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Mould Identification: A Virtual Self Assessment

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Unknown = Rhizopus arrhizus

Case History
A 10 year-old girl with idiopathic aplastic anemia presented with pancytopenia and febrile neutropenia. Immunosuppressive therapy included antihymocyte globin, cyclosporin and high dose methylprednisolone. A progressive swelling with blisters and ulceration developed on her right thigh. Direct examination of biopsy tissue revealed abundant filamentous hyphae and all cultures grew the fungus shown below.
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 (24 hours growth)

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.

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 zygomycosis, 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.

About Rhizopus Back to Virtual Assessment

What is your identification?

Rhizopus microsporus
Rhizopus arrhizus
Rhizomucor pusillus

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School of Biological Sciences



Dr David Ellis