Tiny microbes in your stomach and intestines can make a big, positive difference in your waistline, brain, and immune system. There are about 100 trillion bacteria in or around your body right now. Some estimates say that each human has a pound or two of bacteria living in their guts at all times. But don’t worry, that’s a good thing (even if the idea is a little unnerving)! These organisms play an important role in your everyday health and longevity.

1. What’s in Your Gut May Affect the Size of Your Gut Need to lose weight? Why not try a gut bacteria transplant? New research published in the journal Science suggests that the microbes in your gut may play a role in obesity. However, researchers are still unsure of the extent to which they affect how we metabolize food. For the study, mice were genetically engineered without their own gut bacteria. Half were given bacteria from obese humans while the other half were given bacteria from lean humans. The mice with obese bacteria gained more weight, suggesting that the gut microbes transmitted physical and metabolic traits from their owners. Instead of making bacteria transplants the norm, researchers say that their findings could be an important step toward creating specialized probiotics and food-based therapies for those having difficulty shedding extra pounds.

2. Probiotics May Treat Anxiety and Depression Scientists have been exploring the connection between gut bacteria and chemicals in the brain for years. New research adds more weight to the theory that researchers call “the microbiome–gut–brain axis.” Research published in Proceedings of the National Academy of Science shows that mice fed the bacterium Lactobacillus rhamnosus showed fewer symptoms of anxiety and depression. Researchers theorize that this is because L. rhamnosus acts on the central gamma-aminobutyric acid (GABA) system, which helps regulate emotional behavior. L. rhamnosus, which is available as a commercial probiotic supplement, has also been linked to the prevention of diarrhea, atopic dermatitis, and respiratory tract infections.

3. The More Bacteria the Better While bacteria on the outside of your body can cause serious infections, the bacteria inside your body can protect against it. Studies have shown that animals without gut bacteria are more susceptible to serious infections. Bacteria found naturally inside your gut have a protective barrier effect against other living organisms that enter your body. They help the body prevent harmful bacteria from rapidly growing in your stomach, which could spell disaster for your bowels. To do this, they develop a give-and-take relationship with your body. “The host actively provides a nutrient that the bacterium needs, and the bacterium actively indicates how much it needs to the host,” according to research published in The Lancet.

4. Gut Bacteria Pass from Mother to Child in Breast Milk It’s common knowledge that a mother’s milk can help beef up a baby’s immune system. New research indicates that the protective effects of gut bacteria can be transferred from mother to baby during breastfeeding. Work published in Environmental Microbiology shows that important gut bacteria travels from mother to child through breast milk to colonize a child’s own gut, helping his or her immune system to mature.

5. Lack of Gut Diversity Is Linked to Allergies Too few bacteria in the gut can throw the immune system off balance and make it go haywire with hay fever. Researchers in Copenhagen reviewed the medical records and stool samples of 411 infants. They found that those who didn’t have diverse colonies of gut bacteria were more likely to develop allergies. But before you throw your gut bacteria a proliferation party, know that they aren’t always beneficial.

6. Gut Bacteria Can Hurt Your Liver Your liver gets 70 percent of its blood flow from your intestines, so it’s natural they would share more than just oxygenated blood. Italian researchers found that between 20 and 75 percent of patients with chronic fatty liver disease—the kind not associated with alcoholism—also had an overgrowth of gut bacteria. Some believe that the transfer of gut bacteria to the liver could be responsible for chronic liver disease.