Polynucleobacter necessarius gen. nov., sp. nov., an Obligately Endosymbiotic Bacterium Living in the Cytoplasm of Euplotes aediculatus

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.Endosymbiotic bacteria often are seen in protozoa. However, only a few of them have been studied extensively, and several of these are still referred to by Greek letters, as is customary for cytoplasmic elements. Among the endosymbionts which are well characterized but do not have binary names are omicron and the omicronlike endosymbionts. The purpose of the present paper is to name this group of symbionts in accordance with the rules of the Bacteriological Code. The name Polynucleobacter was selected to indicate a characteristic feature of this genus: all members show multiple nucleoids easily visible in these endosymbionts after staining with acetocarmin, acetoorcein, or deoxyribonucleic acid (DNA)-fluorescent dyes. The epithet necessarius was chosen to indicate the relationship of omicron and its host Euplotes aediculatus, which depend upon each other (3).

Low Intraspecific Diversity in a Polynucleobacter Subcluster Population Numerically Dominating Bacterioplankton of a Freshwater Pond

“…Strains affiliated with this group are thought to be obligate endosymbionts of at least two benthic ciliate species living in fresh and brackish water (26,44). We demonstrated by FISH the presence of free-living PnecC cells in the pelagic zone of a freshwater habitat.…”

“…The PnecC group is a phylogenetically narrow, but potentially ecologically diverse, group (23,26,44). Strains affiliated with this group are thought to be obligate endosymbionts of at least two benthic ciliate species living in fresh and brackish water (26,44).…”

“…to total numbers of bacterial communities has not been observed (17). However, detailed studies on PnecC bacteria have to reveal whether the endosymbiotic stages (26,44) and the observed free-living stages represent two completely independent lifestyles or only two stages of the same lifestyle (i.e., facultative endosymbionts). This knowledge will be essential for the understanding of the reasons for the observed numerical dominance of the PnecC bacteria.…”

“…The isolated strains are closely related to the endosymbiotic type strain of Polynucleobacter necessarius (16S rRNA similarity, 99.2 to 99.4%). This species was described as an obligate endosymbiont of a ciliate (26). Despite the close phylogenetic relationship, the isolates were preliminarily not classified as Polynucleobacter necessarius strains due to the potential differences in lifestyle (obligate endosymbionts versus potentially completely free-living planktonic).…”

Polynucleobacter necessarius , a model for genome reduction in both free-living and symbiotic bacteria

“…Polynucleobacter necessarius was first described as a cytoplasmic endosymbiont of the ciliate Euplotes aediculatus (10,11). Further surveys detected its presence in a monophyletic group of fresh and brackish water Euplotes species (12,13).…”