December 20, 2017

By Betty Zou, PhD

On a blustery autumn day in 1683, a Dutch inventor named Antony van Leeuwenhoek wrote a letter to the Royal Society of London describing something peculiar. He had taken a scraping of dental plaque from between his own teeth and looked at it under a microscope that he had built. What he saw were “many very little living animalcules, very prettily a-moving.”

Unsure of himself, Leeuwenhoek repeated the experiment with dental plaque taken from four other individuals, including two old men who he believed had never brushed their teeth. Again he saw “an unbelieveably great company of living animalcules, a-swimming more nimbly than any I had ever seen up to this time.”

With that, Leeuwenhoek became the first person to see living bacteria.

Today our understanding of these “animalcules”—their sheer numbers, incredible variety and astounding capabilities—has grown beyond anything that Leeuwenhoek could have imagined. We now know that the human body is home to a thriving community of microbes.

Take, for instance, the mouth. What Leeuwenhoek saw in his primitive microscope more than 300 years ago was a sneak peak into the bustling microbial metropolis that is home to over 700 types of bacteria. Collectively known as the oral microbiome, this complex community of bacteria, viruses and fungi is a key player in not just oral health, but also our overall health.

What makes the mouth such an inviting place for microbes? For starters it’s warm, moist and filled with nutrients from saliva and other oral fluids. The mouth also offers a wide variety of habitats that, much like the different neighbourhoods of a city, allow unique bacterial communities to flourish. The hard surfaces of teeth, for example, give bacteria a firm spot to attach to while deep crevices beneath the gums offer a safe haven for oxygen-fearing bacteria.

Not only is there variation in the microbial communities at different sites within the mouth, but there are also differences between the oral microbiomes of different people. The composition of the microbial community in your mouth is shaped by genetics and environmental factors like sugar consumption and whether or not you smoke. The variation among individuals lies in not just who is there, but also how many are there.

In a 2010 study, researchers at Stanford University looked at diversity in the oral microbiomes of 10 healthy individuals. They found that five of the 10 mouths were dominated by a group of bacteria called Streptococcus and two were dominated by Prevotella bacteria. The most common bacteria in the other three mouths were each from a different group. And even though all 10 oral microbiomes had bacteria from the Neisseria family, no one Neisseria species was present in all 10. Despite these apparent differences, all 10 individuals had great oral health.

Many dental diseases and conditions are linked to oral bacteria and arise when the microbial community in your mouth is thrown off balance. Tooth decay, for example, is caused by acid from bacterial fermentation. In addition to these acid-producing microbes, there are also oral bacteria that produce ammonia to help neutralize the acidity and prevent decay. When this delicate balance goes awry—such as when there is a constant source of fermentable carbs for the bacteria to turn to acid—acid levels build up, damaging tooth structure and creating an environment that further favours the acid producers.

All this to say, there is no one “healthy” oral microbiome. A healthy oral microbiome is one that is stable and resilient, able to bounce back from changes caused by eating, drinking, kissing and the like. The mere presence of a particular bacteria species isn’t enough to cause tooth decay or gingivitis. It’s the interactions with the resident fauna and oral environment that dictate whether a microbe becomes a benevolent bystander or a spit disturber that sends you to the dentist.



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