Abstract
The microbial community resident in the human gastrointestinal tract has a major impact on host physiology. Manipulating this complex ecosystem by dietary intervention requires knowledge of the parameters that influence its composition and the activity. More recent developments have taken advantage of culture-independent molecular methods for bacterial identification on the basis of the highly discriminatory sequence database of 16S rRNA. This culture-independent approach was applied to asses the fecal microbiota of human subjects from different European countries in relation to nutrition. In one of two human studies, a positive correlation was observed between the intake of fruits and vegetables and the proportion of bacteria belonging to the Clostridium leptum group. In the second study, the relative proportion of bifidobacteria was much higher in the Italian study group than in any other study group. This finding correlated positively with a consumption of water-soluble fiber. Intestinal bacteria also play a role in the activation and inactivation of plant-derived polyphenolic substances such as lignans and flavonoids. To assess the influence of bacterial metabolism on the bioavailability of flavonoids in the human intestinal tract, two model organisms were used to characterize the transformation of flavones in comparison to that of flavonols. Lignans require activation by intestinal bacteria to exert estrogenic and antioxidant activities. Several bacterial species involved in lignan activation were isolated and the catalytic steps involved were characterized. The results indicate that the activation of dietary lignans involves phylogenetically diverse bacteria, most of which are common members of the dominant human intestinal microbiota.
