Our gut microbes play a crucial role in the development of immune cells that help fight infection, according to a recent study by researchers from the California Institute of Technology and published in the journal Cell Host & Microbe. They began the study by comparing innate immune cells—white blood cells that act as the body’s first line of defense against foreign invaders—in mice both with and without gut microbes. In the germ-free mice, there were less innate immune cells than in mice with gut microbes, which suggests that gut microbes play an important role in the development of these cells.

Next, they investigated whether the difference in number of innate immune cells played a role on protection against infection with the bacterium Listeria monocytogenes, a food-borne pathogen that causes serious infection. As it turns out, the mice with more innate immune cells were better protected against Listeria infection than those mice with fewer immune cells (as a result of a lack of gut bacteria).

In addition, the researchers tested another set of healthy mice (with gut bacteria) by giving them a dose of antibiotics. These mice also had difficulty fighting the Listeria infection. Antibiotics tend to destroy gut bacteria, so the antibiotic treatment may have been enough to alter immune cell function in a similar way as occurred in the germ-free mice.

“Evidence that depletion of the microbiota leads to transient immune suppression suggests factors that disrupt commensal microbes, including clinical antibiotic use, may, paradoxically, be a risk factor for susceptibility to opportunistic pathogens,” noted the researchers. The authors noted the far-reaching effects of gut bacteria, stating, “It’s interesting to see that these microbes are having an immune effect beyond where they live in the gut. They’re affecting places like your blood, spleen, and bone marrow—places where there shouldn’t be any bacteria.”

Our gut bacteria are turning out to be one of the most amazing players in our overall health.