While many people believe dogs' mouths are cleaner than human beings' mouths, the two are not directly comparable, making the common perception impossible to confirm. Dogs' mouths are host to just as many germs as humans' mouths, albeit often different types of germs, according to LiveScience.
It is true that some types of bacteria are found in both canine and human mouths. However, in general, a dog's dental bacteria are so different from that of a human's that there is little in common. The same is true of bacteria that produce tooth and gum disease. Dogs' mouths are more vulnerable to certain types of disease-forming bacteria, while humans' mouths are vulnerable to others.