Basidiobolus ranarum

In 1886, the fungus was first isolated from the dung and intestinal contents of frogs by Eidam.[9] In 1927, it was found in the intestines of toads, slowworms, and salamanders by Levisohn.[10] In 1956, Joe et al. reported and described the first four cases of zygomycosis in Indonesia.[3] Since then, hundreds of the cases of this infection have been reported. In 1955, Drechsler isolated it from decaying plants material in North America.[11] In 1971, it was first isolated by Nickerson and Hutchison from aquatic animals, suggesting that B. ranarum can survive in a wild range of ecological situations.[12]

At room temperature (25–30 °C), colonies of B. ranarum show very rapid growth and are able to reach a diameter of 75–80 mm in a week on suitable growth media.[2] The favored carbohydrate source of this fungus is glucose that can stimulate the growth of its mycelium.[13] Generally, asexual reproduction is favored by glucose and sexual reproduction is favored by acid amines.[14] Primary asexual spores are singly formed on the apices of unbranched hyphae and will then be discharged to form ballistic spores.[15] Secondary asexual spores are singly developed from a hypha that was generated from a geminated ballistic spore.[1][15] Also, sporangiospores can be generated by internal cleave of the cytoplasm and can then be dispersed when the sporangial wall is dissolved. As a result, the ejected asexual spores can form satellite colonies in a distance. After around 10 days of growth, sexual spores, zygospores with 20–50 μm diameters can also be produced.[16] This fungus is believed to have significant protease and lipase activity.[17][18] Its lipase has a maximum activity at 35 °C and pH 6.0[18] while its protease has maximum activity at 30 °C and pH 5.5.[17] Both enzymes might be involved in pathogenesis.[17][18] Light does not affect hyphal growth light but may influence certain aspects of physiology.[19] First, light may stimulate the production of the asexual spores, and certain blue lights (wavelengths 440 nm and 480 nm) may further stimulate the discharge of the those spores.[20] Second, light may also stimulate the induction of aerial hyphae and favor the unicellular configuration of the hyphae while darkness may favor their bicellular configuration.[19]

Gustav Lindau illustration of Basidiobolus ranarum

Colonies of B. ranarum are round, flat, waxy, glabrous and radially folded. And, their color is in a range of yellowish-grey to whitish-grey.[15][21] A one week-old colony can reach 75–80 mm in diameter.[2] A white bloom, consisting of mycelia and sporangiospores, covers the colonies.[2] Under microscope, younger hyphae are wide and have few septa. Older cultures have colorless zygospores (20–50 μm) with smooth, thick walls and abundant large, spherical, darkly coloured chlamydospores.[1][15] The colonies commonly produce a strong Streptomyces-like or benzene hexachloride-like odour.[21]

Basidiobolus ranarum has a worldwide distribution[1] and is capable of living saprotrophically in a broad range of ecological situations,[12] indicating its great ecological and physiological tolerance as well as its ubiquity.[6] Basidiobolus ranarum was widely reported from all parts of the world, especially Asia and Africa.[1] It can saprophytically live in the intestines of vertebrates including amphibians (e.g. frogs, toads, salamanders, mudpuppy), reptiles (e.g. chameleons, wall geckoes, snakes, lizards, turtles), and fishes (e.g. sturgeon).[9][10][12][6][22][23][24][25] In addition, studies also reported occasional presence of B. ranarum in the intestinal contents of mammals such as one bat in India[26] and the kangaroos in Australia.[27] Moreover, other habitats including compost heaps, decaying plant material and soil can also be their place to live.[11][22][28] However, the habitat for B. ranarum is not fixed and a life-cycle illustration of it might provide a better idea of the variation of its habitats. First, insects might eat feces and decaying plant materials in which B. ranarum might be present,[9][10] or insects might have physical contact with the strains so that the strains can attach to the insects externally.[6] Then, those insects might be devoured by predators, such as frogs.[9][10][6] Next, the fungi will travel through the predator’s gastrointestinal tract and might either stay a little bit longer (as long as 18 days) at or leave from the intestine along with the feces. Eventually, the strains in those feces will end up in the soil and some of them will be further transported to decaying plant materials or other organic contents.[6] Also, the tissues that the pathogenic strains of B. ranarum infect can also be considered as its habitats, B. ranarum can also live in both human and non-human animal (e.g. horses,[29] frogs[30]) tissues. However, instead of a worldwide distribution, the pathogenic lifestyle of B. ranarum only exists in tropical and subtropical regions.[6]

Subcutaneous zygomycosis (also known as "entomophthoromycosis basidiobolae", subcutaneous phycomycosis, and basidiobolomycosis)[5][3][4] is a both human and non-human animal disease or lesion caused by the granulomatous infection of subcutaneous tissue by B. ranarum.[5] Several enzymes produced by B. ranarum, including lipase and protease, might hydrolyze and utilize the fatty tissues of the host and contribute to the pathogenesis of the infection.[17][18][31]