Effect of carbohydrate limitation on degradation and utilization of casein by mixed rumen bacteria

Mixed rumen bacteria were incubated in media containing salts, ammonia, vitamins, volatile fatty acids, sulfide, and casein. When initial cell density was 1.0 absorbance unit and mixed carbohydrates (glucose, sucrose, maltose, cellobiose, and soluble starch) were provided at 0, 40, 80, and 160 mg/liter per h, cell growth was limited by carbohydrates, average bacterial growth rate was slow (less than .07/h), and types of bacteria did not appear to change during 7 h. Growth was small if casein was the sole source of energy. Addition of casein to incubations fed carbohydrates caused cell yield to double. Casein hydrolysis was accompanied by marked accumulation of peptides that were metabolized slowly by rumen bacteria. High pressure liquid chromatography indicated that the peptide pool was composed primarily of four fractions. Carbohydrate availability or bacterial growth had little influence on proteolysis or peptide accumulation. Ammonia production was always inversely proportional to rate of carbohydrate feeding. Nitrogen-15 labeling studies indicated that 66% of the cell nitrogen was derived directly from casein at all rates of carbohydrate addition. If average bacterial growth rate was approximately .07/h, little casein entered the ammonia pool even though large amounts of peptide or casein remained undegraded.