Dog mouth microbiome: Is a dog's mouth cleaner than a human's?

Like Mattina, childhood memories growing up in New England during the hot summer months often center around ice cream. With a bunch of cousins the same age as me, I had plenty of family time with them during the summer. However, getting ice cream with my cousins was always something I did not look forward to. Why? My cousin and his dog would always share the same ice cream cone. Even before becoming a microbiologist, this scenario has always bothered me.

A common sight for me growing up. While dog's can certainly have their own cone, my cousins often took turns eating the same cone with their dog. Image credit: Chuck Cutler.

A common sight for me growing up. While dog's can certainly have their own cone, my cousins often took turns eating the same cone with their dog. Image credit: Chuck Cutler.

As a dog person, I am used to treating my pets like family. I am also familiar with receiving friendly licks from my Collie, Chloe. In fact, I often awake each morning getting licked in the face leaving my alarm clock collecting dust. Family or not, though, I believe there should be some boundaries. Boundaries which involve not exchanging licks of the same food with my dog.

'But, my dog's mouth is cleaner than yours!'

Apparently my cousin was not to only one that has used this line to explain their potentially strange behavior to on-lookers. A quick Google search reveals that this statement represents an urban myth. But, is there any truth to this claim? Could a dog's mouth be cleaner than our own despite all the time and energy we spend maintaining our teeth and visiting the dentist? 

According to Discovery Channel's MythBuster's in 2005, a dog's mouth could be cleaner than human's but the results were not conclusive. Let's start by covering some reasons people believe this myth is true.

Discovery Channel's MythBusters explored the idea of microbes with the five-second rule and whether or not a dog's mouth was cleaner back in 2005. We have made some progress since then. Image credit: store.discovery.com.

Discovery Channel's MythBusters explored the idea of microbes with the five-second rule and whether or not a dog's mouth was cleaner back in 2005. We have made some progress since then. Image credit: store.discovery.com.

Dogs are not as susceptible to tooth decay and cavities.

We often brush our teeth multiple times a day with fancy brushes specifically designed to vaporize dental plague – or biofilms – off of our teeth. We even visit specialists that professionally clean our teeth. Despite all these effort, we still get a lot of cavities. Why do dogs and other animals capable of resisting cavities without all this extra effort?

Could we just have dirty mouths?

In reality, this level of occurrence might depend on the high amount of sugar in our diets. The culprit behind cavities is a bacterium called Streptococcus mutans and S. mutans loves sugar. The byproduct of all this sugar consumption is acid, which is what causes tooth decay. With less sugar in their diet's, dogs do not have a lot of S. mutans around or at least not producing as much acid.

Dog saliva has special antibiotic capabilities to keep the mouth clean.

Although this statement seems a bit far-fetched, a few observations can easily lead to this explanation. First, an injured dog will lick their wounds and often these wounds heal very quickly thanks to all this attention. Second, dogs seem to be able to eat anything from dirty water to poop without getting sick. Is there something about a dog's mouth that allows them to handle these tasks?

Probably not. These miraculous feats can simply be explained other ways. For example, licking helps wounds heal faster since they remove dirt and debris to allow healing without the need to be antibiotic. Plenty of dogs also have sensitive stomachs like us as well. Including lactose intolerance, which you should keep an eye open for if you plan to share ice cream with your dog

A dog's mouth is just plain different from a human's.

Especially from a microbial perspective. Although some of the same microbes are present in both the mouths of dogs and humans, experts say there are large differences in the microbes that are present or microbiome. Once again, the difference in the occurrence of cavities demonstrates this fact since S. mutans is more specific to humans. Other microbes that cause gum disease in humans are also not seen in dogs.

With such powerful sniffers, why do they have to be so close?! Regardless, would you want to exchange licks of an ice cream with these dogs after this scene? Sure, you will probably be fine but I still do not like it. Image credit: Tim Dorr with Butting Sniffing Ying Yang.

With such powerful sniffers, why do they have to be so close?! Regardless, would you want to exchange licks of an ice cream with these dogs after this scene? Sure, you will probably be fine but I still do not like it. Image credit: Tim Dorr with Butting Sniffing Ying Yang.

In summary, a dog's mouth is a very different environment and, by extension, contains a vastly different collection of microbes. Since the microbes occupying a dog's mouth are less likely to moon-light as human pathogens, you might actually be more at risk sharing your ice cream with another human rather than a dog. On the other hand, a dog is also much more likely to stick their face into garbage, poop, and other places they do not belong that may contain a variety of nasty, disease-causing microbes specific to humans. As a result, my recommendation is not to share your ice cream with anyone,  even your dog. Just buy him or her their own cone!

I hope this post did not make you lose your appetite and remember, stay hungry!