Sixteen years have passed since a microsporidian was first reported as an opportunistic pathogen in HIV/AIDS. to date, two new genera, Enterocytozoon and Trachipleistophora, and 5 new species, Enterocytozoon bieneusi, Encephalitozoon hellem and E. intestinalis, and T. hominis and T. anthropophthera have been described infecting patients with AIDS. E. bieneusi and E. intestinalis have been among the most common intestinal pathogens diagnosed as causes of diarrhea and wasting in HIV/AIDS.
Members of their phylum, Microspora, have in common a hollow polar tube that is tightly wound up in their spores. The coiling up of the polar tube, just beneath the cell wall, allows a structure that is several times its length to be packaged within a spore. E. bieneusi, which has the smallest of all microsporidian spores, 1 x 1.5 ¼m, overcomes its size limitations by packaging its polar tube in 2 layers of 3 coils each. When conditions are right, this unique structure is extruded from the spore and becomes the “needle of a syringe”.