Nanosynbacter lyticus gen. nov., sp. nov., an ultrasmall, obligate epibiont bacterium and the proposal of Nanosynbacteraceae fam. nov., Nanosynbacterales ord. nov. and Nanosynbacteria class nov.

Abstract

The recently validly published phylum Minisyncoccota (previously Ca. Patescibacteriota or candidate phyla radiation) had long been an enigma, with members having remained uncultivated for several decades since the first SSU ribosomal sequence was recovered. We reported on the first isolated strain of Minisyncoccota, Nanosynbacter lyticus type strain TM7x^T (ATCC TSD-290^T), in 2015, using directed cultivation from human oral samples. Strain TM7x^T had an extremely reduced genome (705 CDS), ultra-small cell size (200-300 nm), and replicated as an obligate epibiont on the surface of another commonly found oral species: Pauljensenia odontolytica_C strain XH001 (host bacterium). TM7x^T also exhibited what has been defined as a parasitic phase, where it disrupts the membrane of the bacterial host cell, resulting in host bacteria lysis. However, free-floating, host-free TM7x^T cells remain viable and can re-infect fresh host bacterial cells when available. This discovery and coculture marked the first concrete evidence of how these ultrasmall organisms within the Minisyncoccota survive and persist despite their reduced genomes missing de novo biosynthesis of many essential compounds, including all amino acids and vitamins. This model dual species system has now allowed a deeper understanding of the TM7x-host bacterium interaction and dynamics, with over 15 publications reporting on various aspects of this strain to date. We propose the name Nanosynbacter lyticus gen. nov. sp. nov. for strain TM7x^T. We also propose Nanosynbacteraceae fam. nov., Nanosynbacterales ord. nov. and Nanosynbacteria class nov. to accommodate the genus Nanosynbacter.