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

The notion that a dog's mouth is inherently cleaner than a human's is a widespread misconception, likely stemming from ancient beliefs and observations of dogs licking their wounds to aid healing. For centuries, folklore, including tales from ancient Greece, suggested canine saliva possessed curative powers. This idea is further reinforced by the fact that many diseases are not easily transmitted between humans and dogs, leading some to believe their mouths must be purer.

However, scientific evidence reveals a different story. Both human and dog mouths are teeming with hundreds of diverse bacterial species, forming distinct ecosystems. While some bacterial families, like Porphyromonas, can cause periodontal disease in both species, the vast majority of microbes are unique to each. For instance, dogs typically harbor around 600 to 714 types of bacteria, while humans have approximately 615 to 700. Only about 15 percent of these oral microbiomes are shared, meaning a dog's mouth contains many bacteria unfamiliar to the human immune system.

Therefore, comparing the two as "cleaner" or "dirtier" is not meaningful. A dog's mouth contains bacteria such as *Capnocytophaga canimorsus*, which is harmless to dogs but can cause severe, even fatal, infections in humans if introduced through a bite or an open wound. Additionally, dogs explore their environment with their mouths, exposing them to pathogens like *Salmonella* or *E. coli* from sources humans typically avoid. While dog saliva does contain some antibacterial properties, this doesn't negate the presence of potentially harmful bacteria.