gen. nov., sp. nov., a mesophilic, obligate parasitic bacterium and proposal of fam. nov., ord. nov., class. nov. and phyl. nov. formerly referred to as Patescibacteria or candidate phyla radiation.

In the domain , one of the largest, most diverse and environmentally ubiquitous phylogenetic groups, Patescibacteria (also known as candidate phyla radiation/CPR), remains poorly characterized, leaving a major knowledge gap in microbial ecology. We recently discovered a novel cross-domain symbiosis between . Patescibacteria and in highly purified enrichment cultures and proposed taxa for the characterized species, including . Minisyncoccus archaeophilus and the corresponding family . Minisyncoccaceae. In this study, we report the isolation of this bacterium, designated strain PMX.108, in a two-strain co-culture with a host archaeon, strain DSM 864 (JF-1), and hereby describe it as the first representative species of . Patescibacteria. Strain PMX.108 was isolated from mesophilic methanogenic sludge in an anaerobic laboratory-scale bioreactor treating synthetic purified terephthalate- and dimethyl terephthalate-manufacturing wastewater. The strain could not grow axenically and is obligately anaerobic and parasitic, strictly depending on as a host. The genome was comparatively large (1.54 Mbp) compared to other members of the clade, lacked some genes involved in the biosynthesis pathway and encoded type IV pili-related genes associated with the parasitic lifestyle of ultrasmall microbes. The G+C content of the genomic DNA was 36.6 mol%. Here, we report the phenotypic and genomic properties of strain PMX.108; we propose gen. nov., sp. nov. to accommodate this strain. The type strain of the species is PMX.108 (=JCM 39522). We also propose the associated family, order, class and phylum as fam. nov. nov., class. nov. and phyl. nov. within the bacterial kingdom .