ABSTRACT
The intestinal microbiota of humans is comprised of a complex ecosystem of metabolically
active microorganisms that reside close to the mucosal surface of the intestine. The bacteria
of the intestine can interact with substrates introduced orally or compounds entering the
intestinal lumen via the bile, mucosal secretions, or systemically from the circulatory
system. This chapter will review the bacterial reactions performed on nutrients and drugs
entering the intestine. The composition and distribution of the intestinal microbiota will not
be discussed, and the readers are referred to other chapters in this book and review articles
that address this topic (1-4). It is, however, important to note that the intestinal microbiota at
any given time weighs approximately 110 to 200 grams and consists of at least 400 different
species. The number of bacterial cells is approximately ten times greater than the total
number of cells comprising the human body. Although the mass of the intestinal
microbiome is equivalent to that of a single kidney, the number and diversity of species
affords the microbiota a diverse metabolic role in the human body. This chapter will review
some of these reactions and implications of these transformations to the host; however, no
attempt will be made to exhaustively review all known reactions carried out by the
microorganisms that inhabit the gastrointestinal tract of humans and animals.
