Role of gut bacterial flora in nutrition and health: A review of recent advances in bacteriological techniques, metabolism, and factors affecting flora composition

In the past decade, there has been a resurgence of scientific interest in man's intestinal microbiota and its role in determining the normal physiology and health status of the host. The vast majority of fecal and intestinal microbes are strict anaerobes, many extremely sensitive to molecular oxygen. Intestinal anaerobes may outnumber aerobes and facultative anaerobes by a factor of 10³ or greater. As a result of newer and more rigorous anaerobic bacteriological procedures for the isolation of gut flora components, viable recoveries of significant fractions (1/3 to 2/3) of the numerous microbes observed by direct microscopic examination of fecal specimens have been achieved. Data from several recent studies put the viable count in normal human feces at 2 to 4 × 10¹¹/g dry weight. Extensive studies of human fecal flora components employing bóth selective media and nonselective random isolation procedures have revealed an unexpected order of complexity both within and between individuals. It is estimated that, while as few as 20 bacterial species may comprise 75% of the viable flora, the remaining 25% may include over 400 additional species or distinguishable types. The dominant organisms in all cases are nonsporing rods both Gram‐negative (particularly Bacteroides spp.) and Gram‐positive (e.g., Bifidobacterium spp. and Eubacterium spp.) This review attempts to bring together some of the voluminous recent literature in this complex and rapidly evolving area. The emphasis is on the role of the bacterial flora in the nutrition and health of normal human hosts, although numerous relevant studies with laboratory animals have been cited. The introductory section describes the role of the flora in nutrition and digestion, absorption and malabsorption, host defenses and longevity, and the known or suspected toxigenic capabilities of the flora. There are also sections on methods of isolation, cultivation, speciation, etc. of normal intestinal flora; the composition and distribution of the gut flora and the effect of diet; descriptions of some important genera and species representative of human lower bowel and fecal flora; and finally selected aspects of gut flora metabolism. Among the latter are the intestinal bacterial metabolism of azo food colors, flavonoids, cholesterol, and bile acids.