Even the most well mannered mouth is like a sewer: it teems with bacteria. But using mouthwash may not be the appropriate response. According to a group of British researchers, the bacteria that live on the back of the tongue, at least, are our friends. They convert food-derived nitrate in saliva into a chemical that kills harmful germs in the stomach.

The news may help restore the good name not only of mouth bacteria but also of nitrate, which has a long history of bad press. The worry about nitrate--a stable compound of a nitrogen atom and three atoms of oxygen--has been that it can lose an oxygen and become nitrite, a much more reactive compound. Nitrite, in turn, can hook up with food components called amines to form nitrosamines. Studies in rats, as well as some epidemiological studies, have linked those compounds to stomach cancer.

Most of the nitrosamines we’re exposed to are probably formed in processed meats that have been treated with nitrates or nitrites as preservatives. But the formation of nitrosamine can also take place in our own bodies because we eat a lot of nitrate, mostly in dark green leafy vegetables. In Europe recently there has been talk of banning the sale of lettuce with high levels of nitrate. As it happens, that tends to be lettuce grown in cloudy northern climes like Britain.

The idea of a lettuce ban seems pretty silly to Nigel Benjamin, a professor of medicine at the University of Aberdeen Medical School in Scotland. Benjamin is one of the people who discovered that most of the nitrate in our bodies is extracted from leafy greens. He also discovered that as much as 25 percent of that nitrate makes its way back to cells in the mouth, where it is secreted in saliva and somehow converted there into nitrite. Why on earth, Benjamin wondered, would the body hold on to potentially harmful stuff that way? What happens to all the nitrite in the quart or so of saliva we swallow each day?

A few years ago Benjamin and his colleagues had a hunch. They knew that in the presence of acid, nitrite can lose yet another oxygen and turn into nitric oxide, an extremely reactive molecule. We knew that nitric oxide is quite toxic to some bacteria, says Benjamin, so we said hey, what may be happening is that nitrite is deliberately being made in the mouth so that it mixes up with the food we eat and gets acidified, which will generate lots of nitric oxide, which will then kill all the nasty germs we eat with our food.

An early test of this hypothesis produced promising results: if salivary nitrite is put in acid, the researchers found, it will indeed convert to E. coli- killing nitric oxide. We proved that if you took bacterial cultures and exposed them to acid, it didn’t do much, says Benjamin. But if you expose them to acid and a little bit of nitrite, it just wipes them out.

That left the question of how exactly the nitrite is made--and according to Benjamin’s most recent experiments, it’s made by bacteria on the tongue. Benjamin and his colleagues studied rats and found that they had lots of bacteria on the rear portion of the tongue, near the throat. Moreover, says Benjamin, the conversion of nitrate to nitrite was happening only on the back part of the tongue, where the bacteria thrive. The nitrite was definitely not being secreted by the tongue itself: a control group of rats raised in a germ-free environment, with no chance of being colonized by bacteria, had no nitrite on the tongue.

Like the human tongue, the rat tongue is covered with tiny fingerlike projections known as papillae. The bacteria that convert nitrate live in clefts between papillae at the rear of the tongue, out of reach of airborne oxygen. They are the kind of bacterium--facultative anaerobes-- that can survive with or without oxygen. We use oxygen to burn up the glucose in our bodies to make energy, Benjamin explains. Whereas if you’re one of these bacteria, you have alternatives. One alternative is to use nitrate. You use that to burn up your fuel. It turns nitrate to nitrite. That’s fine, as long as the nitrite isn’t acidified. If it’s on the surface of the tongue, it isn’t, because the saliva is nice and alkaline.

Tiny glands at the base of the tongue’s papillae secrete bicarbonate, which makes the saliva alkaline and bacteria-friendly. So the bacteria are being bathed with vast amounts of nitrate and converting it to nitrite, says Benjamin. The nitrite is swallowed and hits the acid in the stomach and turns into nitric oxide. It kills germs in the stomach. So we have the ironic situation where we’re using a symbiotic relationship with organisms on the tongue to kill organisms in the stomach.

And not just in the stomach: some of the nitrite in saliva gets converted into nitric oxide around the gums, where acid-producing bacteria live. Benjamin thinks this nitric oxide may protect against tooth decay by killing those bacteria. People who can’t make saliva, he notes, tend to have bad teeth.

Benjamin’s evidence for nitrite-producing tongue bugs is drawn from rats, but he’s confident a similar symbiosis exists in humans. He and his colleagues have found, for example, that humans do indeed have more nitrite in the saliva and nitric oxide in the stomach when they eat more nitrate. The human tongue may harbor slightly different bacteria from those found in rats--perhaps because each type of bacterium has coevolved with its mammalian host. Quite possibly these organisms have evolved to fit a particular niche, says Benjamin. That’s something we’ll be looking into.

Such coevolution might explain why bacteria on the tongue, assuming we have them, are not evicted by the immune system, which would probably attack them if they invaded other parts of the body. It could also explain why babies, who have not yet acquired the bacteria, and patients on antibiotics, who have lost them, are especially vulnerable to stomach infections. Finally, Benjamin and his colleagues are now investigating a particularly interesting possibility: tongue bugs may help protect us against Helicobacter pylori, the bacterium that has been identified as a cause of stomach ulcers and stomach cancer.

Other types of protective bacteria have long been known to flourish in the colon, says Benjamin, but no one has figured out what those bugs do to keep us healthy. I think this tongue study is the first that shows how a true symbiosis is working, he says. We’ve proposed a mechanism and defined the chemical process for how it happens. In addition, he notes, it’s just another factor that makes us think that green vegetables are good for you--even if they’re high in nitrates.