Are Dogs' Tongues Really Cleaner Than Humans'?

Part of the Young Naturalist Awards Curriculum Collection.

by Abby, Grade 8, Iowa, 2011 YNA Winner


Abby and her dog, Lucy

"Abby! Don't let the dog lick you! Her tongue is full of bacteria!" Ever since we got a dog a few years ago, my mom has always been saying that. I love when my dog runs to greet me when I get home from school and says "hello" by giving me licks. What I hated, though, was feeling guilty and disgusted about having her doggy bacteria all over my face. So I decided to do a test to see how dirty dogs' tongues really are.


I hypothesized that human tongues would be cleaner than dog tongues. 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.

Background Research

I read about bacteria on a number of different websites, including the University of California's Museum of Paleontology and Wikipedia. I also read an article from the Journal of the American Academy of Dermatology called "Dog, Cat, and Human Bites: A Review." Lastly, I read an article from the Journal of Clinical Microbiologycalled "Cultivable Oral Microbiota of Domestic Dogs."

Dog 2

What I learned from my bacteriology studies is that bacteria were first called "animalcules" by Antonie van Leeuwenhoek when he discovered them in 1676. Bacteria are single-celled organisms without a nucleus. Because they are so small, bacteria can only be observed with a microscope. These organisms are only a few micrometers long (a micrometer is 1/1000th of a millimeter). There are bacteria covering your skin, inside your body and many other places. Only about 1 percent of bacteria are harmful. Most bacteria are neither harmful nor helpful. For example, useful bacteria cover the inside surface of some of your organs, preventing harmful bacteria from infecting your organs. A lot of neutral bacteria are found in your digestive system, to prevent infection. Many bacteria help your immune system. However, some bacteria cause disease; many infectious diseases are spread by the sharing of saliva infected by harmful bacteria.

Some studies have found differences in the types of infections that happen after being bitten by a human or an animal. But this didn't tell me which bacteria are normally present. Being bitten in the hand by a human will cause infection for a human. Yet only 1 percent of emergency room visits are for infectious bites from dogs. This made me wonder if humans and dogs have significant differences in the type and amount of bacteria in their mouths.

Project Description

A graphic depicting how scientists describe the amount of bacteria on a plate.
How scientists describe the amount of bacteria on a plate.

1. Finding the lab: After deciding that I had a testable hypothesis, I had to find a lab in which to carry out my experiment. I wrote a grant request to the State Hygienic Lab at the University of Iowa explaining what I wanted to do there and what I was trying to find out. My grant was accepted, and I was assigned a mentor to work with me at the facility.

2. Materials:

    • 12 Fisher's Finest swabs
    • 12 gram-negative agar plates
    • 12 blood agar plates
    • Disposable loops
    • Safety equipment: gloves, goggles, lab coat

3. Variables, Controls and Sample Size: The only variable I was testing was the difference between species. The control was the human saliva. I had five human samples and seven dog samples because of the limited materials the lab offered me.

Abby and her mentor, Gabriella Gerken

4. Getting the samples: On June 20-21, 2010, I went around to five of my neighbors who are dog owners and obtained saliva samples. I had the owners of the dogs test their dogs and themselves with separate swabs. I asked the subjects to not brush their teeth or eat prior to the sampling to make sure the tests weren't compromised. This assured that the only variable being tested was the difference between dogs' and humans' mouths. I used Fishers' Finest swabs to swab their tongues. I put the swabs in the refrigerator until I went to plate them on June 21, 2010. I made sure to keep the subjects anonymous by labeling them Human 1 to 5 or Dog 1 to 5.

5. Plating the bacteria: On June 21, 2010, I went to the State Hygienic Lab at the University of Iowa to plate the bacteria on agar. I used blood agar plates that show both gram-negative and gram-positive bacteria, and gram-negative agar plates that have gram-positive antibiotics in them so that only gram-negative bacteria can grow.

The gram stain of bacteria is a way to classify bacteria based their cell walls. To do the gram stain, you put a sample of the bacteria on a plate, stain it purple with a chemical, de-stain it with alcohol, and then stain it with a pink chemical. If the bacteria stain purple, they are gram-positive, which means that their cell walls absorbed the purple chemical. If the bacteria are pink, they are gram-negative, which means that their cell walls didn't take up the purple chemical but did take up the pink. Gram-positive cells are usually found in the mouth or on the skin, and gram-negative cells are usually found in the intestines.

The light pink Petri dishes are the gram-negative plates that have a gram-positive antibody so only the gram-negative bacteria show on the plate.

I first swabbed a fourth of the agar plate, and then I used a loop to spread the bacteria around so that when the bacteria grew, I could get a better idea of their quantity. To plate bacteria, you first streak a fourth of the plate, and then you take some of the bacteria you placed on the plate and rub it along the next side of the plate using an instrument called a loop. Then you disinfect the loop and take some of the bacteria from the first time you used the loop and rub it around on the next quadrant. Finally, you disinfect the loop again and take some of the bacteria from the second time you used the loop and rub it around the rest of the untouched part of the plate (see diagram).

This red Petri dish is an example of a blood agar plate. This means that both gram negative and gram-positive bacteria can grow.

I learned from my mentor the way scientists describe the amount of bacteria on the plates. She taught me that if, after a few days, there were only bacteria cultures in the original place you streaked with the swab, you would say there is a "rare" amount of bacteria on the plate. If there are bacteria cultures on the first side you used the loop on, you would describe the bacteria amount as "few." If there were bacteria cultures on the part of the plate where you used the loop for the second time, you would describe the bacteria amount as "moderate." If there are bacteria cultures on the last place you used the loop, you would describe the bacteria amount as "many."

6. Growing the bacteria: After the cultures were kept in an incubator at 35°C for 24 hours, I observed their growth.

Petri dishes showing human bacteria growth

7. Analyzing the plates: The next day I went back to the lab and analyzed the plates. I described the amount of bacteria using the method described above. I also determined how many bacterial colonies there were by their different shapes, sizes and colors. All observations were recorded in my lab notebook. When I was finished analyzing the plates, they were autoclaved and disposed of by the Hygienic Lab.

Data Table
A spreadsheet with six columns with values related to bacteria quantities in humans and dogs.
Averaged Data
Average Number of Different Bacterial Colonies

The dog Petri dish samples have many different bacteria as can be seen from the different colors and shapes of the bacteria colonies. Most of these plates would be described as having rare or few bacteria colonies because of the position of all the colonies.

The human Petri dish samples mostly have the same brown flat bacteria or white bacteria, unlike the very diverse dog samples. Most of these samples would range from having "few" to "moderate" bacteria levels. These are blood agar plates that allow both gram-negative and gram-positive bacteria to grow.

I recorded the quantity of bacteria on each plate, and I tried to estimate how many different bacterial colonies were on each plate. (I had to estimate because it is nearly impossible to find the exact number.) When I got home, I averaged the data to better understand it. I first found the average quantity of dog and human total bacteria, gram-positive bacteria and gram-negative bacteria. To do this, I assigned a rating scale in which rare = 1, few = 2, moderate = 3, and many = 4. I averaged the results and recorded them in the data table.


I found the average total for human bacteria to be 3, which is a rating of "moderate." The average total for dog bacteria was 2.7 (between "few" and "moderate"). The average total of gram-negative dog bacteria was 0.9, which is "rare." The average total of gram-negative human bacteria was 0.2, which would be considered "very rare." The average total of gram-positive dog bacteria was 1.9, which is close to "few." The average total of gram-positive bacteria for humans was 2.8, which is almost "moderate." (These numbers will be interpreted later in my conclusion.)

The average number of gram-positive bacteria in a human's mouth was 4.2 colonies. Human mouths had more gram-positive bacteria (the type of bacteria usually found in mouths or on skin) than dogs do. Yet human mouths had fewer gram-negative bacteria (this type of bacteria is mostly in intestines) than dogs do? This makes sense, because dogs probably get gram-negative bacteria from sniffing dog waste or dogs' behinds. Humans probably have more gram-positive bacteria because of the environment in their mouths.

I also averaged the quality of the plates, or how many different kinds of bacteria were found in each plate (see graph). The average total for dogs was 5.6 different bacterial colonies. The average total for humans was 4.1 different kinds of bacterial colonies. The average number of gram-negative bacteria in a dog's mouth was 2 colonies. The average number of gram-negative bacteria in a human's mouth was 0.2 colonies. (One person out of five had one colony of gram-negative bacteria.) The average number of gram-positive bacteria in a dog's mouth was 3.7 colonies. (These numbers will also be interpreted in my conclusion.)

The graph shows that human mouths have more gram-positive bacteria yet fewer gram-negative bacteria than dogs do (as shown in the Average Data table). Dogs have a greater total of bacterial colonies. This might be because dogs gather many different types of gram-negative bacteria from sniffing other dogs' waste and bottoms. Dogs also pick up bacteria from the things they sniff and lick. Although humans brush their teeth a lot and clean their mouths often, there are still a lot of bacteria in their mouths. This is probably because there are many harmless bacteria in the world that live naturally in the human mouth.


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.

Dogs have more gram-negative bacteria in their mouths. I think this is because they often sniff and lick things on the ground. Since gram-negative bacteria are usually only found in the intestines, maybe dogs get the bacteria in their mouths because they sniff other dogs' bottoms. When they do that, they may get some gram-negative bacteria on their nose that used to be in the other dog's intestine; then later the dog will lick its nose, getting the bacteria in its mouth. Humans have more gram-positive bacteria in their mouths. I think this is so because the human mouth provides an environment that suits gram-positive bacteria.

In conclusion, will I let my dog continue to lick me? The answer to the question is yes! 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.

Further Research

I would like to research what types of gram-negative bacteria are found in dogs' mouths and where they came from to see if my hypotheses are true. I would also like to see if brushing dogs' teeth daily would help prevent harmful bacteria from growing in their mouths. Conducting this experiment on a larger scale with more subjects will help to support my conclusion. It is known that data is more reliable when there are more subjects being tested.


I would like to thank my mom for encouraging me to write a grant to work at the State Hygienic Laboratory at the University of Iowa. I would like to thank Gabriella Gerken for being a great mentor, and for teaching me so much about plating bacteria and understanding the gram stain. I would like to thank my dad for helping me to understand more about the world of bacteria.

I am so grateful to have been able to experience working in a university lab at the age of 13. I have learned so much working with the agar plates and bacteria. I will be perfectly happy if I don't win, because the experience was incredible and very educational, and I am so happy I get to share my discoveries with the world.


"Bacteria." University of California Museum of Paleontology. 29 November 2009. Retrieved from

"Bacteria." Wikipedia. 29 November 2009. Retrieved from

Brook, I. "Microbiology and Management of Human and Animal Bite Wound Infections." Primary Care 1 (30 March 2003): 25-39.

Elliott, D.R., M. Wilson, C.M. Buckley, and D.A. Spratt. "Cultivable Oral Microbiota of Domestic Dogs." Journal of Clinical Microbiology 43 (November 2005): 5470-5476.

Griego, R.D., T. Rosen, I.F Or, and J.E. Wolf. "Dog, Cat and Human Bites: A Review." Journal of the American Academy of Dermatology 33 (December 1995): 1019-1029.