Rhizopus spp- An Overview

Rhizopus spp are saprophytic fungi found on plants and it is parasitic on animals.

• They are multicellular fungi, with about 8 species.
• They are responsible for causing an opportunistic infection known as invasive mucormycosis in humans and animals.
• Some species are used for industrial importance such as Rhizopus stolonifer which caused bread mold.
• Rhizopus infections may also be a complication of ketoacidosis.
• Phylogenic characterization of Rhizopus categorizes eight species Rhizopus schipperae, Rhizopus delemar, Rhizopus microsporus, Rhizopus caespitosus, Rhizopus arrhizus (Rhizopus oryzae), Rhizopus reflexus, Rhizopus homothallicus,  and Rhizopus stolonifer.
• Medically important groups are Rhizopus oryzae and Rhizopus microsporus.

Image Source: BrainKart and Dr. Gary E. Kaiser.

Habitat of Rhizopus spp

• This species is a cosmopolitan group of fungi.
• It is found on a wide variety of surfaces, on organic substances like soil, decaying fruits, grown fruits and vegetables, jellies, tobacco, peanuts, leather, bread, and syrups.
• They are also found in animal feces

• Rhizopus species grow as filamentous, with branching hyphae which are coenocytic (multinucleated).
• The branched hyphae are of three types: stolons, pigmented rhizoids, and unbranched sporangiophores.

Figure: Macroscopic and microscopic morphology of Rhizopus microsporus CBS 700.68. A: Colony on MEA after 2 days incubation at 30 °C; B: Zygospores with unequal suspensors; B₁–B₃. Azygospore from CBS 344.29. C: Columella; D: Sporangia; E: Rhizoids; F: Columella with a collar. Scale bar = 10 μm. Image Source: Springer Nature.

• They reproduce both sexually and asexually.
• Asexual reproduction produces sporangiospores which are formed in a spherical structure known as a sporangium.
• The sporangiospores are globose to ovoid, single-celled, hyaline to brown, and striate.
• The sporangia are produced in large numbers, which are dark, and are formed on a long stalk conidiophore known as a sporangiophore.
• The sporangiophores arise from root-like rhizoids, and they rounded, responsible for producing numerous multinucleated spores.
• Sexual reproduction produces dark zygospores formed when two compatible mycelia fuse.
• Zygospores germinate forming genetically different offsprings from the parents.
• Colonies are fast-growing, covering the surface of the agar.
• Rapidly growing colonies fade from white to dark during sporulation.
• The colonies have a dense cottony growth or candy flossy or fairly floss in texture.

Figure: Life cycle of Rhizopus. Image Source: BrainKart

Pathogenesis of Rhizopus spp

• Rhizopus spp causes infections in animals, humans, and plants.
• Human infection by Rhizopus spp is known as mucormycosis also known as zygomycosis.
• Plant diseases are collectively known as rots affecting harvested fruits (strawberries), tomatoes, sweet potatoes, tobacco, papayas, and stone fruits.

Pathogenesis in human and animal host: Murcomycosis/Zygomycosis

• The infection is caused by Rhizopus arrhizus, Rhizopus oryzae and Rhizopus microsporus.
• The fungus is transmitted into the host via air spore inhalation which deposits in the paranasal sinuses and the lung.
• Risk factors include diabetes mellitus,  malnutrition, malignancies (lymphomas and leukemias), renal failure, organ transplant, burns, immunosuppressive therapy, cirrhosis, and AIDS.
• Patients with diabetic ketoacidosis and patients on dialysis who receive treatment with iron chelator deferoxamine are also more susceptible to mucormycosis.
• The most common form of mucormycosis is rhinocerebral mucormycosis.

Clinical manifestation of mucormycosis/zygomycosis 

• The infection is characterized by the invasion of blood vessels in humans and animals.
• The infection then progresses to other body areas including the lungs and the brains.
• Generally, these infections are broken down into five presentations: rhinocerebellar, pulmonary, cutaneous, gastrointestinal, and disseminated.

Laboratory Diagnosis of Mucormycosis (Rhizopus spp)

Specimen: Sputum, pus/tissue biopsies, blood/serum, transbronchial biopsy, sinuses biopsy, gastrointestinal tissue biopsy, skin biopsy

Microscopic Examination

• 10-20% KOH wet mount to observe nonseptate or sparsely septate broad hyphae, sporangiophores, root-like hyphae (rhizoids), sporangia, sporangiospores.
• Observe brown colored unbranched sporangiospores occurring in clusters at the tips of the hyphal tubes.
• Singly occurring spores may also be observed which are ovoid-shaped, hyaline to brown colored.
• The spores are rounded with flattened bases.

Figure: Morphological characteristics of Rhizopus oryzae isolated from orange. A: Colony on PDA after 7 days of inoculation, B: Sporangium and sporangiophore, C: Columellum, D: Sporangiospores, E: Rhizoid. Image Source: Korea Institute of Science and Technology Information.

Cultural Examination

• Sabouraud Dextrose Agar (SDA) with cycloheximide at 37°C produces a characteristic dense cottony-fluffy growth with a flossy texture.
• Macroscopic observation of the color of the colonies is initially white and turns grey to yellowish-brown as culture matures, Reverse pigmentation is white to pale

Histological examination

• Using Hematoxylin and eosin stain (H&E stain or HE stain) to observe for wide, thin-walled, hyaline septate or nonseptate hyphae in tissue samples. The hyphae are also observed to be non-parallel and branched

Molecular Diagnosis

• PCR and genomic sequencing for phylogenic identification and differentiation of the Rhizopus specific species.

Differential diagnosis

• Differentiating Pulmonary mucormycosis from other fungal infections such as aspergillosis can also assist in the identification of causative agents.
• This is done during the physical and symptomatic examination of patients inclusive with laboratory diagnosis.

Treatment of Mucormycosis

• Mucormycosis treatment involves combined antifungal treatment along with the surgical procedure of the involved tissues.
• Intravenous administration of amphotericin B is the drug of choice.
• Posaconazole or isavuconazole can be used as second line of tretament after amphotericin B.
• Adjunctive treatment with hyperbaric oxygen is given in combination with primary therapy.
• Other antifungal drugs include voriconazole, fluconazole, and flucytosine can be used depending o the causative agent of mucormycosis.
• Treatments for underlying conditions are mandatory with close observation of patients for contraindications.

Selected species of Rhizopus and industrial uses

There are 8 known species of Rhizopus of which 3 are of industrial and medical importance and they include:

• Rhizopus microsporus var. oligosporus
• Rhizopus oryzae
• Rhizopus stononifer

Rhizopus microsporus var oligosporus

• It is used to produce fumaric acid, lactic acid, and cortisone.
• It is used to make fermented soybeans known as tempeh.

Rhizopus oryzae or Rhizopus arrhizus

• It is used in the production of lactic acid and cortisone.
• It is also used for alcoholic fermentation
• Its also used for the biosorption of heavy metals which is the passive adsorption of chemical contaminants by organisms.

Rhizopus stononifer

• It is also known as black bread mold.
• It causes fruit rot on strawberry, tomato, and sweet potato (soft rot).
• It is used in the commercial production of fumaric acid, lactic acid, and cortisone.

References and sources

1. https://mycology.adelaide.edu.au/descriptions/zygomycetes/rhizopus/
2. https://www.britannica.com/science/Rhizopus
3. drfungus/knowledgebase/rhizopus
4. https://en.wikipedia.org/wiki/Rhizopus
5. https://www.uptodate.com/contents/mucormycosis-zygomycosis/print
6. https://www.ncbi.nlm.nih.gov/books/NBK544364/
7. https://www.sciencedirect.com/science/article/pii/B012227055X011299