Abstract
Approximately 14 of the more than 1,000 species of microsporidia infect humans, only two of which, Enterocytozoon bieneusi and Encephalitozoon intestinalis, cause intestinal microsporidiosis. Clinical isolates of three microsporidia species, E. intestinalis, Encephalitozoon hellem, and the insect parasite, Anncaliia (Brachiola, Nosema) algerae were used in a spore germination assay, and enterocyte attachment and infection assays were performed to model the potential roles of gastric and duodenal environments and host temperature in determining why only one of these microsporidia species causes intestinal microsporidiosis. Enterocyte infection with A. algerae spores was 10% that of the Encephalitozoon species, a difference not attributable to differences in spore attachment to host cells. Prior spore treatment with pepsin in HCl, pancreatic enzymes, or ox bile did not inhibit germination or enterocyte infection by the three microsporidia species. While the Encephalitozoon species differentiated to mature spores within 3 days, the time taken for many enterocytes to turn over, A. algerae took 3-5 days to produce mature spores, near the upper limit for enterocyte turnover in vivo. Thus, host temperature may contribute to A. algerae not causing human intestinal microsporidiosis, but none of the factors tested account for the inability of E. hellem to cause such an infection.