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

It's a widespread notion that a dog's mouth is somehow purer than a human's, a belief perhaps stemming from observations of animals licking their wounds (Review), which sometimes appear to heal without issue. This idea might also be romanticized by the close bond many share with their canine companions, leading to a perception of their pets' inherent cleanliness.

Scientifically, this popular belief doesn't hold up. Both human and dog mouths are teeming with bacteria, each species hosting a unique and diverse oral microbiome. While there might be some overlap in certain bacterial strains, the vast majority are specific to either canines or humans. A dog's mouth, for instance, often contains bacteria like Pasteurella and Capnocytophaga, which are not typically found in the human oral cavity, just as humans have their own distinct microbial residents.

The misconception often arises from the perceived healing properties of a dog's lick. While licking can remove dirt or debris from a superficial wound, and saliva does contain some enzymes, it doesn't possess inherent antiseptic qualities that make it "cleaner." In fact, introducing foreign bacteria, even from a beloved pet, to an open wound can introduce new pathogens and potentially lead to infection, rather than preventing it.

Ultimately, "cleaner" is a misleading term when discussing the complex microbial ecosystems of any mouth. Neither species' mouth is sterile or inherently superior; they simply house different communities of microorganisms. The reality is that both human and dog mouths can harbor and transmit various pathogens, making the idea of one being universally "cleaner" than the other a myth.