Microsporidia are obligate intracellular parasites that lack mitochondria. Until recently, their importance was primarily limited to fish and insects; but with the onset of the AIDS epidemic, understanding their role as human pathogens has become increasingly important. Encephalitozoon microsporidia develop and mature within a membrane bound parasitophorous vacuole (pv). Following initial infection of the host cell, the developing stages (meronts) of the parasite proliferate in close proximity to the pv membrane. Later, they differentiate through a sporogonial stage and finally into environmentally resistant mature spores located more centrally in the vacuole.

Previous work done in our laboratory has shown that the pv membrane does not act as a significant diffusion barrier to the movement of ions between the host cell cytoplasm and the parasite, permitting the free-acid form of calcein, a fluorescent dye, loaded into the host cell to enter the pv.