Impact of commensal microbiota on the host pathophysiology: focusing on immunity and inflammation

All living things on earth are classified into three domains: archaea, bacteria, and eukarya [1]. Bacteria are the oldest and most prosperous among the three; they are estimated to occupy 1/3~1/2 of terrestrial biomass [2]. They inhabit virtually the entire planet, even in extreme environmental conditions such as the stratosphere (ultralow temperature and atmospheric pressure), volcanoes (ultrahigh temperature), and the bottom of deep trenches in the ocean (ultrahigh pressure). Our body is no exception. Many bacteria with a wide variety of species exist on the skin and mucosa. In fact, the colon is the most suitable environment for bacterial growth on earth, with their density reaching as high as 10 super(11-12)/g content; overwhelmingly exceeding the densities of any other known bacterial niches around the world. In the whole gastrointestinal lumen, we retain more than 100 trillion commensal bacteria, gut microbiota, classified into 500-1000 different species [3]. Nevertheless, the diversity ...