Causative agents of microsporosis
■ The clinical significance of the microspora is based mainly on their role as "opportunistic parasites" in HIV patients. The most important species are Enterocytozoon bieneusi and Encephalitozoon intestinalis. Transmission is by characteristic spores. Little is known about the epidemiology of these organisms that are closely related to the fungi. ■
Parasites. The phylum Microspora includes about 140 genera and 1300 species. They are parasites with intracellular development and spore formation. The host spectrum ranges from numerous invertebrates (e.g., protozoa, insects) to many species in all classes of vertebrates. The lack of mitochondria, peroxisomes, and typical Golgi membranes as well as their prokaryotelike ribosomes were previously regarded as characteristics of most primitive eukaryotes. Recently, analyzes of a variety of genes and proteins have revealed a close relationship to fungi. Therefore, some authorities now consider the Microspora to be highly specialized fungi rather than primitive protozoa and place them as a superclass into the subphylum fungi. A notable characteristic of the microspora is the unique morphology of their spores (see below).
Microspora, known since the middle of the last century, have attained attention as human pathogens and opportunistic parasites in the course of the AIDS epidemic. Several genera and species have been identified in humans to date (Table 9.1, p. 477).
Morphology and life cyle. Microspora reproduce intracellularly by means of repeated, asexual binary or multiple fission (merogony), then form spores in a subsequent phase (sporogony) (and sexual stages as well in Thelohania). The developmental stages are located freely in the host cell cytoplasm (Enterocy-tozoon, Nosema) or they inhabit a parasitophorous vacuole (Encephalitozoon); in other genera (Pleistophora, Trachipleistophora) the intracellular stages are separated from the cytoplasm by an amorphous layer (pansporoblastic membrane). Sporogony begins with formation of sporonts, which are derived from merogonic cells and possess a thicker cell wall. The sporonts divide to form sporoblasts, followed by morphological differentiation into spores.
The fine structure of these spores is typical of Microspora (Fig. 9.20a, b). The two-layered spore wall (exospore and endospore) encloses the uninucle-ate (rarely: binucleate) infective parasite stage called a sporoplasm or "ameboid organism" and a complex expulsion apparatus consisting of a coiled polar tubule and the polaroplast, a membranous anchoring component. The size of the spores of Microspora species infecting humans varies between about 1 and 4 im. The number and position of the polar tubule windings as seen on the electron microscopical level (Fig. 9.20a, b) are of diagnostic importance.
The spores are eliminated in feces, urine, or sputum and can remain viable for several weeks outside of the host. Following peroral ingestion by a suitable host, the polaroplast swells up, the internal pressure in the spore in
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