The Answer
Dogs are believed to have fewer species of bacteria in their mouths than humans do. That is offset by the fact that much of that bacteria may not be healthy for humans.
When you find yourself in the presence of a person who lets their dog lick their face (or mouth even), you’re likely to hear that it’s okay because a dog’s mouth is cleaner than a human’s.
Why do people say this?
It is generally believed that fewer species of bacteria are typically found in your pet’s mouth than in our own. This leads many to conclude that our pet’s mouths are “cleaner”.
If anyone knows what’s in a dog or cat’s mouth, it’s Floyd Dewhirst, a bacterial geneticist at the Forsyth Institute and professor of oral medicine at Harvard. … About 400 to 500 bacterial species are common and abundant in the human mouth, he says. So far, Dewhirst and his colleagues have identified around 400 kinds of oral bacteria in dogs and almost 200 in cats, and Dewhirst expects more will be found with further study.
So it’s true?
Not really. The problem with this assumption is that cleanliness is not dictated by a simple count of bacterial species. Some bacteria can be harmful, and others can benign, or even helpful. Humans are well adapted to the bacteria commonly found in our own mouths.
Dogs or other pets may have fewer species of bacteria, but most of that bacteria are species not commonly found in human mouths. We’re not adapted to them and they could make us unwell.
It gets worse
If you’ve been around dogs or cats for any significant amount of time, you’ve probably seen them with their noses and tongues in some pretty disgusting places (their own buttholes, other buttholes…). This can add parasites, fungi and other nasties to the mix – not to mention the fact that it’s just gross.
So, even if there might be fewer bacterial species, it’s a stretch to conclude that dog’s mouths are cleaner than ours.