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Regulation of fermentation in a ruminal bacterium, Streptococcus bovis, with special reference to rumen acidosis

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When high-starch diets are fed to ruminants, Streptococcus bovis often predominates in the rumen and produces lactate as the major fermentation product. Lactate production by S. bovis is regulated by the activity ratio of lactate dehydrogenase (LDH) to pyruvate formate-lyase (PFL) in response to an energy supply or the intracellular pH. The activities of LDH and PFL are affected not only by the amounts of enzyme proteins, but also by the concentrations of allosteric effectors, such as fructose-1, 6-bisphosphate (FBP) and triose phosphates. Synthesis of LDH and PFL is regulated at the transcriptional level in opposing directions. The inverse relationship between LDH and PFL synthesis amplifies a change in the proportion of lactate in the total fermentation products. A high capacity to regulate lactate production in response to low pH allows S. bovis to grow at low pH and as one of the most acid-tolerant bacteria among the prevailing ruminal bacteria, S. bovis also has a high capacity to extrude intracellular H+ by an F1F0 H+-ATPase, which is important for acid tolerance. In addition, S. bovis is able to augment H+-ATPase synthesis at the transcriptional level in response to low pH. The activities of H+-ATPase and pyruvate kinase are enhanced by FBP and strongly inhibited by inorganic phosphate (Pi). Intracellular FBP and Pi concentrations affect sugar metabolism in S. bovis through regulation of the expression of catabolic genes and controlling the activity of sugar transport systems. Synthesis of the enzymes involved in sugar transport and fermentation appears to be regulated by the catabolite control protein A (CcpA) –phosphotransferase system (PTS) system.
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Keywords: Streptococcus bovis; fermentation; lactate production; rumen acidosis; rumen bacteria

Document Type: Review Article

Affiliations: Department of Life Science, Meiji University, Tama-ku, Kawasaki-shi, Japan

Publication date: October 1, 2002

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