Rhizopus
Rhizopus is a genus of filamentous fungi, commonly known as molds, that primarily thrive on dead organic matter, such as decaying fruits, vegetables, and other decomposing materials. While generally saprophytic and non-pathogenic, certain species, particularly Rhizopus arrhizus, can cause zygomycosis—a serious opportunistic infection. This condition is more prevalent in individuals with weakened immune systems due to factors like diabetes, organ transplants, or prolonged corticosteroid use, making them particularly susceptible to infections initiated through inhalation of fungal spores.
Rhizopus colonies can be identified by their cottony appearance, which transitions from white to gray or yellowish brown as they mature and release spores. Infection can manifest in various forms, including rhinocerebral, pulmonary, and gastrointestinal zygomycosis, and may lead to severe complications if untreated. Treatment typically involves antifungal medications such as amphotericin B, along with possible surgical intervention to remove necrotic tissue. Due to the increasing incidence of zygomycosis, early detection and aggressive management are vital for improving outcomes in affected patients.
Rhizopus
- TRANSMISSION ROUTE: Direct contact, ingestion, inhalation
Definition
Rhizopus is a genus of saprophytic 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 saprophyte) 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.
Rhizopus colonies 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 gray 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 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 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. However, it appears to be on the rise in the United States among persons with predisposing factors. These factors include diabetic acidosis; immunosuppression caused by bone marrow transplantation or corticosteroid therapy; and immunodeficiency. Other factors that may predispose a person to develop zygomycosis include treatment with deferoxamine (to remove excess iron), renal failure, extensive burns, trauma, prematurity, and intravenous drug abuse. The body’s natural defense mechanisms against fungal infections have been compromised in persons with these conditions or those receiving these therapies. Additional risk factors include malnutrition, hemochromatosis, organ transplantation, stem cell transplantation, intravenous catheter use, and prolonged use of antibiotics.
The primary route of infection begins with the 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 infections can develop after spores are inhaled from 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. Still, newer azoles, like posaconazole and isavuconazole, have shown increased effectiveness.
Early detection and aggressive treatment are critical to successfully 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, or treatment with posaconazole and isavuconazole must begin. Surgery is usually required to remove infected dead tissue.
Bibliography
Bowers, J. R., et al. "Rhizopus microsporus Infections Associated with Surgical Procedures, Argentina, 2006–2014." Emerging Infectious Diseases, vol. 26, no. 5, 2020, pp. 937-944, doi.org/10.3201/eid2605.191045. Accessed 28 Sept. 2024.
Brown, J. "Zygomycosis: An Emerging Fungal Infection." American Journal of Health-System Pharmacy, vol. 62, 2005, pp. 2593-2596.
Richardson, Malcolm D., and David W. Warnock. Fungal Infection: Diagnosis and Management. 4th ed., Malden, Mass.: Wiley-Blackwell, 2012.
Ryan, Kenneth J., and George Ray. Sherris Medical Microbiology: An Introduction to Infectious Diseases. 5th ed., New York: McGraw-Hill Medical, 2010.
St. Georgiev, Vassil. Opportunistic Infections: Treatment and Prophylaxis. Totowa, N.J.: Humana Press, 2003.