Skin health and its links to the microbiome

It may seem odd to some to link the gastrointestinal microbiome with the skin, but these two parts of the body are actually very closely linked, with the skin and the gut joined to each other, at the mouth at one end and the anus at the other. At these junctions where the skin and gut meet, the bacterial populations can mix, a process boosted by our dog’s habit of frequently all-over body licking and grooming. This means that the microbial populations in our dog’s guts and on their skin are very closely linked, and any imbalance in the gastrointestinal microbiome has the potential to lead to an imbalance in the microbiome of the skin as well. 

As well as being affected directly by changes in the gastrointestinal microbiome, the skin microbiome can be affected by external factors. In fact, the skin is so good at absorbing things, that many health advocates stick by the phrase “if you wouldn’t put it in your mouth, don’t put it on your skin”. The skin is exposed to a lot more external factors than the gut is, such as pollution or toxins, pesticides or insecticides (either applied directly or through environmental contact), antibiotics or steroids, dirt, water, chemicals, etc. All of these can have a huge impact on the bacterial populations of the skin, which can either help improve diversity of the skin biome, in the case of soil, or make it worse, in the case of antibiotics and other chemicals. The skin microbiome will also change in response to both infectious and non-infectious disease. 

And it’s not just general external factors that can influence our dog’s microbes - studies have shown that dogs are more likely to have a similar oral, faecal and skin microbiome to their owners than to other people, with the skin microbiomes being the most similar due to prolonged contact between owner and dog. In humans, this exchange of microbes can be beneficial - for example, evidence shows that children who grow up with pets have a much lower risk of allergies than those who don’t. The theory behind this is that our immune education is improved by exposure to animal microbes during childhood[1]. However, this shared microbiome also means that if you are using a topical antibiotic or steroid cream, or using synthetic chemicals on your skin, then they may be affecting your dog's microbiome as well, and vice versa. So, if one of you has a poorly balanced microbiome, then it could be a problem for you both. 

What does this mean for skin disease? When the gastrointestinal microbiome is out of balance, this can result in immune system dysregulation involving allergic responses as well as the immune system. These changes can leave the body in a state that is more prone to immune-mediated disease, or other diseases caused by dysfunction of the immune system, such as allergies, where the body is reacting to everyday substances as if they are a problem[2]. Immune responses known as Th1 & Th2 are particularly important when we look at one of the most common skin diseases that takes people to their vets regularly – atopic or allergic skin disease. Th1 cytokines (signals) are proinflammatory and responsible for helping to kill intracellular parasites and perpetuating autoimmune responses. Th2 cytokines are anti-inflammatory, or associated with promotion of IgE and eosinophilic responses in atopy/allergy. Ideally these two responses would be well balanced for a normal immune response[3]. Looking at this, it is easy to see how a digestive disturbance or microbiome imbalance, if left untreated, could end up leading to or contributing to atopic skin disease, or worse, to an autoimmune disease.

It is not uncommon for people to complain that their pet has developed a skin lesion or complaint following a course of antibiotics, where the disruption to the microbiome of both the gut and the skin has been enough to cause an acute problem. But it isn’t just gut dysbiosis that can lead to skin disease, as skin dysbiosis itself has been linked with various inflammatory skin diseases, including atopic dermatitis and psoriasis. It is thought that this is contributed to by increased microbial load or locally amplified immune responses to particular skin microbes, along with an impaired skin barrier and genetic predisposition. There are also specific strains of skin microbes that play an active role in the innate and adaptive immune response regulation of the skin, or that produce antimicrobials to protect the skin against opportunistic pathogenic (disease-causing) bacteria. Even microbial metabolites, such as short chain fatty acids, play a role, as they can modulate the inflammatory activity of epidermal keratinocytes (skin cells).2

With these increasingly well-established links between the skin and the gut, it’s becoming clear that many skin disorders are multifactorial and often linked directly to the health of the gut microbiome. So, if your dog has a skin problem, from itchy skin to chronic ear problems, at least part of the answer may lie deep in their intestine, ready to be unlocked by a detailed microbiome analysis, and managed by a program of appropriate digestive supplementation.

References

^[1] Se Jin Song, Christian Lauber, Elizabeth K Costello, Catherine A Lozupone, Gregory Humphrey, Donna Berg-Lyons, J Gregory Caporaso, Dan Knights, Jose C Clemente, Sara Nakielny, Jeffrey I Gordon, Noah Fierer, Rob Knight (2013) Cohabiting family members share microbiota with one another and with their dogs 

^[2] Zheng, D., Liwinski, T. & Elinav, E. Interaction between microbiota and immunity in health and disease. Cell Res 30, 492–506 (2020)

^[3] Berger A. Th1 and Th2 responses: what are they? BMJ 2000; 321 :424