Protists frequently host diverse bacterial symbionts, in particular those affiliated with the order Holosporales (Alphaproteobacteria). All characterised members of this bacterial lineage have been retrieved in obligate association with a wide range of eukaryotes, especially multiple protist lineages (e.g. amoebozoans, ciliates, cercozoans, euglenids, and nucleariids), as well as some metazoans (especially arthropods and related ecdysozoans). While the genus Paramecium and other ciliates have been deeply investigated for the presence of symbionts, known members of the family “Candidatus Paracaedibacteraceae” (Holosporales) are currently underrepresented in such hosts. Herein, we report the description of “Candidatus Intestinibacterium parameciiphilum” within the family “Candidatus Paracaedibacteraceae”, inhabiting the cytoplasm of Paramecium biaurelia. This novel bacterium is almost twice as big as its relative “Candidatus Intestinibacterium nucleariae” from the opisthokont Nuclearia and does not present a surrounding halo. Based on phylogenetic analyses of 16S rRNA gene sequences, we identified six further potential species-level lineages within the genus. Based on the provenance of the respective samples, we investigated the environmental distribution of the representatives of “Candidatus Intestinibacterium” species. Obtained results are consistent with an obligate endosymbiotic lifestyle, with protists, in particular freshwater ones, as hosts. Thus, available data suggest that association with freshwater protists could be the ancestral condition for the members of the “Candidatus Intestinibacterium” genus.

原生生物经常拥有多种细菌共生体，特别是那些属于全孢菌目（ Alphaproteobacteria ）的细菌共生体。该细菌谱系的所有特征成员均与多种真核生物，特别是多种原生生物谱系（例如变形虫、纤毛虫、尾尾动物、眼虫和核虫）以及一些后生动物（特别是节肢动物和相关的蜕皮动物）有明确的关联。 。虽然草履虫属和其他纤毛虫是否存在共生体已被深入研究，但已知的“ Candidatus Paracaedibacteraceae”（全孢菌目）家族成员目前在此类宿主中的代表性不足。在此，我们报告了“ Candidatus Paracaedibacteraceae”家族中“ Candidatus Intestinibacter parameciiphilum”的描述，栖息在草履虫的细胞质中。这种新型细菌几乎是其亲戚“后核肠杆菌”的两倍大，并且不存在周围的光环。基于 16S rRNA 基因序列的系统发育分析，我们进一步鉴定了该属内六个潜在的物种级谱系。根据各个样本的来源，我们调查了“ Candidatus Intestinibacter”物种代表的环境分布。获得的结果与专性内共生生活方式一致，以原生生物，特别是淡水生物作为宿主。因此，现有数据表明，与淡水原生生物的关联可能是“候选肠杆菌”属成员的祖先条件。