8th grader's award-winning experiment finally reveals if dog's mouths are cleaner than ours

Annie Reneau
Wed, 08 Oct 2025 17:45:48 +0000
Pets, Animals, Science, Dog mouths, Dog tongues, Animal research, Dog facts, Dog saliva, Should you let a dog lick your face, Funny, Humor, Dogs

They don't call them Man's Best Friend for nothing. With nearly half of the households in American having at least one dog, there's a lot people need to know about them. Our furry friends come in wide variety of breeds, each with their own unique traits and needs. "Man's best friend" can be a guardian, a helpful worker, a loyal friend, and a snuggly companion, but there's one thing almost all dogs have in common: Licking.

Some dogs lick way more than others, but it's rare to find a dog who never licks anyone or anything. Many dogs communicate and show affection by licking, which is sweet—if a little gross—depending on how slobbery they are. There's a common saying that dogs' mouths are cleaner than humans', which is a bit hard to believe when you see what some dogs put in their mouths, but it is true?

What does science say about dog tongues and saliva? Is a dog licking our face something we should worry about?

We've all heard the myths about dog mouths. Are they true? Photo by James Barker on Unsplash

It turns out, the answer to whether a dog's mouth is cleaner than ours isn't super straightforward.

An 8th grader named Abby tackled this question in a science experiment that won her a Young Naturalists Award from the American Museum of Natural History in 2011. Her family had gotten a dog and her mom kept telling her not to let the dog lick her face because dog mouths are full of bacteria. Instead of arguing, Abby decided to find out herself if this was true.

"I hypothesized that human tongues would be cleaner than dog tongues," she wrote. "I thought this because humans brush their teeth at least once a day. I hypothesized that dogs' tongues would be dirty because they were always licking dirty things like garbage."

After diving into the research about bacteria that live in and on humans and dogs, Abby decided she had a testable hypothesis. But this wasn't any old middle school science experiment. She applied for and got a grant to the State Hygienic Lab at the University of Iowa, where she was assigned a mentor to work with her.

You can read the nitty gritty details of her experiment here, but it involved a lab, swabs, agar plates, and more. Here was the conclusion:

"Will I continue to let my dog lick me? Yes!" Photo by Ashley Levinson on Unsplash

"I concluded that dog and human mouth flora are very different. (Flora means the bacteria found in a mouth or anywhere else.) The bacteria found in human mouths are more similar to another human's oral bacteria than the bacteria found in a dog's mouth.

"I also concluded that dogs' mouths are cleaner than humans' in some ways, and dirtier in other ways. Humans have more bacteria in their mouths than dogs do, based on the total number of bacteria. Most of the humans had a 'moderate' number of bacteria, and most of the dogs had 'few' bacteria. A possible explanation of this might be that dogs pant a lot, and maybe while panting, bacteria falls off their tongues along with their saliva. But dogs had more types of bacteria. The average number of different bacterial colonies in a dog's mouth was about 5.7. The average number of different bacterial colonies in a human's mouth was about 4.1. I think this is so because dogs sniff and lick a variety of things, like carpets, floors, chairs, grass, etc., so they pick up bacteria from many places."

But what about the licking of our faces? That's a bit of a subjective call, but Abby's results gave her some peace of mind:

"In conclusion, will I let my dog continue to lick me? The answer to the question is yes!" she wrote. "I will feel guiltless about letting my dog lick me because I found out that human and dog oral bacteria are different, so my dog's oral bacteria present no harm to me."

Kisses approved by science. Giphy

What do the experts say?

According to Colin Harvey, professor at the University of Pennsylvania’s School of Veterinary Medicine and executive secretary at the American Veterinary Dental College, comparing dogs' mouths to humans' mouth is "like comparing apples to oranges." As Abby found, the microbes in a dog's mouth are very different than those in a human's.

The American Kennel Club elaborates:

"Most of the bacteria in your dog’s mouth aren’t zoonotic, which means you probably won’t get a disease from a big old doggy kiss. There are exceptions to this. Dogs that eat a raw diet are at an increased risk of contracting salmonella, which can be spread to humans. You also probably shouldn’t share kisses with a dog that regularly raids the litter box.

In other words, kissing your dog is less risky than kissing another human, but that doesn’t mean that your dog’s mouth is necessarily cleaner than a human’s—they just have a mostly incompatible set of germs."

Not necessarily cleaner, but different. Photo by Marek Piwnicki on Unsplash

Keeping your dog's mouth healthy through regular teeth cleaning and dental check-ups can also help prevent issues that could potentially come from dog licks.

So there you have it. If your dog doesn't eat a raw diet and doesn't go snacking in the cat box (or some other equally fecal-bacteria-ridden place), their kisses are probably not going to hurt you. Guilt-free pooch smooches for the win!

This article originally appeared in April. It has been updated.