Did you know that the bacteria which cause bad breath feed off protein? This means that if you have excess mucus or a post-nasal drip the mucus coats the back of the tongue and throat, where these bacteria live, and can cause bad breath. The same thing happens when someone has a cold or a sore throat. Post-nasal drip is one of the main causes of bad breath in children. If you have bleeding gums, the blood is also protein so can also cause bad breath.