The new taxonomic classification of Rhinosporidium seeberi: Not a fungus anymore
Rhinosporidium seeberi, first described by Malbran in 1892, is the causative agent of rhinosporidiosis, a chronic subcutaneous mycosis. The infection is characterized by formation of polypoid masses at nasal mucosa, conjunctiva, genitalia, and rectum. Fish and aquatic insects are the natural reservoirs of
R. seeberi.
The taxonomic classification of
R. seeberi has long been controversial so far and has been based solely on morphological properties. Microscopically,
R. seeberi produces spherules in infected tissue and these spherules are filled with endospores. Most interestingly, and complicating the issue, this microorganism cannot be cultured in vitro on artificial media. Based on these findings, while some investigators classified it as an ascomycetous fungus, others preferred the term "fungus-like".
The taxonomy of
R. seeberi was studied in more detail recently by molecular phylogenetic analysis. Interestingly, the results of these studies revealed that the sequence of 18S SSU rDNA from the
R. seeberi isolates of two infected humans, a dog, and a swan proved to be identical to each other and related to a group of fish parasites.
Of note,
R. seeberi was placed within the DRIP clade (Dermatocystidium, rosette agent, Ichtyophonus, and Psorospermium) in 1996. DRIP clade was later renamed as the class Ichtyosporea in 1998, and most recently as the class Mesomycetozoea in 2002. The class Mesomycetozoea includes a heterogeneous group of microorganisms that are at the animal-fungal boundary and consists of two orders; Dermocystida and Ichthyophonida.
While the microorganisms included in the order Dermocystida are either pathogens of fish (
Dermocystidium spp. and the rosette agent) or of mammals and birds (
R. seeberi), those in the order of Ichthyophonida are either pathogens of fish or are saprophytic microorganisms. Thus,
R. seeberi now appears as the only microorganism that is classified in the class Mesomycetozoea and is pathogenic to mammals and birds.
Future studies will hopefully provide more information about the morphological features and pathogenicity of
R. seeberi.
Related Reading
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Mendoza L, Taylor JW, Ajello L. The class mesomycetozoea: a heterogeneous group of microorganisms at the animal-fungal boundary. Annu Rev Microbiol 2002;56:315-44.
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Herr RA, Mendoza L, Ajello L. Rhinosporidium, is it still a fungus? 9th Congress of the European Confederation of Medical Mycology and 7th Trends in Invasive Fungal Infections-Joint Meeting. September 28-October 1, 2003, Amsterdam, The Netherlands, S10.03.
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Herr RA, Ajello L, Taylor JW, Arseculeratne SN, Mendoza L. Phylogenetic analysis of Rhinosporidium seeberi's 18S small-subunit ribosomal DNA groups this pathogen among members of the protoctistan Mesomycetozoa clade. J Clin Microbiol. 1999;37:2750-4.
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Fredricks DN, Jolley JA, Lepp PW, Kosek JC, Relman DA. Rhinosporidium seeberi: a human pathogen from a novel group of aquatic protistan parasites. Emerg Infect Dis. 2000;6: 273-82.
