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Unknown 12 = Rhizopus arrhizus
Direct microscopy: Tissue morphology in zygomycosis showing distinctive infrequently septate thin walled hyphae with focal bulbous dilations and irregular branching, typical for those species belonging to the Mucorales.
Culture: On Sabouraud's dextrose agar, colonies are very fast growing at 25C, reaching 5-8 mm in height, with some tendency to collapse, white cottony at first, becoming brownish grey to blackish-grey depending on the amount of sporulation.
Microscopy: Sporangiophores up to 1500 um in length and 18 um in width, smooth walled, non-septate, simple or branched, arising from stolons opposite rhizoids usually in groups of 3 or more.
Sporangia are globose, often with a flattened base, greyish black, powdery in appearance, up to 175 um in diameter and many spored. Columella and apophysis together are globose, subglobose or oval, up to 130 um in height and soon collapse to an umbrella-like form after spore release. Sporangiospores are angular, subglobose to ellipsoidal, with ridges on the surface, and up to 8 um in length.
Growth temperature studies: No growth at 45C; good growth at 37C.
Key Features: Sporangiospores often higher than 1 mm. sporangia mostly over 100 um diam, growth at 37C but not at 45C.
Comment: Rhizopus arrhizus is the most common causative agent of mucormycosis, accounting for some 60% of the reported culture positive cases, and nearly 90% of the rhinocerebral forms of infection. R. arrhizus has a world-wide distribution with a high prevalence in tropical and subtropical regions. It has been isolated from many substrates, including a wide variety of soils, decaying vegetation, foodstuffs, and animal and bird dung. R. arrhizus is often used in the production of fermented foods and alcoholic beverages in Indonesia, China and Japan. However, it also produces the ergot alkaloid agroclavine which is toxic to humans and animals.
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