Effect of pH and energy spilling on bacterial protein synthesis by carbohydrate-limited cultures of mixed rumen bacteria.

Rumen contents were obtained from a cow fed 2.5 kg concentrates and 2.5 kg timothy hay twice daily, and the mixed rumen bacterial inoculum (pH 6.3) was separated from large feed particles and protozoa by centrifugation. Bacteria were incubated in artificial media at pH 6.7 and 6.0. When starch, sucrose, cellobiose, xylan, pectin, or a mix of carbohydrates were provided to pH 6.7 incubations at 1 mM/h for 10 h, fermentation was carbohydrate-limited, less than 20% of the carbohydrate was converted to lactate, molar ratio of methane to hexose fermented was as great as 49%, and pH remained constant. When initial pH was lowered to 6.0, methane production was nearly eliminated, volatile fatty acid production was reduced, and there was a small decrease (less than .5) in final pH. At pH 6.0, lactate production was increased in all incubations except xylan and pectin, and pH had a greater effect on acid production than the type of carbohydrate provided. Bacterial protein synthesis was reduced 34 to 69% when initial pH was 6.0, but these reductions were greater than decreases in carbohydrate utilization, increases in lactate, and associated decreases in adenosine 5'-triphosphate production. Because less adenosine 5'-triphosphate was used for protein synthesis, it appeared that low pH diverted energy to nongrowth functions.