A low production of saliva causes bad breath or halitosis due to stagnation of food and of the saliva itself. Stagnant saliva undergoes a process of putrefaction and increases the pH of the mouth. With a pH equal to or greater than 7.2, the growth of certain gram-negative bacteria ( Bacteroides melanninogenicus , Fusobacteriym nucleatum and Klebsiella pneumoniae ) increases, which are capable of breaking down protein or fatty foods, as well as the cells themselves and the blood in the mouth, producing compounds derived from sulfur that are volatile and that confer the unpleasant bad smell to the breath: hydrogen sulfide and methyl mercaptan (smell of rotten eggs) .
The bacteria Veillonella alcalescens and Prevotella melaninogenica also produce components of bad odor.
30% of the population complains of halitosis, especially in the morning.
