The skin is one of our body’s first lines of defense, but it’s also home to a rich community of microbes — the skin microbiome.  These bacteria are important in protecting us from pathogens; changes in the skin microbiome are linked with conditions ranging from dermatitis to psoriasis.  In a paper published this month in the journal PNAS, a team of scientists from the University of Pennsylvania showed that these bacteria don’t just live on our skin, but also play a role in regulating our immune response.

The researchers took samples from the skin of laboratory mice and used modern sequencing technology to figure out how many kinds of bacteria were living there and how similar they were to one another. To investigate how the immune system affects the skin microbiome, they then treated some mice with a chemical that blocks a component of the complement system, which is part of the innate immune system, and checked how the bacterial community changed.  Although both groups had roughly the same amount of bacteria on their skin, there were fewer different kinds of bacteria on the skin of the complement-blocked mice.  In other words, they had a significantly less diverse skin microbiome than the untreated mice.  The researchers also found that these mice had lower levels of various genes related to immunity and fewer immune cells in their skin.

The team decided to find out if this interaction was two-way — that is, whether the skin microbiome can affect the immune system instead of just being regulated by it.  To do this, they started with germ-free mice — mice without any bacteria on their skin — and infected them with a skin microbiome.  Examining genes related to the complement system, the researchers found that many of them became significantly more active after the mice gained skin bacteria.

I don’t think this study presents any evidence that the bacteria are actively or directly regulating the immune response; it’s also possible that the changes are because the complement system is responding to the presence of bacteria on the skin. It would be cool if the bacteria were active players, but either way it’s clear that our skin bacteria plays a role in coordinating some of our earliest lines of defense.

Ref
Chehoud C, Rafail S, Tyldsley AS, Seykora JT, Lambris JD, & Grice EA (2013). Complement modulates the cutaneous microbiome and inflammatory milieu. Proceedings of the National Academy of Sciences of the United States of America, 110 (37), 15061-6 PMID: 23980152