Technique reveals organization of tongue bacteria

A consortium from the human tongue, with each type of bacterium assigned a different color. The unlabeled core is made of tongue epithelial cells. Wilbert et al, Cell Reports

Trillions of microbes—microscopic organisms like bacteria, fungi, archaea, protists, and viruses—live in and on your body. Most aren’t harmful. Instead, they play many important roles in your body, like helping with digestion and protecting against harmful infections.

Your mouth is a rich habitat for microbes, with more than 750 species of bacteria. These often live in biofilms, such as plaque on teeth—communities where they provide one another with the nutrients and other molecules they need to thrive. Different types of bacteria thrive in different areas of your mouth.

To decipher which bacteria are living on the human tongue and how their communities are organized, a team led by Drs. Gary Borisy at the Forsyth Institute and Harvard University and Jessica Mark Welch at the Marine Biological Laboratory in Woods Hole, Massachusetts used fluorescence imaging to visualize mouth bacteria from 21 volunteers. The study was funded by NIH’s National Institute of Dental and Craniofacial Research (NIDCR). Results were published on March 24, 2020, in Cell Reports.

Researchers scraped bacteria off the tongues of the volunteers, from back to front. They used Human Microbiome Project data to assess which bacteria would be present on samples from the healthy human tongue. The team found these bacteria organized on the tongue in three ways: as free bacteria, attached to the outer layer of skin cells, or organized into complex biofilms, called consortia, that were multiple layers thick.

To understand the organization of these consortia, the researchers chose the 17 most abundant bacterial genera, found on at least 80% of human tongues. Using a technique Borisy’s group developed, called Combinatorial Labeling and Spectral Imaging–Fluorescence in situ Hybridization (CLASI-FISH), they fluorescently labeled these bacteria to visualize them in consortia. Previous work that used DNA sequencing-based approaches required grinding up the samples and extracting DNA. CLASI-FISH allowed the team to identify the bacteria while preserving spatial structures.

All the participants’ consortia contained bacteria from three genera: Actinomyces, Rothia, and Streptococcus. Actinomyces was mainly found near the core of these structures, which was made of epithelial cells from the tongue. Rothia was mostly found in large patches on the edges. Streptococcus formed a thin crust on the edges as well as veins or patches throughout.

“Our study is novel because no one before has been able to look at the biofilm on the tongue in a way that distinguishes all the different bacteria, so that we can see how they arrange themselves,” Borisy says. “From detailed analysis of the structure, we can make inferences about the principles of community growth and organization.”

—by Tianna Hicklin, Ph.D.