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Why People Who Brush Still Get Cavities

Our dental health changed with our diets. Ancient humans and their hominin ancestors really didn’t have much of a problem with cavities. In fact, finding an ancient skull with a cavity is such a big deal that you can go and publish a whole paper just to show off the discovery. Unhappy smiles became more common only after the advent of farming led to humans eating a lot more carbs — a shift that went even further following the widespread availability of refined flour and sugar in the 19th century.

But in the past decade or so, scientists sequenced the DNA from ancient dental plaque and figured out that something else in our mouths was changing at the same time as our dental health. Turns out, there are specific strains of bacteria — streptococcus mutans, in particular — that are more common in mouths with cavities. And as human diets changed and cavities became more common, those bacteria started taking over our mouths. Our modern communities of oral bacteria are less diverse than our ancestors’ were, and they’re dominated by these cavity-causing strains.

Increasingly, scientists are thinking of cavities as a microbiome problem. The advice you got as a kid — brush your teeth, floss, eat less candy — is still important. But it’s becoming more clear that the types of bacteria inhabiting your mouth matter, too. Some people do all the oral hygiene stuff right and still get cavities because of the bacteria living in their mouths. Which presents a question: If the types of bacteria in your mouth can make you more prone to cavities, could you fix your teeth by getting different bacteria?

I got interested in this question for selfish reasons: Namely, I am one of those people who brush and floss and eat a diet lower in sugar compared with the average American and still  — still — end up with cavities on the regular. Obviously, this is unfair, but even worse, it looks like one of my two children might be in the same position. I wondered if I had caused my kid’s cavities by passing my bacteria on to her and if, somehow, I could solve the problem with … a spit transplant? (I dunno, man, you think wild things at the pediatric dentist.)


We got drunk on margaritas for science

Mouth bacteria are connected to cavities because of the tiny creatures’ digestive process. “Cavity bugs,” as my daughters’ dentist calls them, feed on bits of sugar and carbs stuck to your teeth. The bacteria ferment those things, creating natural acids that start to dissolve the tooth enamel, similar to how water combines with carbon dioxide to dissolve limestone in a cave.

My fear that I had spread cavity-causing bacteria to my kid is, unfortunately, substantiated by science, said Robert Burne, a professor of oral biology at the University of Florida. “You can take an animal that naturally develops cavities and feed it a high-sugar diet, and it will get cavities. And if you house it with animals that seem to be naturally resistant to cavities, they will then develop cavities,” he said. Cavities, in other words, are a transmissible, infectious disease.

What’s more, Burne told me, research has shown that a caregiver’s susceptibility to cavities — be it a parent or a day care provider — can predict how likely a kid is to get cavities.

a still from the Flintstones of a baby riding on top of a dinosaur.

related: Who Took Care Of The First Baby? Read more. »

And transplanting the bacteria of a not-cavity-prone person to someone who’s cavity-impaired isn’t really an option. Rob Knight, director of the Center for Microbiome Innovation at the University of California, San Diego, was once approached by a dentist in Colorado who wanted his help with a study based on that very idea. “He had noticed that some of his patients tended to get cavities at very high rates, even though they were absolutely scrupulous about brushing and flossing, and other patients did basically nothing and they got no cavities at all. So he was hoping to do an oral microbiota transplant, essentially, by transferring saliva from people who did not get cavities to people who did get cavities,” Knight said. The dentist’s proposed mechanism? Hire attractive women with few cavities to kiss cavity-prone participants. The plan never made it past an institutional review board.

Now the basic idea here — to give people better oral health by changing their oral microbiome — isn’t a bad one, Knight said. But the problem (besides the whole bit with the beauty subjectivity  and the kissing) is that scientists simply don’t know enough about the oral microbiome to guess whether a spit transplant would work, would work only temporarily or would make things worse. That’s because the connections between specific bacterial strains and cavities are merely correlations — nobody knows yet if one causes the other. Studies comparing the oral microbiomes of people who have healthy teeth with people who don’t find more streptococcus mutans in the cavity-prone. However, as Knight explained, changing a kid’s microbiome does not necessarily mean changing whether that kid gets cavities. “We might just be seeing a side effect of something else,” he said. Does the bacteria cause the cavities, or are the bacteria and the cavities together a response to some other factor? Nobody knows.

On top of this, your oral microbiome isn’t static. It changes over the course of your life, over the course of the day and even from one part of your mouth to another, Knight said. What does that mean for the idea of changing the oral microbiome? Again, nobody knows. But it does suggest that the issue is more complicated than one that could be solved simply by kissing some strangers with low dental bills.

But there is some good news in all this. Scientists know a handful of surefire ways to make your oral microbiome healthier. Are you ready for this insider knowledge? OK, here goes: You have to brush twice a day, floss, use mouthwash and eat a diet low in sugars and refined carbohydrates. Yeah, the advice you were getting all along is also a form of biohacking. Those activities change the pH levels in your mouth, Burne told me, making an environment that is less acidic and more friendly to the kinds of bacteria that aren’t associated with cavity formation. Decreasing the acidity also helps promote remineralization — basically the process of your teeth fixing themselves. The opposite of this advice — eating a lot of sugar and not reliably cleaning your teeth — creates an environment that is more friendly to streptococcus mutans and its buddies. “It’s a [natural] selection process,” Burne said.

At some point in the future, researchers may well find out enough about the oral microbiome to start hacking it in a more high-tech way — a way that could address the plight of people who do the basics but still get less-than-stellar results. But despite the promises of a plethora of sketchy probiotic “supplements” on the market, we just aren’t there yet.

So the best thing to do is double down on the stuff you’ve already (hopefully) been doing. That is not, shall we say, what I wanted to hear. I wanted some easy solution that would improve my life and my child’s. But in the time it took to research and write this story, I stumbled upon her stash of empty candy wrappers. And that, at least, gives us a place to start making changes. Step one: Don’t give the cavity bugs a buffet.


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Maggie Koerth is a senior science writer for FiveThirtyEight.

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