Virus

The term "viruses" (from the Latin virus = natural viscous moisture, mucus, sap) refers to organic, non-cellular, infectious particles that are not capable of reproducing on their own. The particles are referred to as virions when they are stored extracellularly and as viruses when they are positioned intracellularly. The term "virus" therefore includes the infectious properties of a virion.

Viruses are not counted as living organisms. Viruses are also not cells. They are smaller than any cell (compare: 1nm = 10Å, -C-C- bond = 0.154 nm, DNA helix =3.4 nm, smallest spherical virus = 25 nm, length E. coli = 2000 nm) and can be distinguished from bacteria, fungi and parasites by defining characteristics. Viruses only contain RNA or DNA, whereas eukaryotic cells always contain DNA + RNA.

Although viruses contain the information for reproduction in their genome, they do not contain the necessary biological machinery, i.e. they are not capable of independent replication. Viruses do not have their own metabolism. They require the biosynthetic apparatus and the various morphological compartments of a cell in order to reproduce offspring. This is an important distinction from bacteria:

• Viruses are completely insensitive to antibiotics!
• Viruses also do not reproduce by growing and dividing, but by assembling individual parts (similar to a production line in a car factory), whereby they have to convert the metabolism of an intact host cell for their own replication process. Viruses therefore always contain a selection of the biomolecules found in the living host cells.

For the classification of viruses, see below Viruses, Classification

Since nucleic acids are usually unstable and vulnerable (pH, temperature, nucleases), viruses form capsids from proteins. These proteins encoded on the viral nucleic acid mainly contribute to the mass of an infectious particle. They can be classified as follows:

Structural proteins: Structural proteins serve to package the genome. The packaging must be sufficiently stable to protect the nucleic acid against environmental influences. Furthermore, it must allow the release of the genome after the virus has entered the cell. In the case of tobacco mosaic virus, a virus encodes only a single structural protein. In the case of the poxvirus or the herpes virus, there are more than 30 structural proteins.

Regulatory proteins: These are involved in the replication and transcription of genetic information.

Viral enzymes: Examples of packaged viral enzymes are RNA transcriptases or reverse transcriptase, an RNA-dependent DNA polymerase of retroviruses that can produce a copy of the ss(+) RNA genome in the form of a dsDNA.

In addition to these "nucleic acid-specific" enzymes, there are also structural enzymes, such as neuraminidase, which is expressed by orthomyxoviruses. It serves to cleave off neuraminidase residues at saccharified receptors of the host cell and is consequently also localized in the envelope of the virus.

Functionally analogous host proteins: These are found in viruses with relatively large DNA genomes. Although such proteins are not essential for the replication of the virus, they are important for the survival of the virus in the host cell. These include cytokines or their soluble receptors.

Lipids: Some viruses package their genome with a double lipid envelope (enveloped viruses - in contrast to naked, non-enveloped viruses), which originates from the membrane of the host cell. Viral (transmembrane) glycoproteins are incorporated into this lipid envelope. The glycoproteins have a cytoplasmic, a transmembrane and an extracellular part. Such (lipid) enveloped viruses are sensitive to fat-soluble chemicals and all common disinfectants.

Viruses "misuse" cellular transport and receptor proteins to gain entry into the cell. The high specificity of this recognition is due to the close co-evolution of viral receptor binding domains and structural elements of cellular surface proteins. The effect of viral replication on the host cell itself is called cytopathic effect (CPE), the effects on the entire host organism are referred to as viral disease. There are different types of cytopathic effect:

• cell lysis
• Pyknosis (polioviruses)
• cell fusion (measles virus, herpes simplex viruses, parainfluenza virus)
• intranuclear inclusions (adenoviruses, measles virus)
• intraplasmatic inclusions (rabies virus, poxviruses, papillomaviruses).

1. Adams MJ et al (2017) 50 years of the International Committee on Taxonomy of Viruses: progress and prospects. Arch Virol 162:1441-1446.
2. Hof H (2019). General virology. In: Hof H, Schlüter D, Dörries R, eds Dual series medical microbiology. 7th, completely revised and expanded edition. Stuttgart: Thieme p 168-171
3. Simmonds P et al.(2018) Virus classification - where do you draw the line? Arch Virol 163:2037-2046.