Fischer, 1892
Macroscopic morphology
Colonies on potato dextrose agar at 25°C are woolly and initially white, quickly becoming gray and then developing small black dots in the mycelium which are mature sporangia. Growth is very rapid filling the tube or Petri dish within 2 to 3 days. Growth occurs at 40 °C but fails to grow 45 °C [2202], [531].
Microscopic morphology
Hyphae are hyaline, broad (5-15 µm), ribbon-like, irregularly branched, and aseptate to sparsely septate. Sporangiophores and rhizoids are borne from creeping aerial hyphae known as stolons. Sporangiophores (500 – 3500 µm in length) are mostly unbranched, yellowish-brown to dark brown, are produced singly or in small groups, and bear globose sporangia (50-200 µm in diameter) with ellipsoidal columellae. Sporangiospores are longitudinally striate and angular to lemon-shaped (average size 6-8 x 4.5 – 6 µm). Rhizoids (150 – 300 µm) are mostly simple, may be hyaline to dark brown, and occur at the junctions of the stolons and sporangiophores (nodal). The species is heterothallic, requiring compatible mating strains for the production of zygospores [2202], [531].
Special notes
The synonymous use of the species epithets “arrhizus” and “oryzae” is somewhat confusing, as the taxonomic precedence remains controversial [2048]. Rhizopus arrhizus is the most common etiologic agent of human disease and accounts for approximately 90% of rhinocerebral zygomycosis associated with diabetic ketoacidosis. Individuals with hematologic malignancies, undergoing iron chelation therapy, and those sustaining traumatic injuries are also at risk. A recent report also links long term voriconazole use in immunocompromised patients with hematologic disease as a potential predisposing factor [2323]. In a review of 91 cases of zygomycosis, van Burik et al report the efficacy of posaconazole as salvage therapy [2280].
FTL* in vitro susceptibility data
| AMB | ITRA | VORI | POSA |
|---|---|---|---|
| 0.25 µg/ml=4 | 0.5 µg/ml=1 | >8 µg/ml=2 | 0.06 µg/ml=1 |
| 0.5 µg/ml=6 | 1.0 µg/ml=2 | 0.25 µg/ml=2 | |
| 1.0 µg/ml=1 | 2.0 µg/ml=1 | 2.0 µg/ml=2 | |
| >8.0 µg/ml=2 | 4.0 µg/ml=2 | ||
| 8.0 µg/ml=1 |
| Drug/N | AMB/11 | ITRA/6 | VORI/2 | POSA/8 |
|---|---|---|---|---|
| MIC Range | 0.25-1.0 | 0.5->8.0 | >8.0 | 0.06-8.0 |
| MIC50 | 0.5 | * | * | * |
| MIC90 | 0.5 | * | * | * |
* Fungus Testing Laboratory unpublished data (NCCLS M38-A)