Oral malodor is a remarkably common condition that can affect socialization and self-perception.  Differing from food smells, this malodor is primarily the product of bacterially mediated oral putrefaction.  Common gram-negative anaerobic bacteria, primarily from the posterior dorsum of the tongue and interproximal sites, degrade proteins from saliva, crevicular fluid, and oral soft tissue.  The bacteria metabolites, especially volatile sulfur compounds (VSC) and to a lesser extent volatile amines, are characteristically odiferous.  The resulting malodor may be most evident overnight without cleansing food and drink, often called “morning breath.””
 
From: Compendium of Continuing Education in Dentistry, December 2007; 28(12):65