: Why is saliva smelly? Why does saliva smell only out of the mouth, and not as long it is still inside? What role do oral bacteria play, and which ones?

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The bacteria that produce these foul smelling odors are anaerobic bacteria that live in the oxygen depleted film left on our teeth, tongue, and roof of our mouth if we don't brush.

These bacteria can produce chemicals that cause malodor including:

volatile sulphur compounds (VSCs), mainly methyl mercaptan, hydrogen sulphide, and dimethyl sulphide
diamines (putrescine and cadaverine) and
short chain fatty acids (butyric, valeric and propionic).

It smells more outside because these smelly chemicals are normally diluted by saliva in your mouth but become concentrated when the saliva evaporates.

Imagine your saliva as ocean water. Imagine the salt in the water is the smelly waste of bacteria, and your hand is like a dry shore. When the saliva evaporates on the hand, it leaves only the smelly salt (bacterial waste) behind.

References:

Book Chapter by Suzuki et al., 2012

Review by Scully and Felix, 2005