ENUMERATION OF HUMAN COLONIC BACTERIA PRODUCING PHENOLIC AND INDOLIC COMPOUNDS - EFFECTS OF PH, CARBOHYDRATE AVAILABILITY AND RETENTION TIME ON DISSIMILATORY AROMATIC AMINO-ACID-METABOLISM

Concentrations of phenolic compounds in human gut contents were more t han fourfold higher in the distal colon (6.2 mmol kg(-1)) compared to the proximal bowel (1.4 mmol kg(-1)). Tryptophan metabolites were neve r found in more than trace amounts in large intestinal contents and ph enol substituted fatty acids were the major products of aromatic amino acid fermentation that accumulated in the proximal colon, whereas phe nol and p-cresol were more important in the distal gut, accounting fur 70% of all products of dissimilatory aromatic amino acid metabolism. In vitro incubations of colonic material showed that phenol was produc ed most rapidly (1.0 mu mol g(-1) h(-1)), whereas indole was formed co mparatively slowly (0.06 mu mol g(-1) h(-1)), Most probable number (MP N) estimations demonstrated that large populations of phenol and indol e producing bacteria occur in the large Intestine (range log(10) 9.8-1 1.5 (g dry wt faeces)(-1), mean 10.6, N = 7). With respect to phenolic compounds, phenylacetate and phenylpropionate producers predominated, while indoleacetate-forming bacteria were the major tryptophan-utiliz ing organisms. Quantitation of products of dissimilatory aromatic amin o acid metabolism in MPN tubes showed that phenol and phenylpropionate mainly accumulated at low sample dilutions, whereas phenylacetate, p- cresol, indoleacetate and indolepropionate were formed in greatest amo unts at high sample dilutions. The significance of pH and carbohydrate availability with respect to aromatic amino acid metabolism was shown in batch culture fermentation studies, where net production of phenol ic compounds by mixed populations of intestinal bacteria was reduced b y approximately 33% during growth at pH 5.5 compared to pH 6.8, and by 60% in the presence of a fermentable carbohydrate. Experiments with 1 6 species of intestinal bacteria belonging to six different genera sho wed that environmental factors such as low pH and high carbohydrate av ailability markedly reduced dissimilatory aromatic amino acid metaboli sm in some organisms, but stimulated this process in others. A three-s tage continuous culture model of the colon was used to investigate the effect of system retention time (27.1 or 66.7 h) on aromatic amino ac id fermentation. Qualitative and quantitative increases in phenol prod uction occurred from vessel 1 to vessel 3 in this model. Concentration s of phenolic compounds in vessel 3 were three rimes greater at R = 66 .7 h compared to R = 27.1 h. Phenol and p-cresol were not detected in vessel 1, though formation of these metabolites increased from vessel 2 to vessel 3, in a pattern similar to that observed in the distal col on.