Xenoma on flatfish caused by Glugea stephani
Dictyocoela diporeiae. A, meront and spore; B, spore wall; C, polar filament
A hyperparasitic microsporidian, Nosema podocotyloidis, a parasite of a digenean which is itself a parasite of a fish.

Microsporidia are a group of spore-forming unicellular parasites.

- Microsporidia

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Double-membrane-bound organelle found in most eukaryotic organisms.

Two mitochondria from mammalian lung tissue displaying their matrix and membranes as shown by electron microscopy
Simplified structure of a mitochondrion.
Cross-sectional image of cristae in a rat liver mitochondrion to demonstrate the likely 3D structure and relationship to the inner membrane
Electron transport chain in the mitochondrial intermembrane space
Transmission electron micrograph of a chondrocyte, stained for calcium, showing its nucleus (N) and mitochondria (M).
Typical mitochondrial network (green) in two human cells (HeLa cells)
Model of the yeast multimeric tethering complex, ERMES
Evolution of MROs
The circular 16,569 bp human mitochondrial genome encoding 37 genes, i.e., 28 on the H-strand and 9 on the L-strand.

A large number of unicellular organisms, such as microsporidia, parabasalids and diplomonads, have reduced or transformed their mitochondria into other structures.


Informal term for a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris.

Clockwise from top left: Blepharisma japonicum, a ciliate; Giardia muris, a parasitic flagellate; Centropyxis aculeata, a testate (shelled) amoeba; Peridinium willei, a dinoflagellate; Chaos carolinense, a naked amoebozoan; Desmarella moniliformis, a choanoflagellate
Class Protozoa, order Infusoria, family Monades by Georg August Goldfuss, c. 1844
John Hogg's illustration of the Four Kingdoms of Nature, showing "Primigenal" as a greenish haze at the base of the Animals and Plants, 1860
The ciliate Spirostomum ambiguum can attain 3 mm in length
Isotricha intestinalis, a ciliate present in the rumen of sheep.
Paramecium bursaria, is one example of a variety of freshwater ciliates that host endosymbiont chlorophyte algae from the genus Chlorella
Resting cyst of ciliated protozoan Dileptus viridis.
Life cycle of parasitic protozoan, Toxoplasma gondii
Trophozoites of the amoebic dysentery pathogen Entamoeba histolytica with ingested human red blood cells (dark circles)

A scheme presented by Ruggiero et al. in 2015, places eight not closely related phyla within Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa.

Unicellular organism

Organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells.

Modern stromatolites in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm.
A bottom-dwelling community found deep in the European Arctic.
Paramecium tetraurelia, a ciliate, with oral groove visible
A scanning electron microscope image of a diatom
Transmission electron microscope image of budding Ogataea polymorpha

While there has been considerable debate on the classification of protozoa caused by their sheer diversity, in one system there are currently seven phyla recognized under the kingdom Protozoa: Euglenozoa, Amoebozoa, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa.


Any eukaryotic organism that is not an animal, plant, or fungus.

Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes

The most popular contemporary definition is a phylogenetic one, that identifies a paraphyletic group: a protist is any eukaryote that is not an animal, (land) plant, or (true) fungus; this definition excludes many unicellular groups, like the Microsporidia (fungi), many Chytridiomycetes (fungi), and yeasts (fungi), and also a non-unicellular group included in Protista in the past, the Myxozoa (animal).


Opportunistic intestinal infection that causes diarrhea and wasting in immunocompromised individuals .


It results from different species of microsporidia, a group of microbial (unicellular) fungi.


Organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata.

(A) Electron micrograph of Halothiobacillus neapolitanus cells, arrows highlight carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.

Mitosomes have also been identified in several species of Microsporidia and in Giardia intestinalis.


Close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life.

A fish parasite, the isopod Cymothoa exigua, replacing the tongue of a Lithognathus
Head (scolex) of tapeworm Taenia solium, an intestinal parasite, has hooks and suckers to attach to its host
The parasitic castrator Sacculina carcini (highlighted) attached to its crab host
Human head-lice are directly transmitted obligate ectoparasites
Clonorchis sinensis, the Chinese liver fluke, is trophically transmitted
The vector-transmitted protozoan endoparasite Trypanosoma among human red blood cells
Mosquitoes are micropredators, and important vectors of disease
Life cycle of Entamoeba histolytica, an anaerobic parasitic protozoan transmitted by the fecal–oral route
Cuscuta (a dodder), a stem holoparasite, on an acacia tree
The honey fungus, Armillaria mellea, is a parasite of trees, and a saprophyte feeding on the trees it has killed.
Borrelia burgdorferi, the bacterium that causes Lyme disease, is transmitted by Ixodes ticks.
Enterobacteria phage T4 is a bacteriophage virus. It infects its host, Escherichia coli, by injecting its DNA through its tail, which attaches to the bacterium's surface.
Restoration of a Tyrannosaurus with holes possibly caused by a Trichomonas-like parasite
Wolbachia bacteria within an insect cell
Biologists long suspected cospeciation of flamingos and ducks with their parasitic lice, which were similar in the two families. Cospeciation did occur, but it led to flamingos and grebes, with a later host switch of flamingo lice to ducks.
The protozoan Toxoplasma gondii facilitates its transmission by inducing behavioral changes in rats through infection of neurons in their central nervous system.
Trait loss: bedbug Cimex lectularius is flightless, like many insect ectoparasites.
The dry skin of vertebrates such as the short-horned lizard prevents the entry of many parasites.
Leaf spot on oak. The spread of the parasitic fungus is limited by defensive chemicals produced by the tree, resulting in circular patches of damaged tissue.
The rescuing from extinction of the California condor was a successful if very expensive project, but its ectoparasite, the louse Colpocephalum californici, was made extinct.
Parasites are distributed very unevenly among their hosts, most hosts having no parasites, and a few hosts harbouring most of the parasite population. This distribution makes sampling difficult and requires careful use of statistics.
A plate from Francesco Redi's Osservazioni intorno agli animali viventi che si trovano negli animali viventi (Observations on living animals found inside living animals), 1684
Ronald Ross won the 1902 Nobel Prize for showing that the malaria parasite is transmitted by mosquitoes. This 1897 notebook page records his first observations of the parasite in mosquitoes.
"An Old Parasite in a New Form": an 1881 Punch cartoon by Edward Linley Sambourne compares a crinoletta bustle to a parasitic insect's exoskeleton
Fictional parasitism: oil painting Parasites by Katrin Alvarez, 2011
Idiobiont parasitoid wasps immediately paralyse their hosts for their larvae (Pimplinae, pictured) to eat.
Koinobiont parasitoid wasps like this braconid lay their eggs inside their hosts, which continue to grow and moult.
Phorid fly (centre left) is laying eggs in the abdomen of a worker honey-bee, altering its behaviour.
A hyperparasitoid pteromalid wasp on the cocoons of its host, itself a parasitoid braconid wasp
The large blue butterfly is an ant mimic and social parasite.
In brood parasitism, the host raises the young of another species, here a cowbird's egg, that has been laid in its nest.
The great skua is a powerful kleptoparasite, relentlessly pursuing other seabirds until they disgorge their catches of food.
The male anglerfish Ceratias holboelli lives as a tiny sexual parasite permanently attached below the female's body.
Encarsia perplexa (centre), a parasitoid of citrus blackfly (lower left), is also an adelphoparasite, laying eggs in larvae of its own species

Microsporidia are obligate intracellular parasitic fungi that can also be hyperparasites.

Plasmodium (life cycle)

Living structure of cytoplasm that contains many nuclei, rather than being divided into individual cells each with a single nucleus.

Plasmodiocarp of the slime mold Hemitrichia serpula: the living structure contains many nuclei, not separated from each other by cell membranes or cell walls.

The multinucleate developmental stages of some intracellular parasites, namely Microsporidia (now in Fungi) and Myxosporidia (now in Cnidaria), former cnidosporans, are also sometimes called plasmodia.

Carl Nägeli

Swiss botanist.

In 1857, Nägeli first described microsporidia, the causative agent of pebrine disease in silkworms, which has historically devastated the silk industry in Europe.


Any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms.

Fungal cell cycle showing Dikaryons typical of Higher Fungi
Omphalotus nidiformis, a bioluminescent mushroom
Bracket fungi on a tree stump
In 1729, Pier Antonio Micheli first published descriptions of fungi.
Armillaria solidipes
Mold growth covering a decaying peach. The frames were taken approximately 12 hours apart over a period of six days.
Polyporus squamosus
The 8-spore asci of Morchella elata, viewed with phase contrast microscopy
The bird's nest fungus Cyathus stercoreus
Prototaxites milwaukeensis (Penhallow, 1908)—a Middle Devonian fungus from Wisconsin
Main groups of fungi
Arbuscular mycorrhiza seen under microscope. Flax root cortical cells containing paired arbuscules.
Diagram of an apothecium (the typical cup-like reproductive structure of Ascomycetes) showing sterile tissues as well as developing and mature asci.
A pin mold decomposing a peach
The dark filaments are hyphae of the endophytic fungus Epichloë coenophiala in the intercellular spaces of tall fescue leaf sheath tissue
The lichen Lobaria pulmonaria, a symbiosis of fungal, algal, and cyanobacterial species
The plant pathogen Puccinia magellanicum (calafate rust) causes the defect known as witch's broom, seen here on a barberry shrub in Chile.
Gram stain of Candida albicans from a vaginal swab from a woman with candidiasis, showing hyphae, and chlamydospores, which are 2–4 µm in diameter.
Ergotamine, a major mycotoxin produced by Claviceps species, which if ingested can cause gangrene, convulsions, and hallucinations
Saccharomyces cerevisiae cells shown with DIC microscopy
The mold Penicillium rubens was the source of penicillin G.
A selection of edible mushrooms eaten in Asia
Stilton cheese veined with Penicillium roqueforti
Amanita phalloides accounts for the majority of fatal mushroom poisonings worldwide. It sometimes lacks the greenish color seen here.
Grasshoppers killed by Beauveria bassiana

Phylogenetic analysis has demonstrated that the Microsporidia, unicellular parasites of animals and protists, are fairly recent and highly derived endobiotic fungi (living within the tissue of another species).