Isolation and cultivation of candidate phyla radiation Saccharibacteria (TM7) bacteria in coculture with bacterial hosts

BACKGROUND: The vast majority of bacteria on earth have not yet been cultivated. There are many bacterial phyla with no cultivated examples including most members of the Candidate Phylum Radiation with the exception of human oral isolates from the phylum Saccharibacteria. AIMS: The aims of this research were to develop reproducible methods and validate approaches for the cultivation of human oral Saccharibacteria and to identify the conceptual pitfalls that delayed isolation of these bacteria for 20 years after their discovery. METHODS: Oral samples were dispersed and passed through 0.2 µm membrane filters. The ultrasmall saccharibacterial cells in the filtrate were pelleted, inoculated into broth cultures of potential bacterial host cells and passaged into fresh medium every 2–3 days. RESULTS: Thirty-two isolates representing four species of Saccharibacteria were isolated in stable coculture with three species of host bacteria from the phylum Actinobacteria. Complete genome sequences were obtained for 16 isolates. CONCLUSIONS: Human oral Saccharibacteria are obligate bacterial parasites that can be stably passaged in coculture with specific species of host bacteria. Isolating these important members of the human oral microbiome, and many natural environments, requires abandoning many of Koch’s concepts and methods and embracing novel microbiological approaches.