Our immune system is composed of millions of cells that mobilize to fight infection, but they may not be our only defenders. In our bodies, there are ten times more commensals, or good bacteria, than human cells, yet we know very little about their role in health and disease. Yasmine Belkaid and her collaborators at the National Institutes of Health, aim to change that. Their project explores the immune system’s relationship to the naturally occurring microbes found in our skin. Humbly we have realized that we’re just a minority in our own body, and I think it’s becoming a growing understanding that you can not understand host physiology without taking into account this really enormous pressure of the microbes. One of Dr. Belkaid’s collaborators is Heidi Kong, a dermatologist at the National Cancer Institute. Here she takes a skin sample from a healthy person. This will be used to compare commensals under normal conditions to commensals in people who have skin disorders. As we struggle to try to sanitize our environments and ourselves in certain ways, it is important to keep in mind that there are bacteria that are likely doing important things that are helpful to us, beneficial to us, and to make sure that we aren’t doing away with those bacteria. Another one of Dr. Belkaid’s collaborators is Julie Segre, a geneticist at the National Human Genome Research Institute. She and her lab sequence the DNA from human samples to identify and characterize the diverse bacterial species living in the skin. We need to understand the connection between the immune system and the bacteria to improve health outcomes of a variety of skin disorders, but also other more systemic disorders. To study the role of skin commensals, researchers in Dr. Belkaid’s lab colonize germ-free mice with the bacteria found in the human samples. Although more research is needed, the team’s initial findings suggest that commensals interact with immune cells to protect us from infection. Our skin defenses also may diminish when commensals are removed. I think massive antibiotic treatments or massive change in the way we actually are dealing with those microbes is extremely dangerous, so really trying to understand by which mechanism the microbes influence the immune system, both in healthy state and in disease states, will allow us to devise a better therapeutic approach.