Oral microbiome: why your mouth is a living ecosystem

Your mouth is not a surface to be scrubbed sterile, but a living ecosystem to keep in balance. Where bad breath really comes from, why the myth about the stomach is wrong, and why “kill all bacteria” is exactly the wrong goal.

It is morning. Before you have said a word, you run your tongue along your teeth.

That thin, rough film you feel is called plaque. The scientific term is biofilm: a thin layer of living bacteria. Your first reaction is probably: dirty, get rid of it.

Your mouth is not an operating room that has to be sterile. It is a living ecosystem, a garden, if you like, with hundreds of different bacterial species on your teeth, your tongue and your gums. Researchers have already catalogued more than six hundred of them.¹ And the reassuring insight of modern oral microbiology is this: most of those residents are not your enemy. They actually keep the system healthy.

The first “animalcules”, scraped from his own teeth

Just how alive your mouth is became clear as early as 1683. In Delft, the cloth merchant Antonie van Leeuwenhoek scraped some plaque from his teeth and placed it under a self-made lens. What he saw had never been observed by anyone: moving little creatures he called animalcules, little animals. Astonished, he wrote that there were more of these creatures living in his mouth than there were people in the whole of the Netherlands. His observations appeared a year later in the journal of the Royal Society in London.²

You may have come across his name on this site before: in my article about the gut, van Leeuwenhoek also stood at the very beginning of the story. The same man who made the microbial world of our belly visible did so first in the mouth. And so, quite literally between human teeth, microbiology was born.

Nearly two centuries later, when doctors discovered that bacteria can cause disease, the thinking became one-sided: bacteria are disease-makers, so the fewer the better. That picture has shifted in recent decades, and the new version is more nuanced and more interesting.

An ecosystem, not an operating room

The British oral biologist Philip Marsh captured the insight in a single principle: oral health is not about how few bacteria you have, but about balance. As long as the beneficial species dominate, the system stays healthy. When the balance tips toward the harmful species, we speak of dysbiosis, a disturbed composition that fosters disease.³

Just as important, and chronically underestimated, is your saliva. It rinses away food debris, delivers building blocks to repair damaged enamel, and contains defensive substances that keep bacteria in check. The Danish researchers Anne Marie Lynge Pedersen and Daniel Belstrøm showed that saliva shapes and guards the balance in your mouth.⁴ When that flow fails, with a dry mouth, for instance as a side effect of medication, the mouth loses its most important protective layer and the harmful species get free rein.

Brushing, in other words, is not a war against the life in your mouth. It is maintenance.

The allies and the troublemakers

Most bacteria in your mouth work for you, not against you. They occupy the space so harmful species find no place.³ Some convert the nitrate from vegetables into substances that can help lower your blood pressure and make your mouth more resistant to acid, a direct link between what you eat and your health. For that last effect, the evidence in humans is still limited.⁵ Still others naturally make substances that curb disease-causing bacteria and counter bad breath.⁶

The harmful species, and what they do, you will meet further on in this piece. In short: one group forms acid from sugar and causes cavities.³ A small but influential group drives gum inflammation, with breakdown of gum tissue and jawbone over time.⁷ And on the tongue, yet other bacteria produce the odour compounds of bad breath. Some species, moreover, do not stay in the mouth. One of them is strongly linked in recent research to bowel cancer.⁸ Another has been found in the brains of people with Alzheimer’s: an indication, not proof.⁹

When the balance tips

As long as the balance holds, little happens. Things only go wrong when one group starts to dominate, usually because we feed that group day in, day out. This is how the three best-known oral problems arise.

The cavity. First, an old misconception. Around 1890 the American dentist Willoughby Miller described that it is not the sugar itself that attacks your tooth, but the acid that bacteria form from that sugar.¹⁰ You feed the acid-forming bacteria with sugar; they secrete acid; that acid dissolves minerals from your enamel. If you eat something sweet now and then, your saliva repairs the damage. If you snack all day long, breakdown outpaces repair, and a cavity forms.³

The gums. If plaque stays too long, the gums become irritated: red, swollen, bleeding when you brush. At this stage it is still readily reversible. If you do nothing, the inflammation can spread beneath the gum line: the gum detaches, deep pockets form, and over time a tooth can start to loosen.⁷ Bleeding gums are therefore not an innocent detail that passes on its own. They are the first alarm signal.

The odour. And with that we arrive at the most embarrassing, and most misunderstood problem.

Source: Mahdiabbasinv, Exclusive Lines on the tongue, Wikimedia Commons, 2016, CC BY-SA 4.0. Converted to WebP.

No, it does not come from your stomach

The Dutch researchers Albert Tangerman and Edwin Winkel showed that in 80 to 90% of cases bad breath arises in the mouth itself, not in the stomach.^11,12 In their own study of 58 people who complained of bad breath, the odour came from the mouth in 47 of them.¹¹ Your oesophagus is normally closed; there is no constant stream of stomach air bubbling up. Only about 5 to 10% of cases truly have a cause elsewhere in the body.¹²

So where does that odour come from? From bacteria that break down proteins in oxygen-poor places. That breakdown produces volatile sulphur compounds, the same family of substances that gives rotten eggs their smell.¹²

The largest source lies not between your teeth, but on your tongue. At the back of the tongue, in the small folds, a layer builds up where these bacteria go about their business undisturbed.¹² That is why brushing teeth alone often falls short: the most important spot stays untouched.

And this is not a matter of feeling. In a controlled study of 36 people with gum inflammation, one group received oral hygiene instructions with a tongue scraper, the other without. After a week, the odour compounds had fallen measurably only in the tongue-scraper group. In that group, two inflammatory substances in the fluid around the gums fell as well.¹³ The gums themselves improved in both groups, and the study was small and short. Still: a small tool, a measurable effect.

When we changed, the mouth changed

The ecosystem in your mouth is not new. It travelled with humankind, and it changed precisely when we changed.

In 2013 a team led by Christina Adler did something ingenious. It isolated DNA from petrified tartar on 34 early European skeletons and so reconstructed the oral flora of our ancestors. Two turning points stood out. When humans began farming and eating starch-rich food about 10,000 years ago, the composition shifted toward disease. And again around 1850, when industry brought white flour and cheap sugar, the cavity bacteria gained the upper hand for good.¹⁴

The conclusion is sobering: the modern mouth is poorer in species than that of our ancestors, and according to the researchers, that very impoverishment may contribute to chronic problems.¹⁴ Your mouth is not a separate thing you clean each morning, but an ecosystem we have disturbed over thousands of years by changing what we put into it.

When the mouth affects the rest of the body

It does not always stay in the mouth. In 2019 a research group led by Stephen Dominy found traces of a notorious gum bacterium, and of the toxic enzymes it makes, in the brain tissue of deceased Alzheimer’s patients. In mice, that same bacterium could reach the brain from the mouth and cause damage there.⁹ Worth knowing: a large share of the researchers, including the first author, worked for the company that developed a drug based on this idea.

This is an indication, not proof. A found association and an experiment in mice do not yet mean it works the same way in humans. The drug built on this idea, moreover, failed in a clinical trial in people.¹⁵ Recent review studies do increasingly link the mouth to the rest of the body: heart, gut, inflammatory processes.¹⁶ But an association is not yet a cause.

The mineral that kills nothing

There is one protector in this story that you never see and that nonetheless works with you every day: fluoride. It kills nothing. It strengthens the material of the tooth itself, and precisely for that reason it fits seamlessly with everything above.

Fluoride, too, began with an observant dentist. In 1901 the young American dentist Frederick McKay settled in Colorado Springs and saw something strange: many residents had brown stains on their teeth. Ugly, yes, but those stained teeth turned out to get remarkably few cavities.¹⁷ It took thirty years to unravel the mystery.¹⁸ Only in 1931 did chemist H.V. Churchill identify the cause: a high fluoride concentration in the drinking water.¹⁷ Too much caused stains; the right, low amount turned out to protect the teeth.¹⁸

For a long time afterward people thought fluoride had to be built into the tooth from within. In the 1980s that picture turned.¹⁹ The Amsterdam dental scientist Jacob ten Cate and the Brazilian researcher Marília Buzalaf summarised the consensus: fluoride works mainly locally, while it is present in low concentration in your saliva.¹⁸ It is not a substance that changes the tooth from within, but a protective action at the surface.

That action has three mechanisms, and none of them is “killing bacteria”. First, fluoride slows the breakdown: during an acid attack it binds to the enamel surface and slows the dissolving of minerals. Second, it speeds up the repair: once saliva has neutralised the acid, fluoride helps put the loosened minerals back, and the repaired surface is harder and more acid-resistant than the old one. Third, and much more weakly, at the concentrations found in plaque fluoride curbs the acid production of the cavity bacterium, though it is still uncertain how much that last effect matters in a real mouth.¹⁹

That fluoride truly protects is, moreover, not a loose claim. It is one of the best-studied daily habits in dentistry. In 2019 Tanya Walsh and colleagues pooled 96 studies for Cochrane. Their conclusion: toothpaste with fluoride prevents more cavities than toothpaste without, and a somewhat higher concentration protects slightly better. In young children, the risk of small marks in the enamel does weigh against that.²⁰

Fluoride is therefore not an aggressive mouthwash that scorches everything, but a reinforcer of the tooth exactly at the vulnerable spots.

Finally, a small, almost free detail with surprising effect. Most people rinse their mouth with water after brushing, and so rinse away that very protective fluoride layer. Spit, don’t rinse. A group of researchers, including that same Philip Marsh, laid out the advice: those who do not rinse keep more fluoride in their saliva, and that goes together with fewer cavities.²¹ An honest caveat here too: the evidence is not yet rock-solid and the gain is modest.

Back to the morning ritual

We are back at that moment when your tongue ran along your teeth.

That film is not your enemy. It is the sign of a living ecosystem you carry with you, one that already existed when van Leeuwenhoek saw his first animalcules, and that has travelled with all of humankind. You do not have to fear it or scrub it empty; you have to maintain it: push back what dominates, replenish what dries out, strengthen what is vulnerable, and have it checked now and then.

The greatest influence lies not in an aggressive mouthwash or in fear of bacteria, but in the small, consistent care you begin today.

So the question is not: “how do I get rid of all bacteria?” But: what balance do you maintain, every morning anew, in the small world you carry with you your whole life?

This article is not medical advice. It does not replace a dentist, dental hygienist or GP. Use it to ask better questions, not to make decisions. If you have complaints, bleeding gums, a persistent odour, a dry mouth, discuss them with your dentist or treating specialist.

Related reading

• The organ that never grows old: a short history of your gut
• Polyphenols: why the same blueberry works so differently for you
• Don’t wait until a diagnosis teaches you what health is worth

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Sources

The numbers in the text refer to this list.

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Photo of the tongue: Mahdiabbasinv. (2016). Exclusive lines on the tongue [Photograph]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:زبان.jpg. Licence: CC BY-SA 4.0.

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