It depends where the different mouths have been. Dogs have about 600 different kinds of bacteria in their mouths than 615 or so in a human's. There is only some overlap in the bacteria that both dogs and humans share.Most of the bacteria in your dog's mouth are not zoonotic, which means you probably won't get a disease from a big old doggy kiss. Kissing your dog is less risky than kissing another human, but that does not mean that your dog's mouth is necessarily cleaner than a human's — he has a mostly incompatible set of germs.Information from the American Kennel Club.