1887

Abstract

, a nonruminal cellulolytic mesophilic bacterium, was grown in batch and continuous cultures on cellulose using a chemically defined medium. In batch culture with unregulated pH, less cellulose degradation and higher accumulation of soluble glucides were obtained compared to a culture with the pH controlled at 72. The gain in cellulose degradation achieved with pH control was offset by catabolite production rather than soluble sugar accumulation. The pH-controlled condition improved biomass, ethanol and acetate production, whereas maximum lactate and extracellular pyruvate concentrations were lower than in the non-pH-controlled condition. In a cellulose-fed chemostat at constant dilution rate and pH values ranging from 74 to 62, maximum cell density was obtained at pH 70. Environmental acidification chiefly influenced biomass formation, since at pH 64 the dry weight of cells was more than fourfold lower compared to that at pH 70, whereas the specific rate of cellulose assimilation decreased only from 1174 to 1013 milliequivalents of carbon (g cells) h. The molar growth yield and the energetic growth yield did not decline as pH was lowered, and an abrupt transition to washout was observed. Decreasing the pH induced a shift from an acetate-ethanol fermentation to a lactate-ethanol fermentation. The acetate/ethanol ratio decreased as the pH declined, reaching close to 1 at pH 64. Whatever the pH conditions, lactate dehydrogenase was always greatly in excess. As pH decreased, both the biosynthesis and the catabolic efficiency of the pyruvate-ferredoxin oxidoreductase declined, as indicated by the ratio of the specific enzyme activity to the specific metabolic rate, which fell from 98 to 18. Thus a change of only 1 pH unit induced considerable metabolic change and ended by washout at around pH 62. appeared to be similar to rumen cellulolytic bacteria in its sensitivity to acidic conditions. Apparently, the cellulolytic anaerobes studied thus far do not thrive when the pH drops below 60, suggesting that they evolved in environments where acid tolerance was not required for successful competition with other microbes.

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2001-06-01
2019-10-14
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