Mucor and Rhizopus
Mucor and Rhizopus are environmental molds that cause mucormycosis, an aggressive opportunistic fungal infection. Both show broad, ribbon-like nonseptate hyphae with wide-angle branching and classically affect patients with diabetic ketoacidosis or immunosuppression.
Mucor and Rhizopus are genera of environmental molds in the order Mucorales. They are ubiquitous in soil and decaying organic matter and are harmless to most people, but in vulnerable hosts their inhaled spores cause mucormycosis, a rapidly progressive and often devastating infection.
Microscopically, both molds form broad, irregular, ribbon-like hyphae that are nonseptate (or only sparsely septate) and branch at wide angles, often approaching 90°. This appearance distinguishes them from Aspergillus, whose hyphae are septate and branch at acute angles of about 45° — a comparison exams test repeatedly. On a practical level, the two genera cause the same clinical disease and are managed the same way.
The classic presentation is rhino-orbital-cerebral mucormycosis. The fungi proliferate in the walls of blood vessels, causing vascular invasion, thrombosis, and tissue necrosis. Infection begins in the nasal passages and sinuses, may produce a black necrotic eschar on the face or palate, and can penetrate the cribriform plate to reach the frontal lobes of the brain. Patients with diabetic ketoacidosis are at especially high risk, along with neutropenic and otherwise immunocompromised patients; the acidic, glucose- and iron-rich environment of DKA favors fungal growth. Management combines amphotericin B with urgent surgical debridement.
The USMLE Step 1 exam tests Mucor and Rhizopus in mycology: recognize the DKA patient with facial pain, black eschar, and sinus involvement; the wide-angle nonseptate hyphae on histology; and the contrast with Aspergillus.
Key takeaways
- Mucor and Rhizopus are molds that cause mucormycosis, an aggressive opportunistic infection.
- Histology shows broad, ribbon-like, nonseptate hyphae branching at wide angles (~90°), unlike the 45° septate hyphae of Aspergillus.
- Diabetic ketoacidosis and immunosuppression are the classic risk factors.
- Rhino-orbital-cerebral disease features vascular invasion, necrosis, a black eschar, and possible spread through the cribriform plate to the brain.
- USMLE Step 1 vignettes pair a DKA patient with sinus symptoms and wide-angle nonseptate hyphae.