We hug them, kiss them, sleep with them, and share our food with them – they are, of course, the family dog. In return, they share with us some of the microbes they pick up as they saunter about the neighborhood and places beyond. A study conducted by researchers at the University of Colorado-Boulder reveals that families that own dogs harbor a more diverse and different set of skin bacteria than non-dog owning families. More importantly, the microbial exchange that takes place during the slobbering and paw-petting ritual of canine companionship reminds us of some time-honored interactions with the ancient and very microbial landscape that our western world and culture has walled off – and possibly not in a good way.
Following the observation that the gut microbiota of monozygotic twins (identical twins) are not any more similar than dizygotic twins (fraternal) – suggesting more than genetics determines the selection and source of the microbes on and in your body, the Colorado researchers wanted to explore how co-habiting with other family members and dogs shaped host microbiota. In other words, how does environmental exposure to other people (related and unrelated) and other species (dogs) shape your microbial ecology.
The researchers collected stool, oral, and skin (forehead, right and left palm) samples (swabs) of the humans and the same for the dogs, expect the all four paws of the dogs were sampled. In all, they studied 159 individuals (and 36 dogs):
Group 1: 17 families with children aged 6 to 18 yrs of age.
Group 2: 17 families with one or more dogs, but no children.
Group 3: 8 families with children and dogs.
Group 4: 18 families with no kids or dogs.
Not surprisingly, the researchers found that that family units share similar microbial communities at al sites sampled (stool, skin, oral). The skin was the most similar between family members – as family members often interact with the same objects and surfaces in a home. They also found that parents share similar oral (tongue) and gut communities with children aged 3-18 years in the home, but less so with children <3 years of age – a pattern seen in studies of other populations throughout the world – revealing the dynamic and still maturing nature of the infant oral and gut ecosystem.
They also found that cohabitating partners shared more microbes with one another than say the neighbors next door. They also found that on average, about 11% of the bacteria on someone’s hands (palm) was likely from oral sources, while less than 2% from fecal sources. In both cases, it demonstrates the movement of bacteria between various body sites – which are also ultimately shared with your partner.
They also found that the sharing of microbes between partners is also mimmiced with dogs. They found that dog owners share skin more skin bacteria (skin to paw) with their own dogs, then say with the neighbor’s dogs. But the similarities between human and dog were less pronounced in infants and seniors – possibly suggesting “behavioral differences between age groups.”
In an interesting twist, they found that couples that have a dog share more skin (palm and forehead) microbes between each other than do couples who do not have a dog. Conversely, people who live together but do not have a dog, look less similar to one another. In other words, the dog is the great equalizer and source of novel and rare microbes that are passed to their human companions. Side note: adult females have a greater diversity of bacteria on their hands than do males. Second side note: cats in the house (cats weren’t sampled) had no effect on the diversity of skin bacteria among adults. In addition, dog-owning adults seem to share more skin microbes with their dogs than they do with the children.
Unlike skin microbial communities, dog ownership had no effect on oral and gut communities in humans. Increasing family size had a significant effect on shared skin microbial communities – ie, bigger family, more homogenizing of the skin communities among everyone in the house.
In closing, the researchers note that much of the similarity between the skin microbiota of humans and their dogs can be explained by the oral-skin transfer – a.k.a. licking. In addition, a number of the taxa identified on the paws and forehead of the dogs are commonly associated with soil and water, which the dogs in turn track into the house and onto their owners. The paw and forehead of dogs also showed a greater diversity and evenness of bacteria, consistent with their frequent exposure to a great number of microbial sources.
All in all, the study reveals that our pets harbor a diverse microbial community that ultimately can influence our own microbial community structure and thus our health. The same goes for the people we cohabitate with as well – as their collective and shared exposure outside the home. It’s not much of a stretch to imagine that our overall broad exposure to the larger microbial world has decreased in our modern world. While some of this may be a good thing, we may be depriving our host-associated microbial ecosystems from some health-giving diversity once the norm throughout human evolution. In all things ecological, diversity is a good thing. If you want to channel your ancestors for some wisdom, you might start by hugging an animal or two and getting outside a little more often.
**You can join the American Gut Project to see what microbes you share with your dog – or kids – or your partner.