Impact of Saccharibacteria and their bacterial hosts in Periodontal and Inflammatory Diseases

Abstract
Human oral polymicrobial communities play a significant role in health and disease. Periodontitis is a
polymicrobial disease and is among the most common infections of humans. Recent next-generation sequencing
studies reveal that periodontal polymicrobial communities are comprised of a large diversity of bacterial species;
however, we have little to no understanding of the disease-role of some of the key members. Saccharibacteria
(TM7) is one such bacterial group that has been linked to multiple inflammatory diseases, including periodontitis.
In addition, TM7 belongs to a major bacterial lineage termed Candidate Phyla Radiation. Candidate Phyla
Radiation encompasses one-quarter of all bacterial diversity, yet they have been recalcitrant to laboratory
cultivation. Oral TM7 bacteria were the first to be cultivated, opening the door to many intriguing questions due
to their unique characteristics. TM7 has an ultra-small cell size (200-500 nm) with a highly reduced genome
lacking essential biological functions. More importantly, it is an epiparasitic bacteria that grows on the surface of
Actinobacteria (i.e., Actinomyces, Pseudopropionibacterium), which is another understudied bacterial group in
humans. Actinomyces has been shown to induce inflammatory response both in cultured cells and in animal
models. Our preliminary mouse periodontal studies were designed to investigate the role of TM7 and its host
bacteria, Actinomyces, in vivo. TM7/host bacteria pairs induced less inflammatory bone loss compared to host
bacteria alone, suggesting that oral Actinomyces and related bacteria can be a pathobiont (opportunistic
pathogen), and their pathogenicity is regulated by TM7 bacteria. The current proposal will investigate pathogenic
genes in TM7 host bacteria and how they are regulated by TM7 bacteria via metatranscriptomics. Identified
genes will be further tested by site-directed mutagenesis in the host bacteria (Aim 1). The proposal will also
determine the pathogenic and immunostimulatory activity of TM7 bacteria in epithelial and immune cell
interaction assay and will identify TM7 macromolecules that are involved in their interaction with the eukaryotic
cells (Aim 2). Lastly, this study will expand to culturing and characterizing additional taxonomically diverse TM7
bacteria from the periodontal patients, effectively creating a library of TM7 bacteria. This will allow us to determine
the general immunological behavior of TM7/host bacteria across different TM7 species (Aim 3). Accomplishment
of the proposal aims will improve the currently limited understanding of TM7 bacteria and its interaction with the
host bacteria, as well as generate the first understanding of TM7's role in periodontal and inflammatory diseases.

Grant Number: 5R01DE031274-04
NIH Institute/Center: NIH
Principal Investigator: Batbileg Bor