Rhizopus spp. Taxonomic classification Kingdom: Fungi Phylum: Zygomycota Order: Mucorales Family: Mucoraceae Genus: Rhizopus Description and Natural Habitats Rhizopus is a cosmopolitan filamentous fungus found in soil, decaying fruit and vegetables, animal feces, and old bread. While Rhizopus spp. are common contaminants, they are also occasional causes of serious (and often fatal) infections in humans. Some species are plant pathogens. Species The genus Rhizopus contains several species. The most common ones are Rhizopus arrhizus, Rhizopus azygosporus, Rhizopus microsporus, Rhizopus schipperae, and Rhizopus stolonifer. Some morphological features, such as the length of rhizoids and sporangiophores, the diameter of sporangia, the shape of columellae, and the size, shape and surface texture of sporangiospores aid in differentiation of Rhizopus species from eachother. Maximum growth temperature also varies from one species to other. Pathogenicity and Clinical Significance Rhizopus spp. are among the fungi causing the group of infections referred to as zygomycosis. Although the term mucormycosis has often been used for this syndrome, zygomycosis is now the preferred term for this angio-invasive disease. Rhizopus arrhizus is the most common cause of zygomycosis and is followed by Rhizopus microsporus var. rhizopodiformis. Zygomycosis includes mucocutaneous, rhinocerebral, genitourinary, gastrointestinal, pulmonary, and disseminated infections. Diabetic ketoacidosis and immunosuppression due to various reasons, such as organ transplantation are the most frequent predisposing factors. Desferoxamine treatment, renal failure, extensive burns, trauma, and intravenous drug use may also predispose to development of zygomycosis. While heatstroke has been described as a risk factor for disseminated zygomycosis, contaminated adhesive tapes and wooden tongue depressors have been reported to lead to nosocomial outbreaks of zygomycosis. Vascular invasion that causes necrosis of the infected tissue, and perineural invasion are the most frustrating features of these infections. Zygomycosis is frequently fatal. Macroscopic Features Colonies of Rhizopus grow very rapidly, fill the Petri dish, and mature in 4 days. The texture is typically cotton-candy like. From the front, the color of the colony is white initially and turns grey to yellowish brown in time. The reverse is white to pale. Pathogenic species of Rhizopus can grow well at 37°C. Microscopic Features Nonseptate or sparsely septate broad hyphae (6-15 µm in diameter), sporangiophores , rhizoids (root-like hyphae), sporangia, and sporangiospores are visualized. Sporangiophores are brown in color and usually unbranched . They can be solitary or form clusters. Rhizoids are located at the point where the stolons and sporangiophores meet. Sporangia (40-350 µm in diameter) are located at the tip of the sporangiophores. They are round with flattened bases. Apophysis is absent or rarely apparent and columellae are hemispherical. Sporangiospores (4-11 µm in diameter) are unicellular, round to ovoid in shape, hyaline to brown in color, and smooth or striated in texture. Do send useful feedback.