Normal microbiota, or normal flora, are bacteria that live in or on an organism all the time, while transient microbiota are not always present in the organism. Transient microbiota are a subset of normal microbiota.
Normal microbiota have a symbiotic relationship with the organisms they inhabit. While the host organism provides them with a safe place to grow, they provide the host with several benefits. One of the greatest benefits is preventing colonization by harmful bacteria. The normal microbiota consume nutrients and take up space, both resources that opportunistic, harmful bacteria would otherwise use to harm the host. Certain normal microbiota also produce substances that prevent harmful bacteria from growing.