Saccharibacteria, also known as TM7, a relatively recently discovered group of bacteria, have long evaded laboratory manipulation because they can only grow when attached to host bacteria in a lifestyle known as episymbiosis.
Since their first successful cultivation in 2015, Saccharibacteria have inspired microbiologists at ADA Forsyth to closely consider the importance of polymicrobial interactions, such as episymbiosis, in oral health.
Researchers at ADA Forsyth traverse new frontiers in microbial ecology through further study of episymbionts like Saccharibacteria and their role within the human microbiome.
In a new study, ADA Forsyth researchers performed targeted mutations on an episymbiosis-determining pathway for the first time. They published their findings in The ISME Journal: Multidisciplinary Journal of Microbial Ecology.
“We’re developing innovative tools to look at the symbiosis factors that allow Saccharibacteria to interact with bacterial hosts,” said the paper’s first author Alex Grossman, Ph.D., a postdoctoral fellow in the Bor Lab at ADA Forsyth. “We’ve found that these organisms encode elaborate systems for localization within their environment and on their bacterial hosts.”
In their most significant finding, researchers described how Saccharibacteria use two specific pili – hair-like appendages on the surface of bacterial cells – for motility and for attachment to specific hosts. They show that these pili may act as molecular anchors, tethering episymbionts to their bacterial hosts. The studied Saccharibacteria primarily parasitize periodontitis-associated Actinomycetes bacteria.
Saccharibacteria are tiny – about 70 times smaller than their host bacteria – and scientists haven’t been able to visually observe these interactions in much detail. Until now.
Furthermore, Saccharibacteria change the morphology and metabolism of their host bacteria in ways that may have profound effects on the biology of proinflammatory bacterial hosts and the development of periodontitis. The study of episymbiotic interactions is essential for scientific understanding of how polymicrobial communities form, grow, and transition from a healthy to disease state.
Through a collaboration with Yale microbiologists Jun Liu, Ph.D., and Jack Botting, M.S., the researchers observed the pili visually through cryo-electron tomography, which produces 3D images of biological samples at nanometer resolution. These 3D reconstructions allowed the researchers to precisely measure episymbiosis-driving appendages and observe when the pili had been successfully removed from cells genetically.
“These tiny microbes defy study through standard techniques,” said ADA Forsyth Associate Professor Batbileg Bor, Ph.D. “Our research team did a lot of trial and error using different oxygen concentrations and cultivation strategies over the course of about 18 months before we got the genetics completely functional.”
ADA Forsyth researchers previously found that without the episymbiont attached, Actinomycetes ran rampant in gingival tissue; conversely, when Saccharibacteria were present, they inhibited Actinomycetes pathogenicity and subsequent tissue inflammation. This finding opened a new world of possibilities for Saccharibacteria as engineers of the microbial community and as potential therapeutic agents.
The team hopes to dive deeper into how ultra-small episymbionts attach to host microbes in future experiments. A full understanding of those mechanisms could offer significant opportunities to harness Saccharibacteria in stopping or controlling periodontitis.
“Our research adds a new dimension to the ecology of the microbiome,” Dr. Grossman said. “We’re already familiar with studying the host-microbe relationship as it applies to humans and the microbes that live within us, but we’re far less accustomed to looking at the host-microbe relationship in terms of the bacteria that are parasitizing other bacteria.”
Other collaborators include (from ADA Forsyth unless otherwise noted): Lei Lei, Jett Liu, Nusrat Nahar, Jun Liu, Jeffrey S McLean (University of Washington), Xuesong He.
