Symbiotic Interactions between Colonic Microflora and Protein Metabolism in Infants

ABSTRACT. The utilization of ¹⁵N nitrogen from ¹⁵N-labelled bifidobacteria for whole body protein synthesis was studied in 4 infants by oral single-pulse labellings and in 3 other infants, who had colostomies, by colonic pulse labellings. The bifidobacteria were harvested from a modified Petuely culture medium containing ¹⁵N ammonium chloride and ¹⁵N cystine as the only sources of nitrogen. The tracer dose chosen for the balance studies was 3 mg ¹⁵N/kg. ¹⁵N concentrations in urine and feces collected over 48 hours after the pulse labellings were determined by emission spectrometry. Oral administration of ¹⁵N-labelled bifidobacteria resulted in absorption of approximately 90%, renal excretion of 15%, and fecal excretion of 12% of the tracer dose, respectively. Retention in the protein pool averaged 73%. After colonic single pulse labelling with ¹⁵N-labelled bifidobacteria, the corresponding values were 85.5%, 2.2%, 14.5%, and 83.0%, respectively. Absorption and incorporation of the heavy nitrogen into body proteins were directly demonstrated by increased ¹⁵N atom percent excess values within the trichloroacetic acid (TCA) supernatants and the proteins of the plasma, 0.25 and 0.04 atom %, respectively, at 24 hours after oral pulse labellings. One half of the total ¹⁵N excreted in urine consisted of urea and approximately 8% was eliminated as ammonia.