Volume 29, Number 7—July 2023
CME ACTIVITY - Research
Clinical and Mycologic Characteristics of Emerging Mucormycosis Agent Rhizopus homothallicus
Table 1
Mycologic characteristics of Rhizopus arrhizus and R. homothallicus based on macroscopic and microscopic findings of tissue samples from patients enrolled in a 10-month retrospective study, Chandigarh, India, January–October 2021*
| Characteristic | R. homothallicus | R. arrhizus |
|---|---|---|
| Macroscopic appearance | ||
| Growth rate | Fast-growing colonies and sporulation relatively slower than R. arrhizus | Fast-growing colonies and sporulation relatively faster than R. homothallicus |
| Obverse surface growth |
Cottony, white colonies that turn grayish to olive brown with a variegated appearance; because of the large-sized zygospores, brownish tufts can be seen unevenly distributed throughout the mycelial growth |
Cottony, white colonies that turn gray with typical salt and pepper appearance |
| Reverse macroscopic findings |
Reverse surface has no pigment |
Reverse surface has no pigment |
| Microscopic findings | ||
| Hyphae | Aseptate hyphae, less prominent rhizoids; smooth and thick-walled intercalary chlamydospore | Aseptate hyphae, well-developed nodal rhizoids with occasional intercalary chlamydospores |
| Sporangiophore | Erect, unbranched, or dichotomously branched (length 300 – 2000 µm, width 5–30 µm) | Single or tufts of mostly unbranched sporangiophores (length 1000–2000 µm, width 7–18 µm) |
| Sporangium | Sparsely noted in cultures; when present, appears spherical and greyish brown (20– 140 µm) with conspicuous dark apophysis and subspherical columella | Spherical sporangia (20–250 µm) with short apophysis and spherical columella occupying 50% of sporangium |
| Sporangiospores | Spherical to broadly ellipsoidal (3–5 × 4–8 µm), hyaline, and thick-walled with less prominent striations | Lemon-shaped or subspherical to ellipsoidal (6–8 × 4.5–5 µm) with striations, rough surface |
| Zygospores | Homothallic†; abundant golden brown zygospores (60–100 µm) with stellate spinous projections, attached to large globose suspensor cells that are unequal in size | Heterothallic zygospores not seen on primary culture; red-brown spherical or laterally flattened (60–140 µm) with flat projections; suspensors are unequal, spherical to conical |
*Key features highlighted in bold. †Differentiation from other homothallic Rhizopus species that produce abundant zygospores (e.g., R. sexualis and R. azygosporus) is done based on size of the suspensor cells of the zygospore and by molecular identification using the internal transcribed spacer and 28S sequences of rDNA.
1These first authors contributed equally to this article.
2Current affiliation: Dr. B R Ambedkar State Institute of Medical Sciences, Mohali, India.
3Current affiliation: Indian Council of Medical Research, New Delhi, India.
4Current affiliation: Doodhadhari Burfani Hospital, Haridwar, India.