ABSTRACT
The human body is a wonderland for the microbial world, with harsh uninhabitable lands
in some regions and lush fertile metropolis in others. The normal microbiota of humans is
an extensive and diverse microbial community, which is composed primarily of bacteria
from numerous phylogenetic clusters (1-5). The largest proportion of the human
microbiota is found in the gastrointestinal (GI) tract, or more specifically the colon. Other
regions of the body harboring indigenous bacterial populations include the skin, oral
cavity, upper respiratory tract, and urogenital tract (3). This chapter aims to discuss the
normal microbiota of the human GI tract and our current understanding of its composition
and role in human health. Discussion of the interactions between the gut microbiota and
the host will also abridge the impact of extrinsic factors, such as diet and environment.
