Nitrogen metabolism in rumen bacteria can be characterised by their N isotopic signature

The N use efficiency (NUE) of ruminants is negatively correlated to the natural N-15 enrichment of ruminants' tissues over their diets (Delta(15)Nanimal-diet). The metabolism of rumen bacteria, which strongly discriminates against N-15 utilisation, may contribute to this link but little is known about the role of individual bacteria. In this study, we characterised the Delta N-15 of three metabolically different rumen bacteria, a cellulolytic ammonia-consuming, a hyper-ammonia producing and a proteolytic organism, grown in pure culture with different N sources and levels. The N isotopic signatures significantly changed across the 3 rumen bacteria. Although we failed to observe a higher Delta N-15(bacteria-substrate) when proteolysis and NH3 production was favoured, our results confirm a strong N-15 depletion with the NH3-consuming bacterium. This may suggest that N-15 depleted NH3-consuming bacteria improve the NUE in the rumen.