ABSTRACT
This chapter suggests that a human being is a symbiosis comprised of mammalian and microbial cells with the latter including bacteria, archaea, fungi, viruses, and protozoa. It helps to understand that the composition of the initial microbial community present at a particular body site is dictated by the environmental factors that operate there and that the subsequent development of the community results from changes to that environment brought about by microbial activities. The chapter argues that microbes can live inside mammalian cells as well as grow on a variety of body surfaces as microcolonies or biofilms within which they can communicate with one another. It deals with the epithelium, the mammalian tissue on which most microbial communities reside, and with the ways in which this tissue uses innate and acquired immune mechanisms to exclude or control its microbial residents. In some regions of the body, mechanical forces are generated that can impede microbial colonization.
