Rhizopus is one of the five genera of medically important opportunistic fungi, with the other four being Candida, Cryptococcus, Aspergillus, and Mucor. Rhizopus and Mucor are saprophytic molds but can cause mucormycosis (zygomycosis, phycomycosis) in immunocompromised individuals.
- These saprophytic molds are found widely in the environment worldwide (soil and decaying vegetation). They are not dimorphic fungi.
- Hyphae are nonseptate and right-angle branching or wide-angle branching is seen
- Culture grows mold with black spores.
- Conidia of Mucor and Rhizopus are enclosed within a sac called sporangium. Conidia are spherical, yellow, or brown spores (sporangiospores).
- The sporangium is formed at the tip of the supporting structure called sporangiophore.
Rhizopus and Mucor are transmitted by inhalation of airborne asexual spores. They invade the tissues of patients with reduced host defenses. Patients with leukemia, diabetic ketoacidosis, renal acidosis, burns, or bone marrow transplants are particularly susceptible.
When immunocompromised individuals acquire these saprophytic fungi, these organisms proliferate in the walls of blood vessels, particularly in the paranasal sinuses, lungs, or gut, and cause infarction and necrosis of tissue distal to the blocked vessel. Diabetic patients are particularly susceptible to rhinocerebral mucormycosis, in which mold spores in the sinuses germinate to form hyphae that invade blood vessels supplying the brain. One species, Rhizopus oryzae, causes about 60% of cases of mucormycosis.
Sample: Tissue specimens or exudate from infected lesions
In biopsy specimens stained with calcofluor white or KOH mount, organisms are seen microscopically as nonseptate hyphae with broad, irregular walls and branches that form more or less at right angles. Microscopic findings of organisms resembling zygomycetes (Rhizopus, Mucor, or Absidia) should be notified to clinicians immediately as these organisms grow rapidly and may cause vascular invasion.
Rhizopus and Mucor are rapidly growing fungi that can fill a Petri dish with fluffy, cotton-candy-like growth in under 5 days. Growth is generally whitish in color which can turn greyish-brown with aging as a result of the maturation of the sporangiospores within the sporangium. The reverse remains a pale white. They form characteristic mycelium when cultured on Sabouraud’s agar. Cultures show colonies with spores contained within the sporangium. It is impossible to distinguish between the different genera and species of Zygomycetes based on their colonial morphology.
Serology is not useful for the diagnosis of zygomycosis.
Nucleic acid amplification test (NAATs)
Nucleic acid testing is not routinely used for the diagnosis of zygomycosis.
Treatment of mucormycosis may require surgical intervention; debridement of necrotic tissue where possible followed by treatment with Amphotericin B.