Effect of Carbohydrate Limitation on Degradation and Utilization of Casein by Mixed Rumen Bacteria

Abstract

Mixed rumen [cow] bacteria were incubated in media containing salts, NH3, vitamins, volatile fatty acids, S2- and casein. When initial cell density was 1.0 absorbance unit 3 and mixed carbohydrates (glucose, sucrose, maltose, cellobiose and soluble starch) were provided at 0, 40, 80 and 160 mg/l per h, cell growth was limited by carbohydrates, average bacterial growth rate was slow (< 0.07/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 and 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 4 fractions. Carbohydrate availability or bacterial growth had little incidence on proteolysis or peptide accumulation. NH3 production was always inversely proportional to rate of carbohydrate feeding. 15N labeling studies indicated that 66% of the cell N was derived directly from casein at all rates of carbohydrate addition. If average bacterial growth rate was .apprx. 0.07/h, little casein entered the NH3 pool even though large amounts of peptide or casein remained undegraded.