|Similar Caedibacter, Verrucomicrobia, Polynucleobacter, Hypotrich, Gemmata obscuriglobus|
Epixenosomes are prokaryotes phylogenetically related to the bacterial division Verrucomicrobia.
The generic name is derived by the episymbiotic adaptation of the organisms ( from the ancient Greek επι ξενον σομα = external alien body), indeed they live on the dorsal surface of marine ciliated protozoa of Euplotidium genus. Epixenosomes possess two distinct developmental phases linked to the host cell cycle. Stage I epixenosomes, are spherical, 0.5 μm in diameter and are surrounded by two membranes. They divide by direct binary fission. Stage II epixenosomes, (Fig..) are egg-shaped (2.2 μm in length and 1 μm in width) and show complex organization (more complicated than the majority of prokaryotic organisms) with different cytoplasmic compartments where an extrusive apparatus within a proteinaceous matrix, although apparently not membrane bounded, differs from the remaining cytoplasm. A functional cell compartmentalization has also been evidenced. The ejection process is involved in the defence of the ciliate host against predators. External signals of unknown origin are detected by the membrane receptors located at the top of the organism. The consequent activation of the adenylate cyclase-cAMP system triggers the ejection of the extrusive apparatus, with the formation of a hollow tube, about 40 μm long, terminating in a head mainly consisting of the apical portion of the epixenosome (the region containing DNA). Extrusive apparatus is surrounded by a basket of tubules. Based on experiments with antitubulin drugs and immunocytochemical analyses at the optical and electron microscopical level the possibility that these tubules consist of tubulin (i.e. eukaryotic protein) has been considered. The presence of tubulin genes has been reported in members of free living Verrucomicrobia, in which however tubular structures have not been evidenced.