What is rhizopus?

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Rhizopus is a genus of saphrocytic filamentous fungi (molds) with species that may cause zygomycosis.
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Rhizopus is a genus of saphrocytic filamentous fungi (molds) with species that may cause zygomycosis.

Natural Habitat and Features

Rhizopus is a filamentous fungus found worldwide that lives on dead organic material (as a saphrocyte) in soil, decaying fruit and vegetables, old bread, and animal feces. Rhizopus species are common contaminants that can cause serious, even fatal, infections in humans.

Colonies of Rhizopus mature in four days at 98.6° to 113° Fahrenheit (37-45° Celsius) on a standard agar medium. The texture is typically dense and cottony. From the front, the colony is initially white, turning to grey or yellowish brown with the release of spores. The reverse is white to pale.

On microscopic observation of the colony, broad, thin-walled hyphae (filaments) are observed. They are either not septate (segmented) or sparsely septate. Sporangiospores, specialized structures on the hyphae, carry sporangia (the spores or sporangiospores). The sporangiospores are mostly brown and unbranched. The sporangia are located at the tip of the sporangiospores and are round with flattened bases. They can be solitary or can form clusters. Swelling or projection (apophysis) of sporangia is absent or rarely seen. The sporangiospores are one-celled, round to ovoid, hyaline (transparent) to brown, and smooth or striated.

Other structures observed are rhizoids, which are rootlike hyphae located at the point where the stolons (stems of hyphae) and sporangiospores meet, and columella, which are small, column-like spherical or elongated structures. After the release of spores, apophyses and columella often collapse to form an umbrella-like structure. Features such as the length of sporangiospores; presence, length, and pigmentation of rhizoids; diameter of sporangia; presence and shape of columella, presence of stolons; and the size, shape, and surface texture of sporangiospores help differentiate among the different species of Rhizopus and between Rhizopus and other fungi of the phylum Zygomycota.

Pathogenicity and Clinical Significance

Rhizopus species are among the fungi that cause zygomycosis, a syndrome of invasive, opportunistic infections. This syndrome was formerly called mucormycosis. Other fungi with species that cause zygomycosis include the genera Absidia and Mucor. Among all cases of zygomycoses in humans, R. arrhizus is the most common cause.

Zygomycosis rarely occurs in healthy persons. It does, however, appear to be on the rise in the United States among persons with predisposing factors. These factors include diabetic acidosis; immunosuppression, such as that caused by bone marrow transplantation or corticosteroid therapy; and immunodeficiency. Other factors that may predispose a person to develop zygomycosis include treatment with desferoxamine (to remove excess iron), renal failure, extensive burns, trauma, prematurity, and intravenous drug abuse. In persons with these conditions or in persons receiving these therapies, the body’s natural defense mechanisms against fungal infections have been compromised.

The primary route of infection begins with inhalation of spores that have been released into the air. Initial infection usually occurs in the nasal sinuses or the lungs. Once the infection penetrates the mucosal layer, it invades underlying tissue, nerves, and blood vessels and can disseminate through the circulatory system. Zygomycosis includes mucocutaneous, rhinocerebral, pulmonary, gastrointestinal, and disseminated infections. In rhinocerebral disease, the most common form of zygomycosis, the infection rapidly disseminates from the paranasal sinuses. If untreated, it can reach the brain stem, leading to coma and even death within a few days.

Microsporus and rhizopodiformis are associated with cutaneous infections traced to contaminated surgical dressings and splints in hospital settings. Burn patients are especially vulnerable to these infections, which can lead to gangrene. Gastrointestinal infection can develop after ingestion of spores on spoiled food.

Drug Susceptibility

Little data are available on the susceptibility profile of Rhizopus species, even in the laboratory (in vitro) setting. In one study, the minimum inhibitory concentration for amphotericin B was lower than that of the azoles itraconazole, ketoconazole, and voriconazole against strains of arrhizus. Amphotericin B remains the drug of choice when treating zygomycosis caused by Rhizopus species.

Early detection and aggressive treatment are critical if there is to be success in treating zygomycosis. The first step is to reverse or control the underlying disease, immunosuppression, or other factors facilitating the infection. Amphotericin B at high intravenous doses must be administered. No other antifungal agents are effective against invasive infections caused by Rhizopus. Surgery is usually required to remove infected dead tissue.


Brown, J. “Zygomycosis: An Emerging Fungal Infection.” American Journal of Health-System Pharmacy 62 (2005): 2593-2596. Discusses the growing frequency of zygomycosis cases in the United States

Richardson, Malcolm D., and David W. Warnock. Fungal Infection: Diagnosis and Management. New ed. Malden, Mass.: Wiley-Blackwell, 2010. Chapter 13 contains valuable information related to Rhizopus and other fungi that cause zygomycosis.

Ryan, Kenneth J., and George Ray. Sherris Medical Microbiology: An Introduction to Infectious Diseases. 5th ed. New York: McGraw-Hill Medical, 2010. A first text in microbiology for students in medicine and medical science, with a focus on infectious diseases. Margin notes and a glossary help make the information more accessible. Chapter 45, on opportunistic infections, discusses zygomycosis.

St. Georgiev, Vassil. Opportunistic Infections: Treatment and Prophylaxis. Totowa, N.J.: Humana Press, 2003. Examines zygomycosis as an opportunistic infection. Covers prevention and treatment.