Bacteria can be modified to fight cancer in mice. Human trials are coming.

Most microbiome research has focused on the trillions of insects that live in our guts. But our skin is also home to multiple microbial ecosystems. The community that lives under your armpit can be very different from the community that lives on your eyelids. We still don’t know exactly what these microbes are doing, but they seem to feed on our secretions, possibly producing their own beneficial secretions and protecting us from infection.

They also seem to influence the functioning of our immune system. A growing body of research suggests that microbes living in and on our bodies can amplify or reduce the immune response to something that could potentially harm us, whether it’s an infection, tumor or something more benign.

The simple act of introducing a microbe into an animal’s skin can also trigger an immune response, even if it does not cause all the usual signs of an infection, such as pain, fever or illness. This is somewhat surprising, says Michael Fischbach of Stanford University, because these microbes don’t tend to be harmful: “They are our friends. Adding a microbe to the skin of a mouse, for example, can have a similar effect to vaccinating the same mouse, he says.

Modified microbes

Fischbach and his colleagues wondered if they could hijack this effect to modify the immune response.

The team began the investigation by choosing a microbe commonly found on human skin. S. epidermidis is considered a member of the human microbiome and generally does not cause disease. The microbes used by the researchers were initially collected from behind the ear of a human volunteer, says Fischbach.

The researchers modified these microbes by inserting a new gene into them. The gene codes for a protein found on the surface of some cancer cells. The idea is that if the immune system generates cells that recognize the microbe, those cells will also recognize tumors.

The team then applied these “designer bugs” to mice by wiping them over the animals’ heads with a cotton swab. Another group of mice had regular, unmodified samples of the bacteria coated on them. In both cases, the microbes quickly settled on the mice’s skin, says Fischbach.

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 their surface.