Over millions of years of evolution, humans and their gut microorganisms get on well and have a mutually beneficial relationship. We provide the microorganisms with shelter and nourishment and in exchange, these hundreds of trillions of tiny microbes living inside us contribute to our health and metabolism by performing different and important tasks. One key function is how they help us keep harmful, potentially infection-causing bacteria at bay. And in this important job, they are not alone.

According to a new study published in the journal Immunity by scientists from the University of Chicago, the body’s immune system plays a key role. The study suggests there is a single binding protein in one type of innate immune cells that is in charge of maintaining a balanced microbial community. When said protein (called ID2) is missing, harmful bacteria are more likely to cause serious trouble. Although the reason why is unclear, researchers suggest the immune system may have a way of recognizing the presence of invading gut bacteria.

“Our study reveals how our body’s immune system shapes the gut microbiota to naturally limit infections,” said senior author Yang-Xin Fu, a professor at the University of Chicago Department of Pathology, in a statement.

Fu and his colleagues focused on a kind of immune cell known as type 3 innate lymphoid cells (ILC3s), which are essential for infection resistance in the gut. Through their research, they saw that these cells were less able to fight off harmful bacteria when they lacked the ID2 protein.

Taking their study further, they carried out an experiment with rodents, transplanting microbiota from a mouse with dysfunctional ILC3s that were lacking ID2 into germ-free animals. Then they transferred microbiota from another mouse with healthy and functional ILC3s into a different group of germ-free animals. Thereupon exposed both sets to harmful bacteria. Can you guess what they found?

The first group was highly susceptible to harmful bacterial infection, whereas the second could effectively fight off the bad microbes and avoid infection. The authors say this experiment sheds light on the role the immune system plays in our gut microbiota, acting like a guardian that tries to maintain a proper balance.

“Given the rapid rise of harmful bacteria that are resistant to antibiotics, it is paramount that scientists find methods of limiting harmful bacterial infections without the use of antibiotics. For future patients who are infected with harmful bacteria, it might be beneficial to promote the development of good gut microbiota to indirectly kill harmful bacteria, instead of using antibiotics,” said Fu.

In the study, Fu and his colleagues also suggest that new therapeutic treatments or a microbiota transplantation could be used to promote the development of good gut microbiota and indirectly kill harmful bacteria in patients with recurrent gut infections.

The authors are now focused on trying to isolate specific species that could be used to prevent bacterial infections. They also want to find an answer to a very intriguing question: how do immune cells distinguish between beneficial and harmful bacteria to maintain a healthy microbiota? We may have to wait for their next paper (or longer) to find out the answer…