If this were the case, I’m pretty sure a dog’s mouth would be some sort of medical miracle, considering: they rarely if ever have their teeth brushed or mouths cleaned; they literally put their mouth on everything; and they use their tongue as toilet paper for not only themselves, but also any other dogs they encounter. Mouths are a great environment for bacteria to thrive; so barring some innate very strong anti-bacterial agent in their saliva, this myth just wasn’t going to be true. There are also numerous types of diseases associated with dog bites, including pasteurella, tetanus, rabies, klebsiella, proteus, E. coli, eikenella, etc, which point to the fact that a dog’s mouth couldn’t be sterile, as people often claim.
Don’t just take my word for it though. According to a study on the relative amount of microbes in a human’s mouth vs. a dog’s mouth, dog’s mouths had significantly more bacterial colonies than a human’s mouth. In this study, they measured the number of bacterial colonies per square centimeter in a dog’s mouth vs. a human’s mouth. The samples were taken after both had eaten and without the humans being allowed to brush their teeth before the sampling. The results were scaled down to a 1-4 scale where 1 is the cleanest, with relatively few bacterial colonies (“relative” being the operative word as both human and dog’s mouths are teaming with microbes), and 4 being the grossest, meaning relatively high amounts of different bacterial colonies. According to the study, 10% of the dogs scored a 1 (relatively clean); 30% scored a 3; and the remaining 60% scored a 4 (grossest). With the humans, 70% scored a 1; 20% scored a 2; and 10% scored a 3.
This small study backs up the intuitive notion that if you are someone who goes around eating garbage, poop, and pretty much putting anything and everything you find in your mouth, all the while typically never having your mouth cleaned in any way, then your mouth is going to have quite a lot more types of bacteria in it than someone who brushes their teeth regularly and doesn’t go around eating poop and other such microbial gold mines.
So where did this myth come from? It’s likely that it started from people observing that dogs clean their wounds with their mouths and it is scientifically proven to have less chance of getting infected than if they didn’t lick it. The problem with this is that any wound that is cleaned regularly, by licking or simply running water over it frequently, will give you much less chance of infection than otherwise. There is no strong anti-bacterial agent working here, it is simply a case of cleaning off dead tissue and the like.
Another theory of this myth’s origin is the medical myth that if you are bitten by a human, you’ll have a much greater chance of having that bite become infected than if you are bitten by a dog. This “fact” has been spread around in antiquated medical journals for ages, but has recently been shown to be incorrect. As stated in the 1988 Annals of Emergency Medicine: