Microsporidia: Obligate intracellular pathogens within the fungal kingdom

Microsporidia were initially described about 150 years ago with the identification of Nosema bombycis as the organism responsible for the disease pébrine in silkworms (1). Microsporidia are ubiquitous in the environment and infect almost all invertebrates and vertebrates, as well as some protists (2). Spores from microsporidia are commonly found in surface water (3). These organisms are eukaryotes that have a nucleus with a nuclear envelope, an intracytoplasmic membrane system, chromosome separation on mitotic spindles, vesicular Golgi, and a mitochondrial remnant organelle lacking a genome termed a mitosome (4). For insects, fish, laboratory rodents, and rabbits microsporidia are important pathogens, and they have been investigated as biological control agents for destructive species of insects (2). Several species of microsporidia have caused significant agricultural economic losses including Nosema apis and Nosema ceranae in honeybees (5), Loma salmonae in salmonid fish (6), and Thelohania species in shrimp (7). Franzen (8) published an excellent review of the history of research on these pathogens, and a recent textbook by Weiss and Becnel (2) provides a comprehensive examination of what is known about these organisms. In 1977 Sprague elevated the class or order Microsporidia to the phylum Microspora (9), and a decade later Sprague and Becnel (10) suggested that the term Microsporidia should instead be used for the phylum name. These organisms were previously considered "primitive" protozoa (11), but molecular phylogenetic analysis has resulted in the insight that these organisms are not primitive but instead are degenerate, and that Microsporidia are related to the Fungi, either as a basal branch of the Fungi or as a sister group (12 - 16).