Abstract
Obligately endosymbiotic bacteria living in the cytoplasm of ciliates of the genus Euplotes constitute the new genus Polynucleobacter gen. nov. These endosymbionts are commonly known as omicron and omicronlike particles. The best-studied form, as well as the type species, is omicron from stock 15 of Euplotes aediculatus. We propose the name Polynucleobacter necessarius gen. nov., sp. nov., for this bacterium.