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

The notion that a dog's mouth is somehow cleaner than a human's is a widely circulated misconception. This enduring myth likely originated from observations of dogs licking their wounds (Review), which often appear to heal without significant infection. Additionally, the idea might be bolstered by the fact that many common illnesses are not easily transmitted between humans and canines, leading people to infer a general state of superior oral hygiene in dogs. Historically, the belief in the healing properties of animal saliva, including that of dogs, dates back centuries, even to ancient Greece, which further cemented this popular, yet incorrect, understanding.

However, scientific evidence clearly busts this myth. While both human and dog mouths are teeming with bacteria, they possess distinctly different microbial ecosystems. Studies show that a dog's mouth can harbor around 600 to 714 different types of bacteria, while a human mouth contains a comparable number, approximately 615 to 700 species. Crucially, only about 15 to 16.4 percent of these bacterial species are shared between the two. Dogs, in their natural behavior, explore their environment by sniffing and licking a variety of objects, including waste, which introduces a diverse range of bacteria, some of which are not typically found in human mouths. For example, specific bacteria common in dog saliva, such as *Capnocytophaga canimorsus*, can be dangerous or even fatal to humans if transmitted through a bite or if it enters an open wound.

People often continue to believe this myth because dog saliva does contain certain antibacterial properties and proteins, like histatins, which can help clean minor wounds and prevent *some* infections. The act of licking itself can also help remove dirt and debris. However, these properties do not equate to overall "cleanliness" or mean that a dog's mouth is free from potentially harmful pathogens. The unique bacterial compositions of each species mean that what might be harmless to a dog could pose a significant health risk to a human. Therefore, rather than being cleaner, dog and human mouths simply house different communities of microorganisms, each with their own set of potential risks and benefits.