Viruses – friends or enemies of man?
Viruses have existed on our planet for a long time. He explains the different types of viruses, how they infect humans, and what diseases they cause or cause. the benefits they have for us are explained here.
General
By definition, viruses are infectious organic structures, often referred to as particles and entities, that spread outside of but cannot replicate without suitable host cells. Viruses have been on our planet for a very long time and have played a crucial role in the evolution of life. As is now believed, viruses and their hosts have undergone "co-evolution." Hosts have had to constantly evolve new defenses to ward off viruses, and viruses in turn have had to adapt to them. For this they took up partially from the host in her. But host cells have also acquired viral genes over time. It is amed that viruses, which evolve rapidly, are the main source of new genes in our biosphere [1].
Viruses were not discovered until the end of the 19th century. Viruses were discovered at the beginning of the twentieth century, and since then our view of them has changed constantly. Whereas viruses were long regarded merely as pathogens of animals and plants, we now know that they are much more than that: viruses occur everywhere and have now been identified as the most abundant biological entities on earth [2]. They are tiny structures. Only visible under the electron microscope. Their size varies from 16 nanometers (circoviruses) to about 350 nanometers (poxviruses), where one nanometer corresponds to one millionth of a millimeter. thousandths of a millimeter) large. Thus about 1.000 times larger than viruses.000 times larger than viruses. An exception to the small size are the so-called tupan viruses: these viruses can grow up to 2.3 micrometers in size. With the discovery of these giant viruses, the simple size-based distinction between viruses and cells was abolished [3, 4].
"Der" or "Das"
There is often a debate about whether "der" or "das" is correct in German usage, and the answer is: both are correct. As a technical term, "the virus" initially found its way into the German language, but in everyday language this changed over time to "the virus", which is now also considered correct. In technical language, however, "the virus" is still used. Considered to be the correct form – as in this article.
Dispute: living beings or not?
Experts have long been divided on the question of whether or not viruses are living organisms. The main argument against their classification as living beings is the fact that viruses do not have their own body and cannot reproduce independently. They are "parasitic elements". On the other hand, viruses have played a decisive role in the development of life and still play an important role in living nature, so that they are actually part of life. The question has become a philosophical one over time, and even experts do not agree on this. Despite this fundamental discussion, viruses are usually counted as microorganisms – microscopically small organisms that are invisible to the naked eye. Besides viruses, microorganisms also include archaebacteria (archaea), protozoa, fungi and algae.
Enveloped and non-enveloped viruses
Non-enveloped viruses, also called naked viruses, consist only of their genetic material (DNA or RNA) and a layer of proteins, the capsid, which packages the genetic material. Enveloped viruses additionally have an outer envelope. This can give different viruses a very different appearance.
Figure 1: Representation of a naked (non-enveloped) virus and an enveloped virus with a membrane envelope., Image: Open Science – Life Sciences in Dialogue (CC BY-SA-ND 3.0 AT)
The outer viral envelope consists of a lipid double membrane and viral glycoproteins embedded in it, the envelope proteins. The lipid layer originates from the host cell from which the virus was released and, depending on the virus species, originates from the cell membrane on the cell surface or from membranes inside the host cell. The glycoproteins of the envelope can be seen as surface extensions (spines, spikes) under the electron microscope.
The envelope generally gives the virus better stability against environmental influences. This explains why the newly emerging viruses of the last major pandemics were always enveloped viruses (HI virus, influenza virus, Ebola virus, SARS-associated coronavirus).
As already mentioned, viruses need a host to replicate, i.e. they invade one and use the mechanisms of their host cell there. Enveloped viruses have an advantage over non-enveloped viruses: the envelope allows a virus to change its surface more easily. Among other things, this also influences its uptake into the host cell. The host's immune defenses can be more easily bypassed with an envelope, allowing the virus to better adapt to the host. Whether an infection is an enveloped or non-enveloped virus also determines the fate of the host cell: While naked viruses destroy the host cell when they leave (lysis), enveloped viruses can be released without destroying the cell.
However, the presence of an envelope also makes viruses more vulnerable: the envelope is destroyed during hand washing by the surfactants contained in detergents and soaps. This exposes the virus' nucleic acid, which is no longer infectious. Therefore, washing hands with soap is a good way to protect yourself from viruses. Thorough soaping of hands for about 30 seconds is recommended. Subsequent washing off with warm water.
Course of a virus infection in humans
Viruses can infect a large number of living organisms: Humans, animals, plants, even other microorganisms such as can be infected by viruses. In the case of bacteria, the viruses are then called. Each type of virus uses certain organs and living organisms for its own reproduction.
The replication cycle of a virus starts with its uptake into the host cell and ends with the release of virions – a virion is a complete, infectious virus particle outside a host cell. This process takes place in various steps, illustrated here using the example of an enveloped virus.
Figure 2: Schematic sequence of virus replication using the example of an enveloped virus., Image: User:YK Times, http://creativecommons.org/licenses/by-sa/3.0/ (CC BY-SA)
Attachment (adsorption)A virion attaches to a surface protein on a host cell. Penetration: By endocytosis, the virion is enveloped with the cell membrane of the host cell and thus enters the cell interior. Release (uncoating): The envelope of the virus particle is removed, which is released. Multiplication : The nucleic acid of the – DNA or RNA – is duplicated by the corresponding machinery of the host. Protein biosynthesisThe amplified nucleic acid of the virus is used as a template to synthesize envelope proteins and other components of the virions in the host cell. The host's machinery is also used for this purpose. In the host cell, the nucleic acids are turned into-. protein components new virus particles are assembled. Release: virions are released. In the case of enveloped viruses, this is done by so-called budding, in which the host cell remains intact, and in the case of non-enveloped viruses, by destroying the host cell (cell lysis). The number of newly formed virions of an infected host cell is called burst size. The infection cycle can now start all over again.
A viral infection in humans can take a very different course depending on the type of virus. In principle, there are the following possibilities here: The virus enters the host cell and immediately destroys it, and virions are released. In this case, they are called lytic infection. In a lysogenic infection, however, the host cell is not immediately destroyed. Instead, the virus no longer reproduces and remains dormant in the host. The virus can be reactivated by a certain trigger, which often only happens after many years. This is the case, for example, with herpes simplex viruses, varicella zoster viruses, cytomegaloviruses and Epstein-Barr viruses; most people are latently infected with these viruses.
In general, viral infections – no matter how they progress – always trigger inflammatory reactions in humans and activate the immune system to fight the viruses or. to eliminate the cells that are infected by them. are formed in the blood against the virus. This fact serves as the basis for tests for viral diseases that have passed through, in which the presence of antibodies is checked.
Mutations, epidemics and pandemics
As already mentioned at the beginning, viruses were and are responsible for life as we know it today and are therefore an important part of our planet. However, viruses are usually associated with negative things like disease and death. Viral epidemics and pandemics have killed millions of people throughout history. Examples include Spanish flu, HI-V, Ebola fever, avian flu, swine flu, SARS-CoV, and now SARS-CoV-2. People are therefore afraid of the small, infectious virus particles. You can't see them, yet they spread through the air via droplet infection (z. B. flu viruses) or via contaminated surfaces through smear infection (e.g. B. herpes simplex).
In principle, it is not the aim of viruses to kill their hosts, as this would deprive them of the basis for their own reproduction. Viruses that have had time to adapt therefore cause little damage to their hosts. However, since viruses can change very quickly due to mutations, a harmless one can quickly turn into a more dangerous version. This is also the fear that resonates, for example, with every wave of flu. A certain problem also always resonates here: While with can be fought, these do not work for viruses, and in the case of a viral infection, there is no comparable medicine.
Possible benefits of viruses
Today we know that humans are densely populated with microorganisms. About the advantage of , which are associated with humans, is meanwhile already relatively much known. Although viruses are the largest and most diverse group of microorganisms, we know relatively little about their utility in the human body by comparison. One of the reasons for this is the great diversity of viruses in humans: In addition to viruses that infect humans, we also know of viruses that infect bacteria and fungi in the body. Plant viruses also enter the intestine with food. In the course of the also viral DNA remained in the hereditary property of humans. As to the usefulness of the complex viral society in the body, there are currently the following conjectures: Harmless viral infections that run chronically could be an important training for the immune system and prevent the defense against more severe viral infections. Viruses are also thought to play a role in autoimmune diseases by constantly challenging the immune system, preventing it from targeting the body's own cells. – Viruses that infect bacteria – presumably also control the balance among bacteria in the human body and thus may indirectly have an important role in our health. Although viruses are often linked to the development of cancer, they may also have a beneficial effect in cancer: By preferring cells that divide quickly, they could help the immune system eliminate cancer cells.
Man has already learned to take advantage of viruses. These play an important role in research today. Development as well as in the medical field. Viruses are used, for example, to introduce genetic material into body cells in order to fight diseases. Vien is also used in phage therapy, where bacteriophages are used to fight disease-causing bacteria. Phage therapy has been around for a while, but has seen a resurgence in times of rising.
Whether viruses will continue to be seen only as enemies or also as friends of humans will certainly depend on new insights into their role in our bodies. However, one thing will remain certain: The fear of epidemics and pandemics, such as currently caused by SARS-CoV-2 (the "Corona-"). The better we understand viruses, the better we will be equipped to fight them in the future.
References
References:
[1] Koonin EV, Dolja VV. A virocentric perspective on the evolution of life. Curr Opin Virol. 2013;3(5):546-557. doi:10.1016/j.coviro.2013.06.008
[2] Rosario K., Breitbart M.Exploring the viral world through metagenomics (2011). Curr Opin Virol. 2011; 1:289-297.
[3] Abrahao J., Silva L., Santos Silva L.et al.: Tailed giant Tupanvirus possesses the most complete translational apparatus of the known virosphere (2018). Nat Commun 9, 749 (2018).