The saying “a dog’s mouth is cleaner than a human’s” is a familiar phrase, often used to dismiss concerns about canine kisses. This popular belief suggests dogs have superior oral hygiene, implying less risk from direct contact. However, a scientific understanding of both species’ microbial environments reveals a different reality.
Understanding the Common Belief
This belief often stems from observations and assumptions. People might note that dog saliva appears to aid in wound licking, or they may perceive human mouths as inherently less clean due to habits like smoking or eating diverse foods. It also gains traction because diseases aren’t always easily transmitted between pets and humans. While these observations contribute to the belief, they don’t fully capture the biological reality of oral ecosystems.
The Canine Oral Environment
A dog’s mouth is a complex ecosystem of bacteria, fungi, and viruses. This environment adapts to a dog’s diet and behaviors, such as sniffing, licking surfaces, and consuming items from the ground. The canine oral microbiome includes hundreds of bacterial species, typically 600 to 700 types.
Common bacterial genera in the canine oral cavity include Porphyromonas, Pasteurella, and Capnocytophaga. For example, Pasteurella canis is a common bacterium in a dog’s mouth and is frequently found in people who have been bitten by a dog. Capnocytophaga canimorsus is another bacterium present in healthy dog mouths that can cause serious infections in humans, particularly in immunocompromised individuals. While many are commensal, their presence reflects an oral environment adapted to canine-specific needs, not human standards of cleanliness.
The Human Oral Environment
The human oral cavity also hosts a diverse microbial community, the oral microbiome. This ecosystem comprises over 700 bacterial species, along with fungi and viruses, inhabiting teeth, gums, and the tongue. Its composition is influenced by diet, lifestyle, and oral hygiene practices.
Common bacterial genera include Streptococcus, Actinomyces, and Prevotella. Streptococcus mutans, for instance, is known for its role in dental caries, while other Streptococcus species contribute to dental plaque formation. The human oral microbiome adapts to dietary patterns, such as sugar consumption, which can influence bacterial balance and contribute to cavities and gum disease.
Comparing Microbial Worlds
Comparing dog and human oral environments reveals neither is inherently “cleaner”; each is uniquely suited to its host. Both species harbor hundreds of bacterial types, but most are different and adapted to their specific hosts. Studies indicate only 5% to 16.4% of oral bacterial species are shared between dogs and humans.
This host-specificity means dog mouth bacteria are generally not well-suited to colonize a human mouth, and vice versa. However, zoonotic bacteria can transfer between species, though the risk of transmission through casual contact like licking is low. Dog saliva can transmit bacteria like Pasteurella canis or Capnocytophaga canimorsus, especially through bite wounds. These instances are distinct from general perceptions of cleanliness. The distinct microbial compositions highlight that both human and canine mouths are optimized for their respective inhabitants’ physiology and lifestyle.