Minisyncoccus archaeiphilus gen. nov., sp. nov., a mesophilic, obligate parasitic bacterium and proposal of Minisyncoccaceae fam. nov., Minisyncoccales ord. nov., Minisyncoccia class. nov. and Minisyncoccota phyl. nov. formerly referred to as Candida

In the domain <i>Bacteria</i>, one of the largest, most diverse and environmentally ubiquitous phylogenetic groups, <i>Candidatus</i> 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 <i>Ca</i>. Patescibacteria and <i>Archaea</i> in highly purified enrichment cultures and proposed <i>Candidatus</i> taxa for the characterized species, including <i>Ca</i>. Minisyncoccus archaeophilus and the corresponding family <i>Ca</i>. Minisyncoccaceae. In this study, we report the isolation of this bacterium, designated strain PMX.108^T, in a two-strain co-culture with a host archaeon, <i>Methanospirillum hungatei</i> strain DSM 864^T (JF-1^T), and hereby describe it as the first representative species of <i>Ca</i>. Patescibacteria. Strain PMX.108^T 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 <i>M. hungatei</i> 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^T; we propose <i>Minisyncoccus archaeiphilus</i> gen. nov., sp. nov. to accommodate this strain. The type strain of the species is PMX.108^T (=JCM 39522^T). We also propose the associated family, order, class and phylum as <i>Minisyncoccaceae</i> fam. nov. <i>Minisyncoccales</i> nov., <i>Minisyncoccia</i> class. nov. and <i>Minisyncoccota</i> phyl. nov. within the bacterial kingdom <i>Bacillati</i>.