Tongue, Meet Your Microbes

New images reveal the distribution of bacteria that live on our tongues

Thanks to a new imaging technique, researchers are able to see how the microorganisms that live on our tongues divide themselves into distinct groups by genus.

Scientists at the Forsyth Institute found that when they examined the bacteria that make their home on the human tongue, species were spread out in their own unique communities. “Bacteria on the tongue are a lot more than just a random pile. They are more like an organ of our bodies,” said Gary Borisy, senior author of the study, in the press release.

With samples scraped from the tongues of 21 participants, researches used sequence analysis to identify 17 bacterial genera present on the tongue. The bacteria in the samples form layered, complex biofilms on the surface of the tongue.

In the images, researchers could see a distinct spatial organization of bacterial communities varying in shape and size. They also found that the genera preferred certain locations: Actinomyces was frequently found near the center, whereas Streptococcus tended to form a “thin crust” on the exterior.

The researchers used a technique that was recently developed in the Borisy lab called Combinatorial Labeling and Spectral Imaging — Fluorescence in situ Hybridization, or CLASI-FISH. Rather than use DNA sequencing, which would not have shown the specific locations of the microorganisms, this technique enabled researchers to identify the types of microorganisms present without disturbing their distribution. The paper, published March 24 in Cell Reports, contains startling images of a rainbow of fluorescent microorganisms living on the human tongue.

“Most of the previous work on bacterial communities used DNA sequencing-based approaches, but to get the DNA sequence, you have to first grind up the sample and extract the DNA, which destroys all the beautiful spatial structure that was there,” said Borisy in the press release.

“Imaging with our CLASI-FISH technique lets us preserve the spatial structure and identify the bacteria at the same time.”

This figure illustrates the organization of microorganisms on the tongue.

Scientists are just beginning to investigate the role this curious and diverse microbial community plays in the human body. The oral microbiome is influenced by a combination of factors such as hygiene, saliva, temperature, acidity, oxygen, and abrasions.

Borisy’s lab has been working on visualizing these tongue-dwelling microorganisms for some years now — back in 2017, The Scientist highlighted the researcher’s preliminary work. The images published in Cell Reports this month are the result of that research. Science Friday also recently featured one of the authors of the study, microbial ecologist Jessica Mark Welch, on an episode of their podcast titled “Mapping the Microbiome of Your Tongue.”

“We think that learning who is next to who will help us understand how these communities work,” Mark Welch said in the press release. “The tongue is particularly important because it harbors a large reservoir of microbes and is a traditional reference point in medicine. ‘Stick out your tongue’ is one of the first things a doctor says.”

Moving forward, the researchers are eager to better understand the role these microorganisms play in our mouths. The rest of us may spend an extra minute or two sticking our tongues out at ourselves in the mirror, wondering about the curious microorganisms that call our mouths home.

Science writer wrangling words and horses in the Pacific Northwest. | she/they

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