FORMATION OF PHENOLIC AND INDOLIC COMPOUNDS BY ANAEROBIC-BACTERIA IN THE HUMAN LARGE-INTESTINE

Batch culture incubations were used to investigate the effects of pH ( 6.8 or 5.5) and carbohydrate (starch) availability on dissimilatory ar omatic amino acid metabolism in human fecal bacteria. During growth on peptide mixtures, tyrosine and phenylalanine fermentations occurred o ptimally at pH 6.8, while individual metabolic reactions were inhibite d by up to 80% in the presence of 10 g l(-1) starch. Tryptophan metabo lites were not detected in these experiments. When free amino acids re placed peptides, phenol production was increased during carbohydrate f ermentation, although formation of p-cresol, another tyrosine metaboli te was strongly inhibited. Phenylpropionate, which is produced from ph enylalanine, was unaffected by starch. Tryptophan was fermented in the se studies, although indole production was reduced in the starch ferme nters. The importance of different fermentation substrates (casein, pe ptide mixtures, free amino acids) on aromatic amino acid metabolism wa s investigated in incubations of material taken from the proximal bowe l. The phenylalanine metabolites, phenylacetate and phenylpropionate, were the principal phenolic compounds formed from all three substrates . Phenol was the major tyrosine metabolite produced in casein and pept ide fermentations, while hydroxyphenylpropionate was a more important tyrosine product from free amino acids. Indole was the sole product of tryptophan metabolism, but was formed only from the free amino acid. Bacterial metabolism of individual phenolic and indolic compounds was also investigated. Phenol, p-cresol, phenylacetate, phenylpropionate, 4-ethylphenol, indole, indoleacetate, and indolepropionate were not me tabolized by colonic bacteria. However, hydroxyphenylacetate was hydro lyzed to p-cresol, while hydroxyphenylpropionate was transformed into phenylpropionate. Indolepyruvate was either converted to indoleacetate or metabolized into indole. Indolepropionate, and to a lesser degree indoleacetate were produced from indolelactate. These data show that h uman colonic anaerobes are able to extensively degrade either free or peptide-bound aromatic amino acids, with the concomitant formation of toxic metabolic products. These processes are controlled to a signific ant degree by environmental factors such as pH and carbohydrate availa bility, and this ultimately influences the types and amounts of fermen tation products that can be formed in different regions of the large b owel.