TDO2 and necrotizing enterocolitis: These enzymatic pathways normally reflect the presence of a healthy microbiota characterized by Bifidobacterium longum, Bifidobacterium brevis and Bifidobacterium bifidum. In NEC subjects, however, values three times higher of key enzymes in the tryptophan degradation pathways, such as tryptophanase and indole-pyruvate decarboxylase, were highlighted.