Thanks to Babbel, a language learning app, for sponsoring this episode.

If you’re interested in growing your language skills, Microcosmos viewers get up to 60% off with a 20 day money-back guarantee when you sign up using our link in the description. You may not want to think about it this way, but your mouth is really just one giant, wet cave for microbes.

Sure, it’s also a mouth, which means that it’s a part of a body that does all the mouth things that the body needs it to do. It bites and digests and tastes. But from the perspective of bacteria like these, your mouth is not a tool.

It's a home. It's a place that provides shelter and food, but it is also a place that can pose many threats. And the interplay between our mouths and the microbes that take up residence within them ends up, inevitably, affecting our own health.

We’ll start this episode with a similar disclaimer to the one we made when we talked about dust mites in your home. While watching this video, you may find yourselves suddenly gripped by a desire to clean. Only this time, instead of seeking out a broom to flush the dust mites out of your home, you might find yourself reaching for a toothbrush.

So it is with that in mind that we need to extend a big thank you to Ela, a friend of our master of microscopes. Ela kindly ate a lot of sugary food and skipped brushing her teeth for two days to make this video possible. When Ela texted James that quote, “My mouth is totally gross and perfect for sampling,” the result was this: tartar and plaque scraped off her gums and the spaces between her teeth.

Within these samples, you can see dense populations of bacteria, some of which move around quite a bit, like the corkscrew-shaped spirochetes here. As is so often the case, the microscopist Antonie van Leeuwenhoek got there long before we did. In 1677, he published a paper documenting the “little animalcules” he observed under the microscope.

Among his many illustrations and descriptions is a set of drawings depicting the bacteria that came from his own mouth. In 1889, a dentist named Willoughby Dayton Miller published the book “Die Mikroorganismen der Mundhöhle,” which was translated and published a year later under the English title “The Micro-Organisms of the Human

Mouth: The Local and General Diseases Which Are Caused by Them”. In the first chapter, he wrote: It has been established beyond all question that myriads of micro-organisms are constantly present in the human mouth, and that these, under favorable circumstances are capable of manifesting an action of the utmost significance upon the local as well as the general health of the patient. This statement would be the foundation of a major shift in our understanding of tooth decay. At the time there was a theory that tooth decay was caused by some kind of wiggling worm that needed to be driven out of our mouths.

But Miller developed an alternative explanation called chemo-parasitic theory, where the bacteria in our mouths ferment sugars, but in the process release acids which then cause our teeth to decay. And humans had been cleaning their teeth for millennia, including with ancient tools like porcupine quills. But the chemo-parasitic theory gave a more specific—and accurate—framework to think about how we clean our teeth, and it would shape much of how we approached dental disease in the 20th century, including brushing your teeth, flossing, and the addition of fluoride to toothpaste.

But while Miller’s work helped us uncover the idea of microbes as a mass interacting with our teeth, it turns out that understanding the specific roles of individual microbes within that mass is much more complicated. To understand how complicated, we only need to look at two of the protists we've found on Ela’s teeth. The first is Trichomonas tenax, the gourd-shaped creature swimming here.

Trichomonas tenax likes to take up residence in the teeth, gums, tongue, and saliva. And it is associated with several diseases in the body, but it’s not clear if it’s involved at all in periodontal disease, which you can think of as diseases that involve inflammation of the gums. Scientists have been finding Trichomonas tenax in samples that come from patients who have periodontal disease since the 1960s, but that is not the same as finding out that the microbe itself is causing disease.

And then there’s the blob-shaped Entamoeba gingivalis, which was the first amoeba found inside of humans. It has a name that you might immediately associate with gingivitis, and for good reason: this protist is found in high numbers in people with periodontitis. But again, that doesn’t necessarily mean this protist actually causes the disease.

It might be that these protists are involved in the disease. Or it might be that the changes in the mouth that lead to periodontitis also make for a comfortable home from the perspective of Trichomonas tenax and Entamoeba gingivalis. The oral microbiome is one of the largest and most diverse microbiomes in your bodies, second only to the microbiome of our guts.

And every day that we eat, drink, brush, and floss brings changes to the environment they live in— some of which are too large for the microbial communities to weather. We started by describing your mouth as a big, moist cave for microbes, and the comparison holds when you think of the complex and mysterious ecosystems that live within caves. There are enigmatic creatures that latch onto walls and swim within the waters, that hide themselves and shape the environment around them.

But for the organisms of the cave of your mouth, well, we feed them and we nourish them and then we destroy them because we have to, never fully knowing what it is that we have washed down the sink. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you again to Babbel for sponsoring this episode of Journey to the Microcosmos.

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As a Microcosmos viewer, you’ll get up to 60% off when you sign up using our link. Plus Babbel comes with a 20 day money back guarantee, so you can see where Babbel takes you on our language learning journey. I'd like to say a quick thanks to all the people whose names are on the screen right now who allow us to keep diving deeper into the weird places that we can find microbes on planet Earth.

That, of course, being pretty much everywhere, including right there in your own mouth. If you're interested in helping us be able to continue making this show, you can check out Patreon.com/JourneyToMicro. If you'd like to see more from our master of microscopes, James Weiss, you can check out Jam and Germs on Instagram.

And if you want to see more from us, there's always a subscribe button somewhere nearby.