The idea that a dog’s mouth possesses some kind of purifying, self-cleaning property superior to a human’s is a long-standing cultural belief. This common myth is often invoked to dismiss concerns about sharing saliva, but it overlooks the complex microbial ecology of the oral cavity. Examining the scientific metrics and biological composition of the oral microbiomes of both species reveals that one mouth is not inherently “cleaner” than the other.
What Science Means by Clean
In a biological context, “clean” does not equate to “sterile,” as all healthy organisms host diverse communities of microorganisms. Cleanliness is defined by actions and practices that reduce the spread or transmission of pathogenic microorganisms, limiting the incidence of disease. The scientific metric for overall microbial presence is the total microbial load, which refers to the sheer number of bacteria present. Both human and canine mouths contain billions of bacteria, often belonging to hundreds of different species.
A nuanced scientific understanding differentiates between commensal and pathogenic bacteria. Commensal organisms coexist peacefully with the host, sometimes providing benefits, while pathogenic bacteria can cause illness. Hygiene is not about eliminating all microbes, but about managing the relative risk posed by the microbial community. Saliva contains antimicrobial components that help regulate the oral microbiome, but it does not sterilize the environment.
Comparing the Microbial Populations
The most significant difference between a dog’s mouth and a human’s mouth lies not in the total number of bacteria, but in the specific composition of the species present. Both species harbor a similar number of distinct bacterial species; dogs have around 600, and humans have a comparable number. Although the diversity is similar, the specific types of bacteria are adapted to their respective hosts. Comparing the two microbiomes is often described as comparing apples and oranges because of this host-specificity.
The danger of a dog’s mouth to a human is primarily due to the presence of zoonotic bacteria, which are organisms adapted to a canine host but can cause serious infection when introduced into a human. For instance, Pasteurella canis is common in a dog’s mouth and is frequently isolated in human dog-bite wounds. Capnocytophaga canimorsus is typically harmless to dogs but can cause severe, sometimes life-threatening, systemic infection in humans, particularly those who are immunocompromised.
While some bacterial families overlap, the specific species differ. For example, the Porphyromonas family causes periodontal disease in both species, but the specific organism is Porphyromonas gingivalis in humans and Porphyromonas gulae in dogs. This highlights that a dog’s mouth is not inherently dirtier, but its microbial residents are incompatible with human biology. The risk is less about the count of bacteria and more about the presence of foreign organisms the human immune system is not equipped to handle.
Designing the Scientific Investigation
A scientific investigation into this question would focus on comparing the microbial load and diversity from both human and canine oral samples. Any project involving biological samples, especially those from animals, must be reviewed and approved by a qualified adult, such as a teacher or an Institutional Review Board, to ensure safety and ethical handling. Proper sampling requires using sterile cotton swabs to collect samples from the inner cheek or gums of both human and dog subjects.
The collected samples are transferred to sterile saline solution and diluted so the bacteria are spread thinly enough for individual colonies to grow. These diluted samples are spread onto agar plates, a nutrient-rich gel that provides a food source for bacteria. A negative control plate (left open briefly but not swabbed) and a positive control plate (inoculated with a known source of bacteria) should be included to validate the experimental results.
The plates are incubated at a warm, consistent temperature, typically around 37 degrees Celsius, for 24 to 48 hours to allow the bacteria to multiply. After incubation, the scientist counts the individual spots, known as Colony Forming Units (CFUs), on each plate. The CFU count provides a quantitative measure of the total microbial load, allowing for a direct comparison of viable bacteria. Differences in colony color, size, and shape can also provide a rudimentary assessment of microbial diversity.