A comparison of the chemical composition of mixed bacteria harvested from the liquid and solid fractions of rumen digesta.

Steers, equipped with simple rumen cannulas, were given diets of approximately equal parts of rolled barley and straw supplemented with urea. The diets provided sufficient estimated rumen degradable nitrogen (RDN; RDN:metabolizable energy values of 1:3) to maintain maximum microbial synthesis. In some experiments Na235SO4 was introduced into the rumen to label microbial protein. Rumen digesta samples were taken before feeding and mixed rumen bacteria were separated from the solid (solid-associated bacteria; SAB) and liquid (liquid-associated bacteria; LAB) fractions of digesta. The most effective method of removing SAB from the fibre was a combination of homogenizing and pummelling. This process did not affect the physical form or chemical composition of the bacteria. Samples of SAB contained significantly (P less than or equal to at least 0.05) less ash, total N, RNA and diaminopimelic acid (DAP) and significantly (P less than or equal to 0.01) more lipid than samples of LAB. Concentrations (g/kg dry matter) of ash, total N, RNA, DAP and lipid in SAB were approximately 87, 70, 35, 2.2 and 245 respectively. Corresponding values for LAB were 157, 80, 50, 3.8 and 124 respectively. RNA-N:total N and DAP-N:total N values in SAB were significantly lower than those in LAB (P less than or equal to 0.05 and 0.02 respectively). 35S:total N values were similar in both groups of bacteria. The importance of differences in constituent:total N values in the two groups of bacteria in relation to their use as indices of microbial protein synthesis is discussed.