Tooth terrorist. Bifidobacterium dentium is found in cavities and may help cause them.

Ventura Laboratory

Cavity Causer Goes Under the Microscope

Inside your steamy, warm mouth, a community of tiny organisms is hanging out, grabbing a snack, and--in some cases--rotting your teeth. Scientists have now sequenced the genome of one possible cavity causer, a bacterium known as Bifidobacterium dentium. This little bugger is so successful, its genes reveal, because it can metabolize a wide range of sugars, survive in an acidic environment, and even resist your attempts to murder it with mouthwash.

When Marco Ventura, a microbiologist at the University of Parma in Italy, first started studying B. dentium, he wasn't thinking about cavities. Instead, he was interested in the entire Bifidobacterium genus, which is mostly made up of bacteria that live in the gut and are thought to be good for health. (They're the basis of probiotics, in which bacteria are consumed as dietary supplements, though the practice is controversial) But B. dentium is not so health-promoting. It's found in cavities and is thought to help cause them. To find out what makes B. dentium so different from its relatives, Ventura and colleagues sequenced the bug's genome.

B. dentium's genes reveal some nifty adaptations for living in the mouth, the team reports online today in PLoS Genetics. Compared with its relatives in the intestines, B. dentium has many more genes that make enzymes for breaking down sugars: While bugs in the gut get a steady supply of complex carbohydrates from the stomach, microbes that live in the mouth have to be ready to leap into gear to take advantage of any sugars at any time.

B. dentium's genome also reveals why it is so hard to get rid of. It sports a number of genes that increase their expression in acid environments, which probably help it survive in dental cavities, where acid destroys tooth enamel. B. dentium may even have evolved to protect itself against dental hygiene: When Ventura and his colleagues grew the bug in a variety of mouthwashes and antiseptics, they found that it ramped up the activity of several genes, including those for proteins that bind up toxic compounds and render them harmless.

Now that scientists know how B. dentium works, they may have an easier time killing it, says microbiologist Floyd Dewhirst of the Forsyth Institute in Boston. Researchers could develop a drug, for example, that attacks an enzyme the bacterium uses to keep its internal pH under control.

Still, Ventura notes, there are a number of cavity-causing bugs in the mouth, so wiping out just one is not going to put dentists out of business. And we already know how to prevent tooth decay in most cases, adds Howard Jenkinson, an oral microbiologist at the University of Bristol in the United Kingdom. "The best way to prevent it is through fluoride [and] through educating people not to take sugary drinks," he says. "And yet mothers still give their children bottles of orange juice overnight."