The microsporidia (Microsporida) is a phylum of fungi that brings together more than 1400 species belonging to 200 genera. Its location in the Fungi Kingdom is controversial due to the absence of chitin in most stages of the life cycle, with the presence of chitin in cell walls being a widely used characteristic to define a fungus.
Microsporidia are eukaryotic cells. They have a well-defined posterior vacuole, nucleus, and plasma membrane. They are covered by a protective layer composed of proteins and chitin, which give it a high environmental resistance. They lack some typical eukaryotic organelles, such as mitochondria, the Golgi apparatus, and peroxisomes.
Fibrillanosema crangonycis spore. By No machine-readable author provided. Javier martin assumed (based on copyright claims)., via Wikimedia Commons
Microsporidia are obligate intracellular parasites of vertebrates and invertebrates. The most common species in the digestive system of humans are Enterocytozoon bieneusi and Encephalitozoon intestinalis.
Human infection with microsporidia is called microsporidiosis. It occurs mainly in people who have undergone organ transplants or are immunosuppressed, such as those infected with the Human Immunodeficiency Virus. They also affect children, the elderly or people who wear contact lenses.
The genomes of the species of this phylum are used as models to study host-parasite interactions.
General characteristics
The fungi of the phylum Microsporidia form nonmotile spores that vary in size depending on the species. Spores measuring between 1 and 4 microns have been found in human infections.
The spores have several typical Microsporidia organelles:
- The posterior vacuole that occupies more than a third of the cell volume.
- The polaroplast, a membranous structure located in the anterior segment of the cell.
- The anchor disk, a spiral-shaped structure that wraps around the sporoplasm and attaches the polar tube to the host cell during the infection process.
- The number of spirals that the organelle forms is a diagnostic characteristic of the phylum species.
Taxonomy and systematics
The taxonomy and systematics of the phylum Microsporidia has changed over time and continues to be controversial. It was initially classified in the Protista Kingdom, as a protozoan, due to the fact that they do not present chitin in the structures of most of the stages of the life cycle.
However, the results of studies using DNA techniques suggest that these organisms belong to the kingdom of fungi. Genomic data revealed that Microsporidia contain the genes necessary to produce chitin. In addition, chitin has been found in the resting spore structure.
There is also structural and metabolic evidence that allows Microsporidia to be recognized as true fungi. They apparently share a common ancestor with the phylum Zygomycetes and Mucorales.
The classification of this edge in terms of classes, orders and families is also controversial, so it continues to be reviewed and debated. Recent studies total about 150 genera and more than 1200 species.
Fourteen species have been identified as disease-producing in humans, distributed in the genera Anncaliia, Enterocytozoon, Encephalitozoon, Nosema, Pleistophora, Trachipleistophora and Vittaforma.
Lifecycle
Microsporidia, in the form of spores, can survive in open environments for a long time and under adverse conditions. When spores enter the gastrointestinal tract of a host, they leave their active form. Mainly due to variations in the pH of the environment and due to variation in the cation / anion concentration ratio.
During the activation process, the cell expels the polar tube and penetrates the host cell membrane, injecting it with infectious sporoplasm. Once inside the cell, two key reproductive phases occur in the microsporidium.
On the one hand, reproduction occurs by binary fission (merogony) or multiple (schizogony). During this phase, the reproduction of cellular material occurs repeatedly before cell division occurs, producing rounded forms of multinucleated plasmodia (E. bieneusi) or multinucleated cells (E. intestinalis).
On the other hand, sporogony occurs, a process that gives rise to spores. Both phases can occur freely in the cytoplasm of the cells or inside the vesicle.
When spores increase in number and fill the host cell's cytoplasm, the cell membrane ruptures and releases the spores to the surroundings. These mature spores, in their free state, can infect new cells, continuing the life cycle of microsporidia.
Diseases
Microsporidial infections in humans are known as Microsporidiosis. Gastrointestinal tract infection is the most common form of microsporidiosis.
In the vast majority of cases, it occurs from the ingestion of Enterocytozoon bieneusi spores. Other times it can occur from intestinal Encephalitozoon infections.
Microsporidia spores are capable of infecting any animal cell, including those of insects, fish and mammals. Sometimes they can infect other parasites.
Some species have specific hosts. Encephalitozoon cuniculi is host to rodents, rabbits, carnivores, and primates. E. hellem in birds of the genus psittasis.
E. intestinalis in donkeys, dogs, pigs, cattle, goats, and primates. Enterocytozoon bieneusi in pigs, primates, dogs, cats and birds. Annicaliia algerae is host to mosquitoes.
Infected animals and people release the spores into the environment with feces, urine, and respiratory secretions. Thus, person-to-person infections or contamination of water and food sources can occur, these being the most frequent sources of infection.
Symptoms
Enterocytozoon bieneusi and Encephalitozoon intestinalis infections manifest clinically with watery diarrhea in immunocompetent adults and children, especially in people who reside or travel to tropical countries.
In immunocompromised patients, those with HIV or another type of immune compromise, microsporidiosis presents as chronic diarrhea and wasting syndrome, cholangiopathy, and acalculous cholecystitis.
Other species can cause urinary tract infection, hepatitis, peritonitis, encephalitis, urethritis, prostatitis, nephritis, sinusitis, keratoconjunctivitis, cystitis, cellulitis, disseminated infection, systemic infection, pneumonitis, myositis, and skin infection.
Treatment
In patients with HIV infection, High Efficiency Antiretroviral Therapy (HAART) restores the immune response. It induces the elimination of the microorganism and the normalization of the intestinal architecture.
Albendazole, a tubulin inhibitor, is used in most microsporidial infections and especially species of the genus Encephalitozoon. The duration of treatment depends on the immune status of the patient and the type of infection, whether it is disseminated or localized.
Topical fumagillin is used in keratoconjunctivitis.
Immunocompetent patients can receive short treatments and sometimes the infection is overcome spontaneously, without the need for treatment.
References
- Cali, A., Becnel, JJ, and Takvorian, PM (2017). Microsporidia. In Handbook of the Protists, pp. 1559-1618.
- Cavalier-Smith, T. (1993). Kingdom Protozoa and Its 18 Phyla. Microbiological Reviews, 57 (4): 953-994
- Choappa, RC The phylum Microsporidia. Chilean Journal of Infectology, 35 (1): 73-74.
- Tedersoo, L., Sánchez-Ramírez, S., Koljalg, U., Bahram, M., Doring, M., Schigel, D., May. T., Ryberg, M. and Abarenkov, K. (2018). High-level classification of the Fungi and a tool for evolutionary ecological analyzes. Fungal Diversity 90: 135–159.
- Wikipedia contributors. (2018, September 14). Microsporidia. In Wikipedia, The Free Encyclopedia. Retrieved 07:22, October 18, 2018, from en.wikipedia.org