Most research on the microbiome has focused on the trillions of bugs that live in our gut. But our skin is home to many microbial ecosystems. The society that lives in your armpit can look very different from the society that lives on your eyelids. We still don’t know exactly what these microbes are doing, but they seem to feed on our secretions, perhaps producing some useful fluids of their own and protecting us from disease.
They seem to affect the way our immune system works. A growing body of research shows that the microbes that live in and on our bodies can boost our immune system or respond to things that could harm us, such as an infection, tumor, or other threat.
Just introducing microbes to an animal’s skin can trigger an immune response—even if it doesn’t cause typical signs of infection, such as soreness, fever, or pain. “This is somewhat surprising because these microbes don’t tend to be harmful,” says Michael Fischbach of Stanford University. For example, adding microbes to the skin of a mouse can be similar to giving the same mouse a vaccine, he said.
Fischbach and his colleagues wondered if they could hack this effect to modulate the immune response.
The team began their investigation by selecting the microbes most commonly found on human skin. S. epidermidis It is considered a member of the human microbiome, and does not normally cause disease. The microbes the researchers used were first collected from behind the ears of volunteers, Fischbach says.
The researchers modified these microorganisms by inserting a new gene into them. The gene codes for a protein expressed on some cancer cells. The idea is that if the immune system produces cells that recognize microbes, these cells will recognize tumors.
The team then applied these “designers” to mice by swabbing the animals’ heads with cotton buds. Another group of mice were inoculated with normal and unmodified bacterial samples. In both cases, the microbes quickly made a home for themselves on the mice’s skin, Fischbach says.
At the same time, the mice were injected with skin cancer cells. These cells were taken from other mice with cancer, so they had the target protein on them.