Gram-negative bacteria on the tongue may produce most of the foul odors in breath, but recent research emphasizes that no single type of oral bacterium creates bad breath on its own. Mel Rosenberg, an emeritus professor of microbiology at Tel Aviv University, and his colleague Nir Sterer recently found, for example, that some strains of gram-positive bacteria secrete an enzyme that clips sugar molecules off the proteins found in food, which in turn makes those proteins more digestible for nearby gram-negative organisms. The more proteins the gram-negatives digest, the more odors they emit.
Such interactions illustrate why researchers are increasingly interested in oral ecology, viewing the mouth as a kind of densely populated tide pool. Fresh breath reflects a healthy mouth, which is not necessarily one that lacks “bad” bacteria, scientists are realizing, but rather one in which overlapping bacterial colonies hold one another in check.
Bacterial geneticists contributing to the Human Microbiome Project, funded by the National Institutes of Health, have so far identified about 1,000 species of bacteria that commonly inhabit human mouths. Yet one person's particular mix of “bacterial colleagues,” as Rosenberg calls them, is probably quite different from another's. “Each person has maybe 100 to 200 of those bacterial species colonizing their mouth at any given time,” says Wenyuan Shi, a microbiologist at the University of California, Los Angeles.
During birth our previously sterile mouth picks up some of our mother's bacteria, and in childhood we quickly acquire new microbial colonizers. Studies suggest that a preschooler's population of mouth microbes most closely mimics his or her primary caregiver's. As the years go on, diet, stress, illness, antibiotics and other forces can shift the demographics of an individual's microbial community—and change its collective aroma. When bacteria that release smelly compounds dominate, chronic bad breath may be one of the consequences.
Many current treatments do not improve oral ecology—in fact, they might make matters worse. Although some mouthwashes merely mask unpleasant odors, alcohol-based rinses sold in drugstores and prescription rinses containing chlorhexidine or other antiseptics target all oral bacteria, stinky and otherwise. Shi says that approach has several drawbacks. A chlorhexidine rinse, for example, may improve breath for as long as 24 hours but can temporarily change the taste of food. In one study, 25 percent of subjects experienced a tingling or burning sensation on the tongue after a week of use. Heavy use of rinses with alcohol can dry out the mouth, sometimes exacerbating bad breath. Further, wiping out too many of the mouth's native bacteria could disrupt the usual checks and balances, making way for opportunistic species responsible for gum disease and other infections to move in and take over.
A number of researchers are now working on promising alternatives to basically carpet bombing all oral bacteria. Some new mouthwashes go after the stink rather than the stinkers with ions of zinc or other metals that bind and neutralize sulfur compounds. Rosenberg, who started his career as a petroleum microbiologist, has developed a two-phase oil-and-water rinse that temporarily reduces bad breath by sopping up some of the oral debris and microbes that toothbrushing, flossing and tongue scraping miss.
Other teams are investigating whether probiotics rife with a gram-positive bacterial strain known as Streptococcus salivarius K12 can fight halitosis. A common resident of the mouth and respiratory tract, S. salivarius K12 is benign and known to produce substances that deter harmful bacteria. In a recent study by researchers in New Zealand and Australia, volunteers gargled with a chlorhexidine mouthwash to clear their palate of many native bacteria and subsequently sucked on lozenges laced with K12. Seven and 14 days later they had much sweeter breath. Presumably K12 outcompeted its foul-smelling kin, opening up niches for less offensive species.
At U.C.L.A., Shi and his team are working on a mouthwash that contains a peptide—a chain of amino acids smaller than a protein—tailored to selectively kill S. mutans, the ringleader behind tooth decay. Researchers could develop an analogous peptide to weed out the bacteria behind bad breath, Shi says. A rinse containing such peptides might free up real estate on the tongue for less malodorous microbes, if used in moderation. Rinsing every day risks a sudden and drastic shift in oral ecology that could have unexpected repercussions.
Shi himself brushes and flosses daily but does not use a mouthwash or even a tongue scraper because his family assures him that his breath smells fresh. “I'm one of the lucky ones,” he says. “My goal is to help other people be lucky, too.”