Regulation of lactate production in Streptococcus bovis: A spiraling effect that contributes to rumen acidosis

When Streptococcus bovis was grown in batch culture with 6 g/L glucose at pH 6.7, maximum specific growth rate was 1.47 h(-1), and lactate was the primary fermentation product. In continuous culture at pH 6.7 and growth rate equal to .10 h(-1), little lactate was formed, and formate, acetate, and ethanol accounted for most of the product. When extra-cellular pH decreased to 4.7, intra-cellular pH declined to 5.4, and organisms switched back to lactate production. Intracellular concentration of fructose 1,6-diphosphate of batch culture cells was greater than 12 mM, a concentration that allowed maximal lactate dehydrogenase activity. When Streptococcus bovis was grown in continuous culture at pH 6.7, intracellular fructose-l,6-diphosphate declined to .4 mM, a concentration which gave little lactate dehydrogenase activity at pH 6.5 or greater. Decreasing pH of continuous culture to 4.7 increased intracellular fructose-1,6-diphosphate concentration to .8 mM. This concentration was still limiting if lactate dehydrogenase was assayed at pH 6.5, but nearly maximal activity was obtained when enzyme was assayed at pH 5.5. The small increase in fructose-l,6-diphosphate and decreased requirement of lactate dehydrogenase for fructose-l,6-diphosphate under acidic assay conditions, accounted for increased lactate production during low pH (4.7) continuous culture. These and other aspects of lactate regulation by Streptococcus bovis are discussed as factors leading to rumen acidosis. This pattern of regulation also helps to explain why rumen acidosis is difficult to reverse.