Organisms
Outline
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Rhizopus oryzae
The genus Rhizopus is classified under the family Mucoraceae in the order Mucorales of the phylum Zygomycota. R. oryzae is the most important and representative agent of mucormycosis. R. oryzae and the other members of Mucorales are recovered in profusion from decaying vegetables, fruits and their seeds, grains, compost piles, soil, animal excreta, and molding bread. Since it is ubiquitous in nature, mucormycosis cases are reported worldwide. The majority of mucormycosis patients have a serious underlying condition, such as diabetes mellitus, immunosuppression, starvation, burns, or other major trauma. Pathologically, mucormycosis is characterized by vascular invasion with hyphae, infarction and necrosis of tissue, and by an acute or subacute course of infection. Of the many forms of clinical manifestation, the most common form caused by Rhizopus species is the rhinocerebral and craniofacial mucormycosis. This originates in the paranasal sinus but presents in contiguous structures of orbit, palate, face, nose, or brain. Ordinarily death occurs in untreated cases within 4 weeks of onset.
The most prominent predisposing factor for the facial cranial mucormycosis is diabetes mellitus, and the diabetic population is on the rise worldwide. The second most common form of mucormycosis is pneumonia, which occurs most frequently among patients with hematologic disorders, lymphoma, severe nutropenia, or history of deferoxamine therapy. Infection in these patients has been observed to be fatal in a very short period of time in all cases reported to date. Although the number of cases has been small, children appear overrepresented among the mucormycosis patients without a known underlying disease. Rhizopus species also can cause skin and soft tissue infection in the setting of local trauma or by the hematogenous route.
The zygomycetes are evolutionarily divergent from ascomycetes. R. oryzae, like other members of Mucorales, is a rapidly growing mold that propagates by hydrophobic sporangiospores that readily disperse after maturation. It is one of the common laboratory contaminants due to its ubiquity in soil and decomposing organic material. It is different from ascomycetes or basidiomycetes in that its hyphae are tube-like, without septation, and the cell wall contains chitosan and chitin instead of glucans, mannans, and chitin. Asexual spores are produced within sporangia and are released upon maturation. The fungus can also undergo sexual reproduction and produce zygospores upon mating between the positive and negative strain on an appropriate mycological medium.
R. oryzae occurs as a haploid. Although R. oryzae has a defined sexual cycle involving mating between the positive and the negative strains, classical genetic approaches have rarely been applied to study this organism. That is because germination of zygospores takes months and the rate of germination is erratic. Molecular biological approaches have been developed, including transformation systems and gene disruption by homologous integration. In addition, episomal plasmids, cDNA expression libraries, and animal models are available. It is now possible to identify the genes of interest and study their impact on pathobiology.
Rhizopus oryzae mitochondrial sequence
The mitochondrial sequence was assembled separately from the genomic sequence. For submission to Genbank, however its supercontigs (81 and 82) were assigned numbers consecutive to the genomic numbering.