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Rhizopus Mold Species

The U.S. Government's Occupational Safety and Health Administration [OSHA] classifies Rhizopus species as an allergen and irritant and a cause of Hypersensitivity Pneumonitis and Dermatitis.


Rhizopus species is a causative agent of Zygomycosis: Systemic Zygomycosis and Rhinocerebral Zygomycosis.

(Information from @ 2005)



Taxonomic Classifications


Kingdom: Fungi
Phylum: Zygomycota

Order: Mucorales

Family: Mucoraceae

Genus: Rhizopus



Rhizopus Mold Pictures


Rhizopus microscopic morphology

(Image Courtesy of @ 2005)


Microscopic morphology of Rhizopus oryzae.


Sporangiophores up to 1500 um in length and 18 um in width, smooth walled, non - septate, simple or branched, arising from stolons opposite rhizoids usually in groups of 3 or more.  Sporangia are globose, often with a flattened base, grayish black, powdery in appearance, up to 175 um in diameter and many spored. 



Rhizopus colony morphology

(Image Courtesy of @ 2005)


Rhizopus oryzae on Sabouraud's dextrose agar.


Colonies are very fast growing at 25C, about 5 - 8 mm high, with some tendency to collapse, white cottony initially becoming brownish grey to blackish - grey depending on the amount of sporulation.


Rhinocelebral zygomycosis from Doctor Fungus

(Image Courtesy of @ 2005)

Rhinocerebral zygomycosis caused by Rhizopus oryzae extensive involvement of the orbit and associated MRI image.





Rhizopus is a cosmopolitan filamentous fungus frequently isolated from soil, decaying fruit and vegetables, animal feces, and old bread. Aside from being known as common contaminants, Rhizopus species are also occasional causes of serious, and often fatal, infections in humans.  Certain species are plant pathogens as well.





The genus Rhizopus contains several species.  The most common ones are, namely, Rhizopus arrhizus, Rhizopus azygosporus, Rhizopus microsporus, Rhizopus schipperae, and Rhizopus stolonifer.


Morphological features, such as the length of rhizoids and sporangiophores, the shape of columellae, the diameter of sporangia, and the size, shape and surface texture of sporangiospores, help in species differentiation of Rhizopus.  Maximum growth temperature also varies from species to another.



Pathogenicity and Health Effects


Rhizopus species are among the fungi causing the group of infections referred to as zygomycosis Zygomycosis is now the preferred term over mucormycosis for this angio invasive disease.  Rhizopus arrhizus is the most common cause of zygomycosis and is followed by Rhizopus microsporus var. rhizopodiformis.

Zygomycosis infection includes mucocutaneous, rhinocerebral, genitourinary, gastrointestinal, pulmonary, and disseminated infections.  The most frequent predisposing factors for zygomycosis include diabetic ketoacidosis and immunosuppression due to various reasons, such as organ transplantation and other factors such as desferoxamine treatment, renal failure, extensive burns, trauma, and intravenous drug use which may also predispose to development of zygomycosis.  Heatstroke has been described as a risk factor for disseminated zygomycosis as well.  Contaminated adhesive tapes and wooden tongue depressors have been reported to lead to nosocomial outbreaks of zygomycosis.  Vascular invasion that causes necrosis of the infected tissue, and perineural invasion are the most frustrating features of these infections. Zygomycosis is frequently considered as fatal infection.



Macroscopic Appearance


     Growth rate is very rapid and colonies are typically cotton candy like in texture;

     The surface colony color is initially white becoming gray to yellowish brown in time while reverse is white to pale; and

     Pathogenic Rhizopus species can grow well at a temperature of 37C.



Microscopic Appearance


      Non septate or scarcely septate broad hyphae with diameter ranging from 6 15 m, rhizoids, sporangiophores, sporangia, and sporangiospores are present;

      Sporangiophores are usually unbranched, brown in color, solitary or appear in clusters;

      Rhizoids are found at the point where the stolons and sporangiophores meet;

      Sporangia are round with flattened bases, located at the tip of the sporangiophores, and with diameter ranging between 40 - 350 m;

      Sporangiospores are unicellular, hyaline to brown in color, smooth or striated in texture, and with size ranging between 4 - 11 m in diameter; and

      Apophysis is absent or rarely evident while the columellae are hemispherical.



Laboratory Precautions


General laboratory precautions are required, no special safety measures needed.





Limited data are available on the in vitro susceptibility profile of Rhizopus species.  Amphotericin B, based on the study comparing the in vitro activity of amphotericin B, ketoconazole, itraconazole, and voriconazole against Rhizopus arrhizus strains, yielded low MICs.  The MICs of ketoconazole, itraconazole, and voriconazole were similar to one another while slightly higher than those of amphotericin B.  Considerably high MICs were detected against Rhizopus arrhizus by fluconazole.


Caspofungin and anidulafungin appeared to have limited activity against Rhizopus species.  Azasordarin derivatives and posaconazole, on the other hand, were found to be active in vitro against Rhizopus arrhizus.  Appeared to be active against Rhizopus species were posaconazole and ravuconazole compared to voriconazole as well. 


Treatment of Rhizopus infections remains difficult due to its ability to invade vascular tissues, infarction of the infected tissue is common and mortality rates are very high.  Surgical debridement or surgical resection and well as antifungal therapy are usually required.  The most commonly used antifungal agent is amphotericin B.   In some cases of zygomycosis, liposomal amphotericin B and other lipid based amphotericin B formulations such as amphotericin B colloidal dispersion have also been used as treatment.

Frequently, clinical response to therapy is unsatisfactory in zygomycosis.  Enhanced clinical response has been anecdotally associated with adjuvant therapy with cytokines, especially the colony stimulating factors.  For further validation is the successful use of fluconazole and terbinafine as treatment for zygomycosis.  Furthermore, a combination of fluconazole with trovafloxacin or ciprofloxacin proved to be effective in a murine model of pulmonary zygomycosis.


The mycological information gathered and
organized in this extensive research on different
Pathogenic Molds was sourced out from the list of
informative websites and reference below:

A Clinical Laboratory Handbook:
Identifying Filamentous Fungi by
St. Germain, Guy and R. Summerbell.



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