Breath's Chemical Code
Bad breath has, of course, plagued humans for ages. Young girls, Hippocrates advised, should regularly rinse their mouths with wine, anise and dill seed. By the early 1970s Joseph Tonzetich of the University of British Columbia had begun to tackle the problem with technology. He used his lab's gas chromatograph, a machine that separates a complex gaseous bouquet into its constituent compounds, to tease out reeking breath's signature chemicals.
Sulfur compounds that easily vaporize were among the stinkiest chemicals Tonzetich identified in bad breath, especially hydrogen sulfide, which smells like rotten eggs, and methyl mercaptan, which smells like rotten cabbage. Since then, scientists have detected around 150 molecular components of human exhalations, many of them putrid. Dimethyl sulfide (think rotten seaweed) and the tellingly named cadaverine, putrescine and skatole are just a few such pungent molecules. Still, hydrogen sulfide and methyl mercaptan stand out: in study after study, the higher the levels of these two molecules in breath, the more that breath offends the human nose.
These smelly compounds are waste products released by the millions of bacteria feasting on particles of food and tissue in our mouth. Above the gum line, gram-positive species, which have relatively simple cell walls, dominate dental plaque—the living film of bacteria coating teeth. Streptococcus mutans and other sugar-loving gram-positives spew acid and dissolve enamel but are not heavy producers of foul-smelling compounds. In contrast, gram-negative bacteria—which have an extra cell wall layer—live mostly below the gum line and are much gassier. Some of these resilient bacteria, including Porphyromonas gingivalis, Treponema denticola and Prevotella intermedia, thrive in gaps between the gum and tooth and in the mosh pit crevices of the tongue.