FIG 1.

Microsporidial life cycle as exemplified by E. bieneusi and E. intestinalis. Spores are the infectious form of the organism (1). Spores infect cells when the polar filament protrudes from the spore and forms the polar tubule, which inserts into the eukaryotic host cell (2). The infective sporoplasm is injected through the polar tubule into the host cell, where it replicates asexually by binary fission (merogony) or multiple fission (schizogony) (3 to 5). Some species divide directly in the host cell cytoplasm (e.g., E. bieneusi), while others are found within a parasitophorous vacuole (e.g., Encephalitozoon spp.) or sporophorous vesicle (e.g., Pleistophora spp.). Mature spores are produced by a form of sexual reproduction called sporogony (5) and eventually, following cell disruption, are released into the surrounding environment (6) to infect new cells. (Courtesy of the CDC DPDx Service.)