The human body is home to a vast number of bacterial species; in fact, it is thought that the number of bacterial cells in a human body far outnumbers the human cells themselves. Many of these bacteria live in the gut, while others live in saliva, the inside of the eyelids, or the skin.
While most of them have no known effect on us, some are useful to their human hosts. For example, several studies have suggested that gut bacteria help our immune systems protect us against pathogens. A recent paper by Naik et al. in Science suggests that at least some of the bacteria living in our skin do the same.
These authors found that ‘germ-free’ mice without resident bacteria have reduced levels of interleukins (chemicals produced by white blood cells that help fight infections) in their skin tissues. When bacteria were introduced into the gut of these mice, interleukin levels in the gut increased, but levels in the skin did not. On the other hand, when a species of bacteria that normally resides in skin was introduced into the skin of germ-free mice, the interleukin levels in the skin increased, as did the immune response to the parasite Leishmania major.
It has long been known that gut bacteria produce vitamins that are important for human health. In recent years, a variety of studies have linked resident bacteria to obesity, disease, prevention of allergies, and even behavior. The finding that bacteria in our skin may be important for the human immune system is another step in understanding the importance of this vast and diverse population of cells.