Dogs' mouths are not cleaner than human mouths. Both dog and human mouths contain billions of bacteria with roughly around 600-700 bacterial species each, but these species differ between dogs and humans. Although some bacteria families like Porphyromonas, which cause periodontal disease, affect both, dogs have a different bacterial makeup and generally a greater variety of bacteria. The idea that a dog's mouth is cleaner is a myth as bacteria in both mouths can cause dental and health issues. Thus, neither is necessarily cleaner; they just harbor different bacterial populations.