Rhizopus oryzae is a widespread microscopic fungus, a common mold found globally. This filamentous microfungus is a saprotroph, meaning it obtains nutrients by decomposing organic matter. It is a well-studied fungus, recognized for both its beneficial industrial applications and its potential as a spoilage agent and opportunistic pathogen.
Biological Characteristics and Natural Occurrence
Rhizopus oryzae is classified within the genus Rhizopus, which belongs to the phylum Mucoromycota. This mold is characterized by its rapid growth and typically appears as fuzzy, brownish-grey to blackish-grey colonies. It develops root-like structures called rhizoids, which anchor the fungus and absorb nutrients from its surroundings.
Reproduction in Rhizopus oryzae occurs through both asexual and sexual methods. Asexual reproduction involves the formation of spores within spherical structures called sporangia, which are supported by stalks known as sporangiophores. These sporangia are globose and black when mature, with air-dispersed sporangiospores. Sexual reproduction involves the fusion of two compatible mycelia, leading to the formation of a dark zygospore.
This fungus naturally occurs in various environments worldwide, particularly in tropical and subtropical regions. It is commonly found in soil, dung, and decaying organic matter, including fruits, vegetables, and seeds. For example, R. oryzae has been isolated from peanuts in East Asia, with a reported 21% isolation rate from peanut kernels in Indonesia. It can also be found on a variety of other tropical foods such as maize, beans, sorghum, and potatoes.
Beneficial Applications
Rhizopus oryzae holds significant value in various industrial and traditional applications. It is notably used in the production of tempeh, a traditional Indonesian fermented soybean food. During tempeh production, R. oryzae mycelium binds soybeans together into a cake, and its enzymes break down complex molecules, improving digestibility and nutrient bioavailability. This fermentation process can enhance the content of B group vitamins, such as riboflavin and thiamine, and reduce anti-nutritional factors like phytic acid.
The fungus is also used for the industrial production of various enzymes and organic acids. It produces carbohydrate-digesting enzymes like amylases, cellulases, and proteases, which are valuable in food processing. R. oryzae produces organic acids such as L(+)-lactic acid and fumaric acid, with some strains demonstrating resistance to high concentrations of lactic acid. Furthermore, it is employed in the production of alcoholic beverages in parts of Asia and Africa.
Impact as a Pathosis and Spoilage Agent
Rhizopus oryzae is a common opportunistic human pathogen, responsible for mucormycosis, also known as black fungus disease. This infection is characterized by the rapid growth of fungal hyphae within and around blood vessels. Mucormycosis predominantly affects individuals with weakened immune systems, such as those with uncontrolled diabetes mellitus, neutropenia, or those undergoing chemotherapy.
The fungus can also produce toxins, such as agroclavine, which can be harmful to humans and livestock. Mucormycosis can affect various body parts, including the sinuses, brain, and lungs, and requires prompt diagnosis and treatment due to its rapid progression. In addition to its pathogenic role, Rhizopus oryzae is a spoilage agent for many food products.
It causes soft rot in many fruits and vegetables, leading to visible mold growth and decay. Common affected items include sweet potatoes, sugar-beet, groundnuts, strawberries, tomatoes, and bread. This spoilage results in post-harvest losses, sometimes reaching up to 50% in certain fruits. The molds produce acidic compounds that alter the color, texture, and taste of the affected produce.
Understanding and Managing Its Presence
Identifying Rhizopus oryzae in different contexts relies on both macroscopic observation and microscopic examination of its characteristic structures. In industrial settings for beneficial applications, conditions like temperature ranges between 30°C to 35°C and controlled water levels promote its growth. For preventing food spoilage, proper food storage and hygiene practices are important, as the fungus thrives in moist environments and on organic substances.
In cases of pathogenic infection, early detection improves outcomes. Diagnostic methods include quantitative polymerase chain reaction (PCR) for detecting circulating fungal DNA and the identification of specific antigens. Treatment for mucormycosis involves antifungal medication like amphotericin B and posaconazole, alongside surgical removal of infected tissue. Managing underlying predisposing factors, such as controlling blood sugar levels in diabetic patients, also supports treatment success.