Influence of Dietary Fiber on the Intestinal Environment 1

Abstract

The human colon contains one of the most complex bacterial ecosystems known. At least 400 distinct species of bacteria are represented, although 15 major species account for nearly two-thirds of the isolates (1). The concentration of bacteria in the colon is very high, about 10¹⁰ per gram wet weight. Approximately onethird of colon contents is bacteria (1). The bacterial flora of the colon is presumably in equilibrium with its human host. We assume that this equilibrium is a dynamic one because the composition of the host's diet and the physiological state of the host can change. We also assume that the predominant species of colon bacteria are uniquely adapted to survive and flourish in the colonic environment. However, little is known either about the specific features of the interaction between host and flora or about the particular characteristics that are most important for the survival of organisms in the human.

To understand how colonic bacteria interact with their human host, we need to be able to answer three types of questions: First, which species of bacteria are found in the colon; i.e., what is the species composition of the colonic flora? Second, what are the bacteria in the colon actually doing; i.e., what are their metabolic activities? Third, where are the bacteria located; i.e., what is their environmental niche? To date, most research has focused on the first of these questions, despite the fact that answers to the last two questions are actually more important for understanding the interaction between the microbial flora and its host. The purpose of this review is to summarize recent research on the composition and activities of the colonic flora and to assess what progress has been made toward being able to answer all of the questions outlined above.