Ultrasmall episymbiont Nanosynbacter lyticus employs multiple ATP-generating metabolic pathways during horizontal transmission.

Saccharibacteria (formerly TM7) are a group of environmentally diverse, ultrasmall bacteria with highly reduced genomes belonging to Patescibacteria (formerly Candidate Phyla Radiation), a newly identified bacterial lineage accounting for over a quarter of microbial diversity. Nanosynbacter lyticus strain TM7x was isolated from the human oral cavity and was the first culture representative of Saccharibacteria. It displays an obligate episymbiotic lifestyle where TM7x lives on the surface of its bacterial host Schaalia odontolytica strain XH001. Saccharibacteria rely on host bacteria for growth. TM7x multiplies through budding division, and daughter cells can disassociate from host bacteria during their horizontal transmission stage and establish symbiosis with new bacterial hosts. However, how these metabolically constrained symbionts maintain their viability and infectivity during their horizontal transmission phase, when they are disassociated from hosts, remains poorly understood. By applying targeted mutagenesis using recently developed genetic tools for Saccharibacteria, we demonstrate that the TM7x-encoded arginine deiminase system (ADS) plays a critical role in ATP production and impacts TM7x-host bacterium interaction. Furthermore, we present the first empirical evidence showing that TM7x can uptake and utilize glucose via the glycolysis pathway. Glycolysis is particularly important for episymbiont ATP production under anoxic conditions during horizontal transmission between hosts. Our study demonstrates that TM7x employs two ATP-generating metabolic pathways, ADS and glycolysis, to ensure its viability and infectivity under different microenvironments when disassociated from its hosts during horizontal transmission, a critical phase of its life cycle.