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Rhizomucor spp.

 

Taxonomic classification

Kingdom: Fungi
Phylum: Zygomycota
Order: Mucorales
Family: Mucoraceae
Genus: Rhizomucor

Description and Natural Habitats

Rhizomucor is a cosmopolitan filamentous fungus found in soil and decaying fruit and vegetables. While Rhizomucor spp. are often isolated from fermenting and composting organic matter, they are also rare causes of serious (and often fatal) infections in humans. Rhizomucor spp. other than Rhizomucor variabilis are thermophilic in nature and can grow at temperatures as high as ~54 °C [531, 1295, 2144, 2202].

Species

The genus Rhizomucor contains three species; Rhizomucor pusillus, Rhizomucor miehei, and Rhizomucor variabilis. Rhizomucor variabilis is phylogenetically very close to Mucor hiemalis [2341]. Maximum growth temperature, biochemical assimilation profile, thiamine dependency, and the diameter of the sporangia aid in differentiation of the three Rhizomucor species from each other. Rhizomucor miehei is homothallic while Rhizomucor pusillus is homo- or heterothallic [531].

See the list of synonyms for Rhizomucor spp.

Pathogenicity and Clinical Significance

Rhizomucor spp. are among the fungi causing the group of infections referred to as zygomycosis. Although the term mucormycosis has often been used for this syndrome, zygomycosis is now the preferred term for this angio-invasive disease. Vascular invasion that causes necrosis of the infected tissue, and perineural invasion are the most frustrating features of these infections. Zygomycosis is often fatal [696, 1918].

There are few reports on human infections due to Rhizomucor spp. Cutaneous, pulmonary, rhinofacial, and disseminated zygomycosis due to Rhizomucor pusillus have so far been reported in neutropenic patients with hematological malignancies [251, 2143] and diabetes mellitus [2427]. Noteworthy, primary cutaneous infections due to Rhizomucor variabilis have been reported in otherwise healthy individuals [2493, 2494]. In contrary to the rare human infections, animal infections, particularly bovine mycotic abortion due to Rhizomucor are common [1219].

Macroscopic Features

Colonies of Rhizomucor grow very rapidly, fill the Petri dish, and mature in 4 days. The texture is typically cotton-candy like. From the front, the color of the colony is white initially and turns grey to yellowish brown in time. The reverse is white to pale. Significantly, Rhizomucor spp. other than Rhizomucor variabilis are thermophilic and yield good grow at temperatures as high as ~54 °C [531, 1295, 2144, 2202].

Microscopic Features

The microscopic morphology of Rhizomucor appears to be intermediate between that of Rhizopus and Mucor. Nonseptate or sparsely septate broad hyphae, rudimentary rhizoids, sporangiophores, sporangia, and sporangiospores are visualized. Rudimentary rhizoids, if exist, are few in number and are located on stolons between the sporangiophores. Sporangiophores are irregularly branched and end in sporangia at their apices. Sporangia (40-80 µm in diameter) are brown in color and round in shape. Apophysis is absent. Columellae, on the other hand, are prominent and spherical to pyriform in shape. Sporangiospores (3-4 µm in diameter) are small, unicellular, and round to ellipsoidal in shape. Zygospores, if present, are formed in the aerial hyphae. They are round to slightly compressed and dark brown to blackish brown in color [531, 1295, 2144].

Histopathologic Features

Broad, thin-walled, hyaline, often aseptate or sparsely septate hyphae are observed in the infected tissues. The contours of the hyphae are typically non-parallel and branches are irregular. Invasion of blood vessels is noteworthy [462, 763]. See also our histopathology page.

Compare to

Rhizomucor spp. must be differentiated from the other genera included in the phylum Zygomycota:

Absidia
Apophysomyces
Rhizopus
Mucor
Mortierella

Rhizomucor differs from Mucor by growing at 50-55°C and by having rhizoids and stolons; from Rhizopus by having branched sporangiophores and rhizoids not arising opposite the sporangiophores; and from Absidia by having globose sporangia and sporangiophores that are not swollen where they merge with the columellae. The features that help in differentiation of these genera are summarized in the table below [531, 1295, 2144, 2202].

Genus Best growth Sporangi-
ophore
Apophysis Columella Sporan-
gium
Rhizoid Stylo-
spore
Absidia 45°C Branched, hyaline Conical, not prominent Dome-shaped Pear-shaped +, but usually indistinct -
Apophysomyces >42°C Usually unbranched, grayish-brown Bell-shaped, not prominent Usually dome-shaped, rarely elongated Pear-shaped + -
Mortierella 40°C Branched, hyaline - - Spherical + +/-
Mucor <37°C Branched or unbranched, hyaline - +, in varying shapes Spherical - -
Rhizomucor 54°C Branched, brown - Spherical Spherical + -
Rhizopus 45°C Unbranched and brown mostly Not prominent Spherical or elongated Spherical + -


Laboratory Precautions

No special precautions other than general laboratory precautions are required.

Susceptibility

No in vitro susceptibility data are available for Rhizomucor spp.

Similar to zygomycosis due to other fungi of class zygomycetes, treatment of Rhizomucor infections consists of antifungal and surgical therapy. Amphotericin B is the most commonly used antifungal agent. Liposomal amphotericin B has also been succesfully used in some cases with zygomycosis due to Rhizomucor [251]. Early diagnosis and treatment are crucial and mortality rate is high [1918].

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Rhizomucor miehleri
Rhizomucor miehei is often homothallic
absidia corymbifera
Primitive rhizoids of Rhizomucor pussilus are visible at the tips of the vegetative hyphae



References

251. Bjorkholm, M., G. Runarsson, F. Celsing, M. Kalin, B. Petrini, and P. Engervall. 2001. Liposomal amphotericin B and surgery in the successful treatment of invasive pulmonary mucormycosis in a patient with acute T- lymphoblastic leukemia. Scand J Infec Dis. 33:316-319.

462. Collier, L., A. Balows, and M. Sussman. 1998. Topley & Wilson's Microbiology and Microbial Infections, 9th ed, vol. 4. Arnold, London, Sydney, Auckland, New York.

531. de Hoog, G. S., J. Guarro, J. Gene, and M. J. Figueras. 2000. Atlas of Clinical Fungi, 2nd ed, vol. 1. Centraalbureau voor Schimmelcultures, Utrecht, The Netherlands.

696. Eucker, J., O. Sezer, B. Graf, and K. Possinger. 2001. Mucormycoses. Mycoses. 44:253-260.

763. Frater, J. L., G. S. Hall, and G. W. Procop. 2001. Histologic features of zygomycosis - Emphasis on perineural invasion and fungal morphology. Arch Pathol Lab Med. 125:375-378.

1219. Knudtson, W. U., and C. A. Kirkbride. 1992. Fungi associated with bovine abortion in the northern plains states (USA). J Vet Diagn Invest. 4:181-5.

1295. Larone, D. H. 1995. Medically Important Fungi - A Guide to Identification, 3rd ed. ASM Press, Washington, D.C.

1918. Ribes, J. A., C. L. Vanover-Sams, and D. J. Baker. 2000. Zygomycetes in human disease. Clin Microbiol Rev. 13:236-301.

2143. St-Germain, G., A. Robert, M. Ishak, C. Tremblay, and S. Claveau. 1993. Infection due to Rhizomucor pusillus: report of four cases in patients with leukemia and review. Clin Infect Dis. 16:640-645.

2144. St-Germain, G., and R. Summerbell. 1996. Identifying Filamentous Fungi - A Clinical Laboratory Handbook, 1st ed. Star Publishing Company, Belmont, California.

2202. Sutton, D. A., A. W. Fothergill, and M. G. Rinaldi (ed.). 1998. Guide to Clinically Significant Fungi, 1st ed. Williams & Wilkins, Baltimore.

2341. Voigt, K., E. Cigelnik, and K. O'Donnell. 1999. Phylogeny and PCR identification of clinically important zygomycetes based on nuclear ribosomal-DNA sequence data. J Clin Microbiol. 37:3957-3964.

2427. Wickline, C. L., T. C. Cornitius, and T. Butler. 1989. Cellulitis caused by Rhizomucor pusillus in a diabetic patient receiving continuous insulin infusion pump therapy. South Med J. 82:1432-1434.

2493. Zheng, R.-y., and C.-q. Chen. 1993. Another non-thermophilic Rhizomucor causing human primary cutaneous mucormycosis. Mycosystema. 6:1-12.

2494. Zheng, R.-y., and C.-q. Chen. 1991. A non-thermophilic Rhizomucor causing human primary cutaneous mucormycosis. Mycosystema. 4:45-57.



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