User:Visviva/Blastomyces

Blastomyces is a genus of fungi in the order Onygenales. Most species in this genus are known to cause the fungal disease blastomycosis, which can be fatal to humans, dogs, and other mammals.

Blastomyces fungi are dimorphic, with a mold phase and a yeast phase. The ability to transition between these two phases is crucial to their ability to infect people and animals. In the soil the fungi take the form of a filamentous mold. But when a Blastomyces spore enters a mammal's body, typically through the respiratory tract, it transitions to a yeast.

Several species of Blastomyces exist, most of which were only recently discovered. The best-known species is Blastomyces dermatitidis, found in much of eastern North America. Blastomyces species are also found in western North America, Africa, the Middle East and possibly India.

The natural ecology of the Blastomyces fungi is poorly understood, but they are known to prefer moist soils near watercourses, and to tolerate high levels of ammonia.

Description


The Blastomyces fungi are dimorphic: as temperatures increase, by approximately 37°C they transition from their mycelial form and become yeast-like cells. This transition typically happens after a mammal inhales Blastomyces spores, and leads to a blastomycosis infection.

In the mold phase, Blastomyces forms colonies that are yellowish-white to ochre in color. Under a microscope, it consists of long filaments called hyphae, with stalks called conidiophores branching off from the hyphae. The conidiophores bear one or more spores, or conidia. In the best-known species of Blastomyces, the hyphae are hyaline, and the conidiophores are narrow, unbranched, and run perpendicular to the hyphae.. There is considerable variation between species. For example, the conidiophores of the primarily North American species B. dermatitidis and B. gilchristii end in a single spore so that the conidiphore resembles a lollipop, while those of the primarily African species B. percursus end in a branching cluster that resembles a floret.

In the yeast phase, Blastomyces has a distinctive appearance. Its cells are typically in the 10–15 micron range and have thick, double-contoured cell walls and form a single bud with a broad base.

The ability to transition between the yeast and mold phases is crucial to Blastomyces' ability to infect its human and animal hosts. Strains of Blastomyces that lack the necessary genes for the phase transition have very little ability to cause disease. In the B. dermatitidis species complex, the most important protein for the phase transition is BAD1 (Blastomyces adhesin 1). BAD1 is secreted outside the cell; it helps the yeast cells to attach to the host cells and also inhibits the production of tumor necrosis factor by neutrophils and macrophages.

In the mold phase, Blastomyces typically reproduces asexually through airborne spores known as conidia. However, it is also capable of sexual reproduction. The sexual form, or teleomorph, was formerly considered a separate genus, Ajellomyces.

Blastomyces grows slowly, and cultures may take 4-6 weeks before they grow sufficiently to be diagnosed.

Taxonomy
Despite widespread use, the genus Blastomyces was formerly considered invalid under the International Code of Botanical Nomenclature. This is because under Article 53.1 of the Code, a taxon name is illegitimate if it "is spelled exactly like a name based on a different type that was previously and validly published for a taxon at the same rank", and the name Blastomyces had previously been published for the fungus now known as Chrysosporium. However, the genus name has subsequently been conserved.

Along with other important genera of human-pathogenic fungi such as Histoplasma and Paracoccidioides, Blastomyces belongs to the family Ajellomycetaceae. The three principal pathogens in this family, including Blastomyces, all share the property of switching from a mold-like growth form in the soil to a yeast-like growth form in a warm-blooded animal host.

Species
Prior to 2013, Blastomyces was considered monotypic, containing only the species B. dermatitidis. B. dermatitidis remains the type species of the genus. Together with B. gilchristii, it makes up the "Blastomyces dermatitidis species complex".

The known Blastomyces species include the following:


 * Blastomyces dermatitidis was the first Blastomyces species described, in 1898. For more than a century it was the only fungal species known to cause blastomycosis.


 * Blastomyces emzantsi is found in South Africa. It was discovered in 2020 based on genetic analysis of African samples from 1967 to 2014. It chiefly causes infections of the skin and bone rather than the lungs. In contrast to the "lollipop" conidiophores of B. dermatitidis and B. gilchristii, B. emzantsi has branching conidiophores that form clusters of conidia resembling florets.


 * Blastomyces gilchristii was described in 2013 based on phylogenetic analysis. Some strains that were previously assigned to B. dermatitidis are now recognized as B. gilchristii. B. gilchristii differs from B. dermatitidis in having a much more limited geographic range, and in being more associated with large blastomycosis outbreaks. However, there no known clinically significant differences between B. gilchristii infections and B. dermatitidis infections.


 * Blastomyces helicus is found in western North America, outside the range of B. dermatitidis. It typically infects people with weakened immune systems, often spreads throughout the body, and has a high fatality rate. B. helicus differs from many other Blastomyces fungi in its morphology: in the mold phase, it has helically curled hyphae, and does not appear to form conidia; in the yeast phase, it forms smaller cells than B. dermatitidis and forms multiple buds per cell. Exactly how B. helicus spreads in the environment without spores is unclear. The earliest known sample of B. helicus dates to 1970, but the species was only discovered in 2015 through genetic analysis. The species was initially named Emmonsia helica because the Blastomyces taxon had not yet been conserved. The research that led to the discovery of B. helicus was inspired by a 2014 tweet that showed what was described as an African form of Emmonsia isolated from a patient in the western US.


 * Blastomyces parvus, formerly known as Emmonsia parva or Haplosporangium parvum, was discovered in 1942 by Chester Wilson Emmons and Llewelyn L. Ashburn. It was assigned to Blastomyces in 2018. It primarily infects rodents but can also infect humans. It is clinically distinct from other Blastomycosis species: the disease it causes is known as adiaspiromycosis, and is caused by spores that swell but do not replicate when they enter the lungs.


 * Blastomyces percursus was discovered in 2017 through phylogenomic analysis. It is the chief cause of blastomycosis in Africa and the Middle East. It has been identified in samples from Angola, the Democratic Republic of Congo, Israel, Morocco, Mozambique, South Africa and Uganda. In the mold phase, it shares the morphology of B. emzantsi, with floret-like conidia. In the yeast phase, it resembles B. dermatitidis and B. gilchristii, with relatively large cells and broad-based budding.


 * Blastomyces silverae, discovered in 2018, is named after the mycologist Eleanor Silver Keeping. It is not known to cause disease in humans. The type specimen was taken from coyote dung in Alberta. Like B. parvus, in the yeast phase it mostly produces giant cells.

Distribution and habitat
Blastomyces is considered an endemic fungus, a dimorphic pathogenic fungus found in certain restricted geographic areas. Other endemic fungi include Histoplasma and Emergomyces.

Ecology
The natural ecology of Blastomyces is poorly understood. In the mold phase, it is believed to be most common in "moist, acidic, sandy soils enriched with decaying organic matter and animal droppings".