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.
Rhizopus contains several species. The most common ones are, namely,
Rhizopus arrhizus, Rhizopus azygosporus, Rhizopus
microsporus, Rhizopus schipperae, and Rhizopus stolonifer.
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
species are among the fungi causing the group of infections referred to as
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.
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.
Growth rate is very rapid and colonies are typically cotton – candy like
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 37°C.
– septate or scarcely septate broad
hyphae with diameter ranging from 6 –
15 µm, rhizoids, sporangiophores, sporangia, and sporangiospores are
Sporangiophores are usually unbranched, brown in color, solitary or appear
Rhizoids are found at the point where the stolons and sporangiophores
Sporangia are round with flattened bases, located at the tip of the
sporangiophores, and with diameter ranging between 40 - 350 µm;
Sporangiospores are unicellular, hyaline to brown in color, smooth or
striated in texture, and with size ranging between 4 - 11 µm in diameter;
Apophysis is absent or rarely evident while the columellae are
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.
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.
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