The bacteria which cause bad breath and sour/bitter/metallic tastes are anaerobic sulfur-producing. Their goal in life is to break down the proteins in foods that we eat. However, under certain conditions, they will start to break down proteins in mucous and phlegm.
Therefore, the people who suffer with post nasal drip, sinus problems, and other similar issues are more prone to bad breath and awful tastes because the bacteria will start to extract sulfur compounds from the amino acids that make up these proteins.
Scientifically, they feed upon the amino acids cysteine and methionine, which are common to mucous/phlegm and dairy foods. In fact, many people notice that when they drink too much milk or eat too much cheese that they end up with more mucous or phlegm in their throat. This is a natural reaction for many people and unfortunately, ends up causing more bad breath and lousy tastes.
If you still have your tonsils, you may be harboring a higher number of the bacteria which can lead to a misunderstood phenomenon called tonsilloliths. They are stones in the tonsils that are produced by the conglomeration of mucous draining down the back of the throat and volatile sulfur compounds produced by the bacteria which easily end up in the “nooks and crannies” of the tonsils, every time one swallows.