A map of microbes living in our language

Our body is not only ours: it is the habitat in which thousands of microorganisms live, feed and reproduce. Some are beneficial to us and help us perform our basic functions, others do not affect us, and a few cause us problems when there is some kind of imbalance.

One of these communities, that of our language, forms a very complex ecosystem, a whole microbial universe with a very precise structure. Although the components of this particular world are more or less well known until now less attention has been paid to the spatial organization of the world. Are the microorganisms randomly distributed or can we rather assign areas where each particular group 'resides', so to speak?

The spatial organization of these miniature cities is affected by variables such as temperature, humidity, saliva flow, pH, oxygen and the frequency of disturbances such as the arrival of toothpaste and other oral hygiene products. In addition, microbes influence their neighbours by acting as sources and sinks of metabolites, nutrients and molecules. By occupying space, microorganisms can physically exclude each other from desirable habitats, but their surfaces also have binding sites to which other microbes can adhere.

Now, a team of researchers has just published in the journal Cell Reports a series of images, obtained with a new fluorescence imaging technique, that shows the spatial organization of the tongue microbiome with very good resolution. To make these unique maps, the scientists used samples from the tongue scraping of 21 people and managed to identify the 17 most abundant genera of microorganisms, all present in at least 80% of the participants in the study.

A very well-defined organization

The team managed to identify three types of samples: bacteria that live freely, bacteria that are attached to the epithelial cells of the tongue, and bacteria organized in consortia, which would be a kind of biofilm that is structurally very complex and formed by multiple layers. The study of these consortiums is very interesting: they were generally dominated by a specific type of microorganism, organized around a nucleus of epithelial cells and with a very well defined perimeter.

The results also revealed that the languages of all the participants showed consortiums of three main bacterial genera: Actinomyces, Rothia and Streptococcus. Actinomyces frequently appeared near the nucleus, while Rothia was often seen in large patches outside the consortium. In turn, Streptococci formed thin crusts on the outside of the consortia and veins or patches on the inside.

For the authors, these results give us an idea of how these well-structured bacterial communities came to be formed. First, the bacteria bind to the surface cells of the tongue (this layer is called the epithelium), and they do so either individually or in groups. As the community grows, the different types of bacteria compete with each other, so that each group ends up proliferating more in the microenvironments that best meet their physiological needs. This differential growth results in this patchwork organization, which is observed in mature consortia.

Bacteria, our allies

Other information that can be inferred from the spatial organization of the bacterial communities of the language has to do with the reduction of nitrates, a function that is not encoded in the human genome and that we are therefore unable to perform ourselves. The images revealed that some genera that do have this capacity, such as Actinomyces, Neisseria, Rothia and Veillonella, are prominent in language consortia. This raises the possibility that some of the small bumps found on the surface of the tongue are structured in such a way as to stimulate the growth of the bacteria that convert the nitrate in saliva into nitrite.

"Understanding who lives next to whom in these communities will help us know how they work," explained Jessica Mark Welc, one of the authors. "Language is particularly important because it holds a large reservoir of microbes and is a traditional reference point in medicine. One of the first things a doctor says to you in the office is, 'Stick out your tongue'," she concludes.

Explanation of top image: Human epithelial tissue forms a central (grey) core. The colours indicate different bacteria: Actinomyces (red) occupies a region near the nucleus; Streptococcus (green) is located in the outer crust and forms stripes on the inside. Other taxa (Rothia, cyan; Neisseria, yellow; Veillonella, magenta) are present in groups and stripes that suggest that the community grows outwards from the central core.

Reference: Wilbert et al. 2020 "Spatial Ecology of the Human Tongue Dorsum Microbiome" Cell Reports.