Host cell
A host cell is a living cell that can be infected by a virus , an intracellular bacterium or an intracellular parasite . Viruses are completely dependent on host cells because they do not have their own metabolism and use that of the host cell to realize their genetic material and replication. In the case of other intracellular pathogens , in addition to reproduction, protection from the host's immune system or (in the case of infection of mobile host cells such as macrophages, etc.) the spread of the pathogen in the host organism can play a role in thePlay infection .
Host cells are also cells that have absorbed ( phagocytosed ) one or more other cells as 'endosymbionts' within the framework of an endosymbiosis without dissolving them, so that an intracellular life partnership ( symbiosis ) was created for mutual benefit. Examples of endosymbionts are in particular chloroplasts and mitochondria ; an intermediate form to parasitism occurs in kleptoplasty .
In genetic engineering , cells into which plasmids or, in general, foreign DNAs are smuggled and replicated there or also produce certain proteins , are also referred to as host cells. Host cells are used in genetic engineering to produce and store genetic vectors such as plasmids and viruses.
Intracellular pathogens
Pathogens that can penetrate a host cell are called intracellular pathogens. A distinction is made between obligatory and facultative intracellular pathogens. The former are in any case dependent on a host cell, the latter can use it, but also survive and multiply outside of a host cell (extracellular).
Obligatory intracellular pathogens :
- All viruses
- Bacteria:
- Mushrooms:
- Protozoa:
Optional intracellular pathogens :
- bacteria
- Mushrooms:
Infection of a host cell
Infection of a host cell begins with a pathogen attaching to and invading the cell. Depending on the pathogen, specific host cells are infected, with both eukaryotic (animals, plants, fungi) and prokaryotic cells (bacteria) occurring as host cells in viruses. With this specific susceptibility to infection, one speaks of the pathogen's tropism in relation to a host cell.
After penetrating the host cell, the pathogen must survive the cellular defense mechanisms, for which the pathogen has various mechanisms (e.g. pathogenicity factors ). In the case of viruses, for example, these are the switching off of cellular apoptosis , in bacteria and parasites, various mechanisms against cellular digestion in endosomal vesicles and resistance to the acidic environment in the endosomes. Facultative and obligatory intracellular bacteria express so-called invasins in order to penetrate the host cell . The penetration can be an active process induced by the pathogen, or it can take place passively via endocytosis . Some pathogens (e.g. Toxoplasma spp.) Prevent the phagocytic endosome ( phagosome ) from fusing with lysosomes in order to avoid destruction. In Listeria, the phagosome membrane is lysed, preventing the pathogen from digesting. For these mechanisms to establish an infection of the host cell, not only invasins but also aggressins (damage to the host cell), impedins (inhibition of the immune response ) and modulins (induction of cytokines ) are expressed.
The host cell can be lysed by the pathogen in the course of the infection , the cell is thereby destroyed and the cell content is released together with the newly formed pathogens. With some viruses the genetic material remains in the host cell, with retroviruses as viral DNA ( provirus ) integrated into the host genome or with hepadnaviruses and herpes viruses as episomal cccDNA . With some bacteria such as chlamydia , dormant forms can remain in the cell. Based on these latency stages, the pathogen can start reproducing again by reactivating the pathogen.
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- C. Mims, HM Dockrell and others: Medical microbiology / infectious diseases. Elsevier, Munich 2006, ISBN 3-437-41272-8 .
- H. Hahn, D. Falke, SHE Kaufmann, U. Ullmann: Medical microbiology and infectious diseases. 5th edition. Heidelberg 2005, ISBN 3-540-21971-4 .