Dogs sometimes chew their own faeces, frequently insert their snouts into each others' bottoms, and always use their tongues as personal hygeine tools.

Also, they never brush their teeth.

Let's face it... dogs are filthy. Letting a dog lick your mouth is one of the most disgusting things a person can do.

However, as Yams has quoted, the bacteria in human mouths are human-specific. Dogs carry dog-germs, people carry people-germs.

So basically... if your dog hasn't been munching on poo or filth or rotting meat, and hasn't been cleansing his groin with his tongue recently, then just maybe his saliva won't be quite so fetid.

However, since most dogs do all those unspeakable things on a regular basis, I'd say you're better off getting bitten by a human than by a dog.