Symbiotic interactions between colonic microflora and protein metabolism in infants

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.