Microsporidia

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Microsporidia
Fibrillanosema crangonycis

Fibrillanosema crangonycis

Systematics
Domain : Eukaryotes (eukaryota)
without rank: Amorphea
without rank: Opisthokonta
without rank: Nucletmycea
Empire : Mushrooms (fungi)
Department : Microsporidia
Scientific name
Microsporidia
Balbiani

The microsporidia , also called microspora , are unicellular parasites belonging to the fungi and causative agents of microsporidiasis . They can reach a size of a few µm (2–12 µm). Microspora parasitize mostly intracellularly in representatives of many animal phyla, less often in other protists from the Sar -called clade of eukaryotes. It is transmitted by ingestion of unicellular spores (mostly orally, i.e. through the mouth).

Life cycle

Microsporidia are obligatory parasites. They live inside a cell of their host. Unlike many other cell parasites, they do not live in a separate vacuole , but directly in the cytoplasm . Infection occurs from a spore, the only stage that occurs outside the host cell. The cytoplasm (called sporoplasm) of the parasite is separated from that of the host cell by its cell membrane, which is often surrounded by a shell consisting mainly of complex carbohydrates called glycocalyx . At this stage, the parasite does not noticeably damage the host cell (occasionally there are functional restrictions in muscle fibers), it multiplies in parallel with the host cell through cell division ( merogony ), the stage is also called meront . Its position within the cell makes it invisible to the host's immune system. The structure of the parasite cell is extremely reduced and its basal cell functions are completely dependent on the host cell. At first it contains only one nucleus (rarely doubled, dikaryon ), tightly packed ribosomes and various membrane components that are not organized into organelles, neither a Golgi apparatus nor functioning mitochondria are recognizable. Organelles called mitosomes are reduced relics of mitochondria, in which only about 20 functional proteins are detectable (normal mitochondria contain about 1000). They no longer have an independent genome. The genome of the nucleus of the microsporidial cell is also greatly reduced. There are only about 2000 genes left, so that many metabolic products have to be taken over from the host cell. The genes for nucleotide synthesis and many essential components of the respiratory chain are missing . They are even partially dependent on the direct import of adenosine triphosphate (ATP) from the host cell for energy supply, the metabolism of which they heat up for this purpose. Although the functional scope of the ribosomes is also reduced, the parasite has an extremely large number of them. These enable it to grow very quickly at the expense of its host. Its ability to defend itself against the infection through programmed cell death ( apoptosis ) is deliberately sabotaged by signal substances. As a result, the infected host cell swells to many times its normal size. Some microsporidia (especially fish parasites) form huge, spore-filled inclusion bodies called xenosomas inside the host cells , which are often fused to form a syncytium .

The microsporidia within the host cell start producing new spores very quickly, often within three days of infection. The spore-forming stage within the cell is also called the sporontium . In the groups in which meiosis occurs, it occurs immediately before spore formation. The spores of the microsporidia serve both to infect new cells within the host and to infect new hosts. They have developed a specific mechanism for the infection process. The infection of new cells takes place via a thin infection tube which, compared to the cell length, can reach an unusual length (up to over 100 micrometers). Like an injection syringe, the infection tube can penetrate tissue and cells, where the spore contents are transferred to the new host by a massive increase in pressure within the spore. Two types of spores are normally always formed, one that infects new cells very quickly and another that does this only with a delay. With some types of microsporidia, these are also morphologically distinguishable.

The compactly built, usually round, oval, pear-shaped or rod-shaped spore has a thick cell wall that is made up of two layers. The inner (endospore) layer consists of proteins and chitin , the outer only of proteins. The inner layer presumably serves primarily to withstand the extreme pressure that is built up during infection (more than 7 megapascals are reached), while the outer layer is used for cell contact, e.g. for host recognition. In addition, they often have spiky appendages. Inside the spore, in addition to the cell nucleus and a little cytoplasm, there is primarily the injection apparatus that forms the infection tube and a storage organelle called polaroplast for membrane components. The overpressure during infection is created by a vacuole in the rear section, which increases rapidly in size and thus builds up pressure. The infection tube at the front end of the spore looks like a thread, hence also called polar filament, and is usually rolled up like a loop. In the bee parasite Nosema apis , the spore is 5 micrometers long, the injection tube (polar filament) inside is 300 micrometers long. Instead, about a fifth of the microsporidia genera have a short, rod-like injection tube that is only about the length of a spore. With them it is anchored in a mushroom-shaped structure, the polar cap. The main component of the infection tube is a protein called PTP1. This turns out to be a strong antigen in hosts (including humans) , so that the immune response directed against it can make the host resistant to infections. During the infection process, the cell contents are pressed into the tube by the expanding vacuole in the spore. The polaroplast, which at rest consists mainly of a folded membrane, provides the necessary material for the new cell membrane in the tube. The infection tube penetrates adjacent tissue and other obstructions, including any other microsporidial spores that would be in the way.

Numerous types of microsporidia produce several spore shapes, usually two (dimorphic), rarely three or more (polymorphic). Different forms either serve as an alternative for rapid infection within a host and delayed germination to infect new hosts, or different spore forms are formed in different host species. In others, all spores look the same and only differ in the speed at which they are ready to germinate. New hosts are infected by slowly germinating spores, usually after ingestion through the mouth and digestive tract (orally). In aquatic microsporidia there are hyperparasitic species in which the infection tube penetrates the body or intestinal wall of the host in order to parasitize the parasite living inside.

The newly formed spores mostly end up in the open air through the faeces, urine, or only after the host's death . In many hosts, including most vertebrates, the infection is chronic and spores are excreted over long periods of time. In insects, after a delayed and often relatively symptom-free onset, more severe infections can occur, which can result in the death of the host. Even with chronic infections, the lifespan and general condition of the host species is usually impaired.

Host loyalty

Most species of microsporidia are host-specific; they only affect a single host species or a few closely related species. Within the host, they are usually specialized in certain tissues. Microsporidia of the genus Nosema , the pathogens of nosemosis , are not only specialized in honey bees of the genus Apis , but also exclusively attack cells of the midgut. Often closely related host species have their own, equally closely related parasites (called “co-cladogenesis”). In many cases the host specificity is dictated by environmental factors; H. under experimental conditions, in the laboratory, species can also be infected in which this never occurs in the field. Other species have a wide host range, e.g. B. all mammals that use them opportunistically.

Microsporidial infections are among the most common parasitic diseases in the animal kingdom. Microsporidia were registered as parasites in about half of the strains in the animal kingdom. However, only the relatively few groups of animals in which the infection leads to economic or health damage that is significant for humans have been better investigated. It is therefore assumed that by far most of the species are still undiscovered and undescribed. Most of the known host species are insects and crustaceans. Microsporidia can cause considerable damage: Pébrine's disease caused by Nosema bombycis led to the collapse of European silkworm breeding around the middle of the 19th century . However, other species, such as mosquito parasites, also have positive effects on humans as regulators or in the context of biological pest control. About 160 species from 17 genera infect fish species. In humans as hosts, 14 microsporidia were registered, not a single one of which were host-specific. Species of the genus Endoreticulatus are opportunistic pathogens that occur in humans and other vertebrates as well as in insect species.

Important parasites and their hosts

Microsporidiosis is the generic term for clinical pictures caused by these organisms.

Systematics and nomenclature

The nomenclature status of microsporidia is not clear. They were treated as a strain according to the International Rules for Zoological Nomenclature (ICZN) (at that time assigned to the Sporozoa ), but the author citation is not clear. It is also questionable whether the name is valid according to the International Code of Botanical Nomenclature (ICN). The affiliation of the microsporidia to the mushrooms and therefore the responsibility of the ICN emerged in 2007. The assignment to Balbiani (CR Acad. Sci Paris 95: p. 1168, 1882) is therefore provisional, but still common today.

Phylogeny

The division (or stem, phylum) of Microsporidia form them with a clade of little-known organisms around the genus Rozella (referred to as Rozellida or Cryptomycota) and the Aphelidea , a species-poor group of parasites of unicellular algae, a group that has been named Opisthosporidia is classified in the classical system as an overstem (superphylum). The Opisthosporidia belong to the group of mushrooms (in the broader sense).

In the meantime a number of protists have been found which have numerous characteristics in common with the Microsporidia, but differ from them in some characteristics. Among other things, there are representatives with a less strongly reduced genome, which also have functional mitochondria. This group includes the genera Mitosporidium , Paramicrosporidium and Nucleophaga . Another group, including Amphiamblys and Amphiacantha , summarized as "Metchnikovelliden", are hyperparasites in, even parasitic, single cells of the Apicomplexa . In addition, numerous, as yet unknown organisms are genetically similar, of which only their base sequence, obtained from environmental DNA , is known without the associated organism and its biology having been known to date; these are sometimes very common in environmental samples. Almost the entire group, often summarized as Rozellomycota or Cryptomycota, has been shown in more recent analyzes to be more closely related to the Microsporidia than to Rozella , so that an expanded group of Microsporidia including these groups has been proposed.

The division of microsporidia into groups is unclear according to the current state of knowledge. A proposal to divide them, also based on genetic data, into three classes of Aquasporidia (especially in freshwater), Marinosporidia (especially marine) and Terresporidia (especially in terrestrial habitats) is ultimately not accepted due to different, more recent data found.

Research history

In the 1850s, Pébrine disease ravaged European silkworm farming. The Swiss researcher Carl Wilhelm von Nägeli discovered infectious "globules" in it in 1857, which he described as Nosema bombycis . Their way of life was then enlightened in 1870, above all by the famous Louis Pasteur, and countermeasures were recommended so that the industry recovered. Nägeli classified his find in the “split fungi” or schizomycetes , a now abandoned grouping of “lower” fungi and bacteria that are not closely related to one another. Édouard-Gérard Balbiani placed Nosema and relatives in the group of Sporozoa in 1882 and coined the name Microsporidia ("microsporidies"). He set up a group of "Cnidosporidia" for them, which also included some groups not related to what we know today, including the Myxozoa ( cnidarians according to what we know today ).

Further knowledge about the microsporidia only came after the invention of electron microscopy in the 1950s. Now it became clear that the Microsporidia lack numerous characteristics that are otherwise almost universally distributed within the animals. Neither mitochondria nor a Golgi apparatus or peroxisomes exist in the cells, flagellum cells with flagellum or structures that can be derived from them do not exist in any stage of life . Thomas Cavalier-Smith then developed the hypothesis that microsporidia belong to a (paraphyletic) group of primitive eukaryotes that split off from the common family tree before these structures had evolved. This group called "Archezoa" was also well supported by the first genetic analyzes. In the mid-1990s, numerous working groups discovered features, mainly from different protein families, in close succession, which instead suggested a closer relationship between the microsporidia and the fungi. Ultimately, the support for the Archezoa through better genetic methods turned out to be a so-called “long-branch attraction” artifact, in which DNA sequences that differ greatly from the other groups are erroneously pushed outwards by the sorting algorithm and thus simulate a basal position. For a while it was unclear whether the microsporidia should be regarded as a sister group of the mushrooms or whether they belong to the mushrooms. The new position was only clarified after 2010, through further improved analysis methods and newly discovered organisms of the kin group.

See also

literature

  • Jiří Vávra, Julius Lukeš: Microsporidia and 'The Art of Living Together'. Chapter 4 in D. Rollinson, D. (Editor): Advances in Parasitology 82. Academic Press (Elsevier), 2013. pp. 253-320. ISBN 978-0-12-407706-5 .
  • Marianne Abele-Horn: Antimicrobial Therapy. Decision support for the treatment and prophylaxis of infectious diseases. With the collaboration of Werner Heinz, Hartwig Klinker, Johann Schurz and August Stich, 2nd, revised and expanded edition. Peter Wiehl, Marburg 2009, ISBN 978-3-927219-14-4 , p. 292 f.
  • Heinz Mehlhorn: Outline of Zoology; Chapter: logs and blueprints. Spectrum Berlin reprint 2001; P. 71 ff.
  • Alexander Mathis et al .: Zoonotic Potential of the Microsporidia . Clinical Microbiology Reviews, July 2005, p. 423-445, vol. 18, no. 3 PMID 16020683
  • Eva Heinz et al. "The Genome of the Obligate Intracellular Parasite Trachipleistophora hominis: New Insights into Microsporidian Genome Dynamics and Reductive Evolution." In: PLoS Pathogens 8.10 (2012): e1002979. doi: 10.1371 / journal.ppat.1002979
  • Louis M. Weiss, James J. Becnel (Eds.): Microsporidia: Pathogens of Opportunity. [728-page reference work]. Wiley-Blackwell, 2014. ISBN 978-1-118-39522-6 (print); ISBN 978-1-118-39526-4 (eBook)

Web links

Commons : Microspora ( Microsporidia )  - collection of pictures, videos and audio files

Individual evidence

  1. ↑ DS Hibbett et al .: A higher-level phylogenetic classification of the Fungi . In: Mycological research , May 2007; 111 (5): 509-547. Epub 2007 March 13, 2007. PMID 17572334
  2. ↑ Sergey A. Karpov, Maria A. Mamkaeva, Vladimir V. Aleoshin, Elena Nassonova, Osu Lilje Frank H. Gleason (2014): Morphology, phylogeny, and ecology of the aphelids (Aphelidea, Opisthokonta) and proposal for the new superphylum Opisthosporidia . Frontiers in Microbiology 5: 112. doi: 10.3389 / fmicb.2014.00112
  3. ↑ Sina M. Adl et al. (2018): Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes. Journal of Eukaryotic Microbiology 66: 4-119. doi: 10.1111 / jeu.12691
  4. ↑ David Bass, Lucas Czech, Bryony AP Williams, Cedric Berney, Micah Dunthorn, Frederic Mahe, Guifre Torruella, Grant D. Stentiford, Tom A. Williams (2018): Clarifying the Relationships between Microsporidia and Cryptomycota. Journal of Eukaryotic Microbiology 65: 773-782. doi: 10.1111 / jeu.12519
  5. ↑ Charles R. Vossbrinck & Bettina A. Debrunner Vossbrinck (2005): Molecular phylogeny of the microsporidia: ecological, ultra structural and taxonomic considerations. Folia Parasitologica 52: 131-142.
  6. ↑ Section after: Nicolas Corradi & Patrick J. Keeling (2009): Microsporidia: a journey through radical taxonomical revisions. Fungal Biology Reviews 23: 1-8. doi: 10.1016 / j.fbr.2009.05.001