Wikipedia
Polynucleobacter
Polynucleobacter is a genus of Proteobacteria, originally established by Heckmann and Schmidt (1987) to exclusively harbor obligate endosymbionts of ciliates belonging to the genus Euplotes.
Recently, four new Polynucleobacter species were described, which all represent free-living (i.e. not host-associated) planktonic freshwater bacteria. Furthermore, the first described species covers both obligate endosymbionts and free-living planktonic strains.
Free-living Poynucleobacter bacteria represent important members of bacterioplankton in freshwater systems such as lakes, ponds, and streams.