NITROGEN-METABOLISM BY RUMINAL MICROORGANISMS - CURRENT UNDERSTANDINGAND FUTURE PERSPECTIVES

This review presents an outline of our current understanding of rumina l nitrogen metabolism from three perspectives: proteolytic microorgani sms and their enzymes, intraruminal recycling of microbial protein, an d enzymes of ammonia assimilation. Some of the pending advances and fu ture research opportunities in these areas are also discussed. The 'sm ugglin' concept appears to offer the potential to inhibit peptide-util izing bacteria selectively in the rumen, as demonstrated by initial st udies with Prevotella ruminicola. The relative contributions of protoz oa-, bacteriophage-, and self-mediated lysis of bacteria to intrarumin al recycling of microbial protein are not yet quantified, and further efforts to understand the biology and dynamics of ruminal bacteriophag e and protozoa populations are warranted. In Ruminococcus flavefaciens and Prevotella ruminicola, glutamate dehydrogenase (GDH) appears to b e the predominant route of ammonia assimilation irrespective of ammoni a concentration, and peptides modulate GDH activity in P. ruminicola. The physiological basis behind the difference between optimal ammonia concentrations for ruminal fibre digestion and microbial protein synth esis remains unclear. Molecular biology techniques extend beyond their application in pursuit of the 'superbug' concept, by offering new and exciting opportunities to understand better microbial physiology, div ersity, and ecology. Fundamental research in these areas must be conti nued if further advances in feed utilization and nutrient retention ar e to be realized.