Commensal bacteria are microorganisms that live in close association with host organisms, such as humans. These bacteria typically coexist with their host without causing harm, and in some cases may even provide benefits to the host. Commensal bacteria play important roles in maintaining the host's health, such as by aiding in digestion, protecting against harmful pathogens, and regulating the immune system. Research in this area focuses on understanding the interactions between commensal bacteria and their host, as well as the potential therapeutic applications of manipulating these microbial communities to improve health and prevent disease.