphology provides the basis for grouping viruses into families.

Naming

They are named according to their presence and structure. For instance, the name Reoviridae is derived from respiratory, enteric, and orphan viruses as they were found in both respiratory and enteric specimens.

Similarly, Retrovirus is from reverse transcriptase and Filoviridae is derived from the Latin filum meaning “thread” or “filament”.

Several viruses are still unclassified due to difficulty in obtaining sufficient samples.

External References:

https://www.ncbi.nlm.nih.gov/books/NBK21523/

https://microbiologysociety.org/why-microbiology-matters/what-is-microbiology/viruses.html