 As humans, we live and draw resources from communities of people. Interestingly, our bodies also house their own complex communities of human cells and microorganisms that work together. In fact, our bodies are made up of more bacterial cells than human cells. All of the microorganisms that live in and on our bodies compose the microbiome, including bacteria, fungi, and viruses. Our intestines host the largest microbial community, 100 trillion microorganisms that make up the gut microbiome. So what is the role of these gut microorganisms in our bodies? The gut microbiome, for a long time ignored, is now considered an organ with key functions in promoting health. Actually, it promotes the development of a functional intestine and helps digest food to provide the nutrients necessary for our growth and well-being. Through an interactive connection between host cells and the gut microbiome, helpful microorganisms selectively colonize the gut and participate in the development and maintenance of a well-balanced immune system. The gut microbiome modulates the secretion of certain hormones and vitamins, like serotonin or vitamin K. The gut microbiome also regulates energy extraction from the ingested food. Finally, the gut microbiome provides an effective barrier against infection by harmful pathogens in our intestine. So where does this microbiome come from? At birth, the guts of newborns are sterile, and according to the mode of delivery, they are colonized by microorganisms with which the babies were first in contact. Then, the gut microbiome composition fluctuates in a dynamic process with selective pressure induced by diet, genetics, and environmental exposures. Infections and the use of antibiotics can cause significant and sometimes irreversible effects on the future adult gut composition. From the age of three, the gut microbiome is becoming closer to the adult final composition, becomes mature, highly diverse, and relatively stable. Some factors can still affect its composition, for instance, diet changes and transient use of antibiotics. Fortunately, our gut microbiome is very resilient and can return to its original composition after a disturbance. However, chronic alteration of the gut microbiome can cause lasting changes to its composition, including reduced diversity or specific destruction of some species, which may then contribute to disease development. Because our gut microbiome serves to regulate our immune system, alterations in its composition can promote inflammation and immune system dysregulation, such as in Crohn's disease or allergies. Changes in the gut microbiome composition could promote metabolic conditions, such as obesity, diabetes, or metabolic induced hepatitis. Because these microbes act as a barrier to harmful pathogens, persistent damage to the community can be linked to the development of some infectious or hospital acquired diseases. Finally, changes in the gut microbiome are also believed to influence chemistry in the brain and contribute to CNS diseases like depression or Parkinson's disease. So as you can tell, the gut microbiome influences biology from our liver to our brains and has far-reaching implications for our health. There's still much we need to understand about the role of the different microorganisms and what defines a diseased versus healthy microbiome. Much progress is being made around the world to better understand this intriguing community that lives in our bodies. This video has been provided to you by Eureka Science. To stay in touch with Eureka Science, like us on Facebook, follow us on Twitter, subscribe to our YouTube channel, or visit us at www.eurekascience.com. Thank you for watching!