The role of the gut microflora in the utilization of dietary urea by the chick.

1. In a preliminary experiment, growth of conventional chicks given a basal diet containing adequate amounts of all the essential but none of the non-essential amino acids was improved by supplements of 10-3 g urea or 50-4 g glutamic acid/kg diet or both. 2. In the main study the effects of supplementing the basal diet with 20-6 g urea/kg were compared in groups of sixteen germ-free and conventional chicks. 3. The germ-free chicks did not benefit from the urea supplement whereas the conventional birds showed improved food conversion efficiency and significantly better growth. 4. In both environments nitrogen retention ((mg N intake-mg N excreted)divideg food intake) was higher in the birds given urea, but N utilization ((mg N intake-mg N excreted)dividemg N intake) was reduced. This reduction was greater in the germ-free birds. 5. There was a small increase in plasma ammonia concentration in the germ-free birds given urea but a significantly greater increase in the corresponding conventional group. 6. Plasma uric acid concentrations were variable in both groups, and much lower than the normal range. They followed a similar pattern to the plasma ammonia values. 7. More insoluble N was excreted by the conventional chicks given urea than by the corresponding germ-free group, or by either group given the basal diet. 8. It was concluded that the gut micro-organisms are responsible for the growth-promoting effect of urea, presumably through release of ammonia by bacterial ureas (EC 3.5.1.5) and its consequent incorporation into amino acids.