What causes tooth decay?

Okay, everyone knows that sugary foods cause tooth decay.

But how?

Well, it’s an interesting story. First, a note on tooth anatomy. Enamel, the outermost covering of the tooth, is made up almost entirely (96%) of minerals. The most abundant mineral, by the way, is hydroxyapatite (a complex molecule made up of calcium and phosphate). Minerals in the enamel layer dissolve when the pH drops below 5.5. Which we’ll talk about in a minute.

We have tons of bacteria in our mouths. Anaerobes, spirochetes, Neisseria…but the most important of these is a group of bacteria called Streptococcus viridans (which contains a bunch of species, like S. mutans, S. salivarius, S. sanguis, and S. mitis). By the way, viridans (the root of which means “green” in a bunch of different languages) is a great name for these bugs – because they generally display alpha (partial, or greenish) hemolysis on a blood agar plate. Anyway, when these bacteria encounter dietary sugars, they convert the sugars into acids (like lactic acid). The acid lowers the pH in that region, and the enamel starts to dissolve (scary!).

After a couple hours (or if you go brush your teeth), the pH returns to normal, and the dissolving process stops. You can even remineralize the surface to some extent. However, if you eat frequent sugary meals (without allowing enough time for the pH to get back to normal), there won’t be any chance for remineralization…and you’ll get a cavity.

Strep mutans is the biggest culprit in caries (the fancy name for The Bacterial Disease that Causes Cavities). Why? Several reasons. First, it is a major part of the normal flora. Second: it makes a ton of lactic acid in a short amount of time. Third: it tolerates low pH and high sugar concentration better than other bugs. Fourth: it attaches really well to the tooth surface (it has an enzyme on its surface, glycosyl transferase, that helps attach the bug to the tooth surface). Fifth: it’s great at making glucans (sticky glucose polymers that attach the bugs to each other, and to the tooth, making plaque) using that same glycosyl transferase.

It’s bad enough that Strep viridans species cause cavities. But they also are responsible for a much scarier disease: bacterial endocarditis. When the sadistic dentist dislodges plaque and jabs his or her long metal pick into your tender gums, the bugs can get into the bloodstream and settle on heart valves. Patients with valve abnormalities (like mitral stenosis) are at greater risk. Just another reason your mom was right when she told you to brush your teeth after every meal.

Yummy donut photo by D. Sharon Pruitt.