DE-NOVO SYNTHESIS OF AMINO-ACIDS BY THE RUMINAL BACTERIA PREVOTELLA-BRYANTII B(1)4, SELENOMONAS-RUMINANTIUM HD4, AND STREPTOCOCCUS-BOVIS ES1

The influence of peptides and amino acids on ammonia assimilation and de novo synthesis of amino acids by three predominant noncellulolytic species of ruminal bacteria, Prevotella bryantii B(1)4, Selenomonas ru minantium HD4, and Streptococcus bovis ES1, was determined by growing these bacteria in media containing (NH4Cl)-N-15 and various additions of pancreatic hydrolysates of casein (peptides) or amino acids. The pr oportion of cell N and amino acids formed de novo decreased as the con centration of peptides increased. At high concentrations of peptides ( 10 and 30 g/liter), the incorporation of ammonia accounted for less th an 0.16 of bacterial amino acid N and less than 0.30 of total N. At I g/liter, which is more similar to peptide concentrations found in the rumen, 0.68, 0.87, and 0.46 of bacterial amino acid N and 0.83, 0.89, and 0.61 of total N were derived from ammonia by P. bryantii, S. rumin antium, and S. bovis, respectively. Concentration-dependent responses were also obtained with amino acids. No individual amino acid was exha usted in any incubation medium. For cultures of P. bryantii, peptides were incorporated and stimulated growth more effectively than amino ac ids, while cultures of the other species showed no preference for pept ides or amino acids, Apparent growth yields increased by between 8 and 57%, depending on the species, when 1 g of peptides or amino acids pe r liter was added to the medium. Proline synthesis was greatly decreas ed when peptides or amino acids were added to the medium, while glutam ate and aspartate were enriched to a greater extent than other amino a cids under all conditions. Thus, the proportion of bacterial protein f ormed de novo in noncellulolytic ruminal bacteria varies according to species and the form and identity of the amino acid and in a concentra tion-dependent manner.