Rhinosporidium seeberi: a century-old puzzle of classification
For more than a hundred years, microbiologists argued about what kind of organism Rhinosporidium seeberi actually is. It has been called a fungus, a protist and even a bacterium. The story of that argument is a lesson in why classifying a living thing can be so hard.
The organism
Wikipedia's article on Rhinosporidium seeberi describes it as a eukaryotic pathogen responsible for a disease called rhinosporidiosis, which affects humans, horses and dogs and, to a lesser extent, cattle, cats, foxes and birds. The article states that it is most commonly found in tropical areas, especially India and Sri Lanka, that the pathogen was first identified in 1892, and that it was comprehensively described in 1900 by Seeber, whose name it carries. A 2012 critical review in the Revista Iberoamericana de Micología by Vilela and Mendoza describes the disease as one of the mucous membranes and, infrequently, of the skin and other tissues of humans and animals. This page is about the organism and its classification; it says nothing about the disease beyond that.
The same Wikipedia article is candid about how much remains unknown. It lists the pathogen's natural habitat, aspects of its life cycle, its immunology and the reasons for difficulty in growing it in the laboratory among the aspects that remain problematic and enigmatic.
Why it resisted classification
Classifying a microbe traditionally means growing it, watching it reproduce, and comparing its structures and chemistry with known groups. Rhinosporidium seeberi made every one of those steps difficult. The review by Vilela and Mendoza begins from exactly this point: because the organism resists culture, its true taxonomic identity had been controversial for more than 100 years. An organism that cannot be grown reliably on a plate cannot be studied in the usual way; researchers were largely limited to what they could see in samples taken from infected tissue.
What they saw was a set of large, round structures filled with smaller bodies. Structures of that general kind occur in several unrelated groups, including some fungi and some aquatic organisms, so appearance alone could support more than one interpretation. Different teams read the same pictures differently, and for most of the twentieth century the question stayed open. The Wikipedia article summarises the situation: for most of the 20th century the classification was unclear, with the organism considered either a fungus or a protist.
Three hypotheses
The review by Vilela and Mendoza sets out three views introduced in the years before it was written: that a cyanobacterium in the genus Microcystis is the cause of the disease; that R. seeberi is a eukaryotic pathogen in the group called the Mesomycetozoa; and that it is a fungus. Having reviewed evidence from electron microscopy, from the study of infected tissue and, more recently, from several molecular studies, the authors conclude that the evidence strongly supports the view that R. seeberi is a eukaryotic pathogen, but not a fungus.
They explain their reasons in their abstract. A fundamental point against the bacterial theory is the presence of nuclei, reported by numerous authors; bacteria do not have nuclei. The suggested resemblance to certain bacteria and fungi, they write, was merely hypothetical and lacked the rigour needed to validate those proposals, and key traits of those groups cannot be found in the organism's parasitic phase. They regard the amplification of a bacterial DNA sequence from cases of the disease as an anomaly, perhaps due to contamination with environmental Microcystis or perhaps reflecting plastids acquired long ago from cyanobacterial ancestors, and note that even if the organism carries some prokaryote DNA, this would not make it a cyanobacterium. They conclude that placing it within the fungi is scientifically untenable, while calling for further studies on questions that still need careful scrutiny.
Where it sits today
Wikipedia's article reflects the same outcome. It states that carefully performed molecular studies showed the definitive identity of the organism, and that it was shown to be part of a group called the Mesomycetozoea, also known as the DRIP clade, which includes a number of well-known fish pathogens. The classification box in the article places it in the class Ichthyosporea, within the Holozoa. That group sits near the base of the animal branch of the tree of life, a surprising home for an organism long thought to be a fungus.
What the story teaches
The Rhinosporidium debate shows several general points about biology. Appearance can mislead, because similar structures evolve in unrelated groups. An organism that cannot be cultured is hard to study, and old methods may simply lack the power to settle a question. Molecular data, reading the organism's own DNA and comparing it with others, can resolve disputes that decades of microscopy could not, but only when samples are handled with care, because contamination can make a sample appear to contain something it does not. The reviewers' warning about controls is advice that applies well beyond this one organism.
Finally, the story is a reminder that classification is a working hypothesis, open to revision as better evidence arrives. The current placement rests on the best evidence available, and the reviewers themselves pointed to questions still needing careful scrutiny.