If you brush, you floss, and your breath still stinks like a steam vent, it could be from last night's garlic-onion-anchovy pizza or, perhaps, from a medical problem.
His bad breath isn't entirely his fault.
Or maybe, new research suggests, you happen to be colonized with an especially foul-smelling mix of mouth bacteria.
Yoshihisa Yamashita and colleagues at Kyushu University in Fukuoka, Japan, studied spit samples from 240 healthy, clean-living people with chronically bad breath.
Like beachcombers taking the census of a tide pool, the microbiologists counted and categorized the critters they found in each mouth — noting different strains of bacteria instead of starfish.
The problem with these healthy people with chronically bad breath wasn't infection, the researchers found. It was the particular mix of indigenous bacteria entrenched in each mouth. The people with the worst breath, they found, were colonized with a mix dominated by harmless but particularly stinky bacterial strains known to emit lots of volatile sulphur compounds. Think rotten eggs.
The results appear in the May issue of Applied and Environmental Microbiology.
Some day, the scientists say, it may be possible to tweak mouth ecology so that you can nudge your own native mix in a way that will help fresher-smelling critters dominate. Until then, limiting the stinkers' food supply may be the best approach: Keep brushing and flossing.