Rhizopus

Rhizopus species are ubiquitous saprophytic molds found in soil, decaying organic matter, and bread (black bread mold). They are classified under the order Mucorales, class Zygomycetes, and are the most common cause of mucormycosis (formerly called zygomycosis). Key virulence relates to angioinvasion, leading to tissue infarction and necrosis.

Pathophysiology: Rhizopus thrives in high-glucose, acidic, and iron-rich environments—explaining its predilection for patients with diabetic ketoacidosis (DKA), as acidosis impairs neutrophil function and increases free iron availability (a key growth factor for Mucorales via ketone body-mediated release of iron from transferrin). Other risk factors include neutropenia, hematologic malignancy, immunosuppressive therapy, deferoxamine use (which paradoxically supplies iron to the fungus), and corticosteroid use.

Clinical syndromes: - Rhinocerebral mucormycosis: Most classic presentation, especially in DKA patients. Presents with sinusitis, black necrotic eschar on nasal turbinates or palate, orbital cellulitis, and can extend intracranially causing cavernous sinus thrombosis or stroke. - Pulmonary mucormycosis: Seen in neutropenic and transplant patients. - Cutaneous, GI, and disseminated forms also occur.

Histopathology: Broad (6-25 μm), ribbon-like hyphae with rare septations and right-angle (90°) branching—contrasts with Aspergillus, which has septate hyphae with acute-angle (45°) branching. Angioinvasion causes thrombosis, infarction, and tissue necrosis, contributing to rapid progression and high mortality.

Diagnosis: Tissue biopsy with histopathology (KOH prep, silver stain) showing characteristic hyphae; culture on Sabouraud agar showing fast-growing, cottony colonies. Imaging (CT/MRI) for extent of invasion.

Treatment: Urgent surgical debridement is critical due to poor vascular penetration of antifungals into necrotic tissue. First-line antifungal is IV amphotericin B (liposomal formulation preferred to reduce nephrotoxicity). Posaconazole or isavuconazole may be used for step-down or salvage therapy. Reversal of underlying risk factors (e.g., correcting DKA, reducing immunosuppression) is essential.

High-yield board points: Classic vignette is a poorly controlled diabetic with DKA presenting with facial pain, black necrotic nasal/palatal eschar, and orbital/cerebral involvement. Remember: Rhizopus = right-angle branching, non-septate hyphae, DKA association, and angioinvasion leading to necrosis.