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

The idea that a dog's mouth is inherently cleaner than a human's has a long and interesting history, likely stemming from observations of dogs instinctively licking their wounds (Review) and sometimes recovering without apparent infection. This ancient belief was even present in cultures like the Egyptians and Greeks, who incorporated dogs into their healing practices, believing their saliva possessed curative powers. People may have also noticed that the mechanical action of a dog's tongue could help remove debris from a wound, contributing to the perception of a "cleaner" effect.

However, scientific understanding reveals a different picture. Comparing the bacterial landscape of a dog's mouth to a human's is truly like comparing apples and oranges. Both species harbor a vast and diverse array of bacteria, with dogs having around 600 to 714 different types and humans possessing a similar number, approximately 615 to 700. While there's some overlap, many of these bacterial species are unique to each animal. For instance, specific bacteria found in dog mouths, such as *Capnocytophaga canimorsus* and *Pasteurella canis*, can be quite dangerous or even fatal if transmitted to humans through bites or open wounds.

People often continue to believe this myth because, in many everyday interactions, a dog's lick doesn't immediately result in illness. This is partly because most bacteria in a dog's mouth are not zoonotic, meaning they typically don't cause disease in humans. Additionally, dog saliva does contain some compounds with antibacterial properties that can fight off certain types of germs. Yet, these properties are limited, and a dog's habit of exploring its environment with its mouth—licking everything from the ground to other animals—means their mouths can harbor a wide range of pathogens, including those that can cause illnesses like salmonella. Ultimately, while a dog's lick might offer some minor benefits in ancient, unhygienic contexts, it is not a safe or effective method for wound care today.