“A dog's mouth is cleaner than a human's mouth.”

It's a widely held belief that a dog's mouth is somehow purer than a human's, perhaps stemming from observations of dogs licking their wounds (Review) or the common phrase "licking it clean." This notion often implies that canine saliva possesses antiseptic qualities, making their mouths a sterile environment compared to our own. However, this popular idea is a significant misunderstanding of microbiology.

Scientifically, both human and dog mouths are teeming with a vast array of bacteria, each species hosting its own unique microbial communities. While some bacteria are specific to dogs and others to humans, neither mouth is inherently cleaner. In fact, a dog's mouth can harbor bacteria like Pasteurella, Capnocytophaga, and even Salmonella, which can be harmful or even dangerous to humans, especially if transmitted through a bite or deep lick. The idea that dog saliva has potent healing properties for humans is largely unfounded, and any perceived benefits are often outweighed by the risk of infection.

The persistence of this myth likely comes from a combination of factors. Our affection for our canine companions often leads us to romanticize aspects of their biology. Additionally, some people might compare a dog's mouth, which doesn't typically suffer from cavities in the same way humans do, to a human mouth that might have poor oral hygiene. However, the absence of certain dental issues in dogs doesn't equate to a sterile oral environment. It's crucial to remember that "clean" is a relative term, and from a health perspective, a dog's mouth poses just as many, if not more, risks for cross-species bacterial transmission as a human's.