(A) A host cell infected with Brachiola algerae. Note the arrangement of spores around the host (E6 cell) nucleus (N). A single spore is probably in the process of infecting an adjacent cell (arrowhead). Magnification, x1,200 (B)A spore with an everted polar tubule. Magnification, x1,200 (Visvesvara, Clinical Microbiology Reviews, July 2002, p. 401-413, Vol. 15, No. 3)

    The microsporidia, which form a phylum including about 1200 known species, are obligate intracellular eukaryotic parasites capable of parasitizing practically all members of the animal kingdom, thus demonstrating formidable capacities for adaptation. These parasites create major problems in human and veterinary health.

The Encephalitozoon cuniculi sequencing project has been completed, and has provided information on the novel structure of microsporidian chromosomes.
In order to obtain more in-depth knowledge of these pathogens (metabolic capacities, host-parasite relationships, genome structure), the sequencing of the genome of the microsporidian, Brachiola algerae (synonym of Nosema algerae) has been undertaken.
Brachiola algerae was discovered in the larvae of Anopheles mosquitoes (Vavra and Undeen, 1970). The spores are ovoid and are about 4.4 x 2.8 um. As in all microsporidia, Brachiola algeria possesses a polar tube rolled up in the spore which permits invasion of a new host. This structure for invasion, which is unique in the living world, facilitates the passage of the infectious sporoplasm (nucleus, cytoplasm) from the parasite to the host cell.

Both phases take place in the same host cell (Desportes, Évolution des Parasites et des relations hôtes-parasites, 2-3-4 mai 2001)

The "diplokaryotic nucleus" is present during all the developmental stages (merogonie: cell multiplication and division and sporogony: differentiation of the spore). Vegetative multiplication and differentiation occur directly in contact with the cytoplasm of the host cell. It is interesting to note developmental differences with the first microsporidium sequenced, Encephalitozoon cuniculi. E. cuniculi has a monokaryotic nucleus, the development cycle takes place in the inside of a parasitophoric vacule, and the spores are smaller (2.5 x 1.5 um).
Since microsporidia are intracellular parasites, the developmental temperature depends directly on the body temperature of the host. Therefore, microsporidial parasites of insects develop at a temperature below or equal to 35 degrees C, whereas microsporidial parasites of mammals develop at a temperature above or equal to 37 degrees C. Studies on Brachiola algerae have shown that development is also possible in vertebrate cell culture at temperatures from 24 to 38 degrees C (see Lowman et al., 2000, for a review). Furthermore, B. algerae is capable of infesting the tail and foot pads of athymic mice (Trammer et al., 1997), and more recently, it has been identified in the cornea of an immunocompetent patient (Visvesvara et al., 1999). This human isolate has been used to infect immunodeficient mice (Koudela et al., 2001). After application to the eye, the development of the parasite causes a severe infection of the liver of the mouse, without causing a lesion in the eye. The physico-chemical conditions in the eye permit a metabolic adaptation of this microsporidium which mainly infects poikilothermic organisms (insects), making the infestation of homeothermic organisms possible also (mammals). This species therefore has great adaptation potential concerning the host range (insects, mammals), the type of cell infested (muscle, kidney, cornea, intestine, and this is probably far from an exhaustive list) and is capable of multiplying and differentiating over a large temperature range (24 to 38 degrees C).
Other studies have demonstrated that Brachiola algerae infection of a mosquito parasitized by Plasmodium falciparum leads to a reduction in development of the causative agent of malaria, suggesting a possible biological control (Margos et al., 1992).
It seems reasonable therefore to concentrate our efforts on Brachiola algerae (adaptation capacities), a mosquito parasite which is also capable of infesting mammals and notably humans, in the context of the sequencing of the genomes of Plasmodium falciparum (causative agent of malaria), the Anopheles mosquito (vector of P. falciparum) and humans.