Carbohydrate utilization by Clostridium thermocellum: Importance of internal pH in regulating growth

Abstract

Clostridium thermocellum can utilize cellulose, cellobiose, glucose, and fructose as carbon sources, although initial growth on the monosaccharides from an inoculum grown on cellobiose occurs only after a long lag period of 180–200 h. The lag phase, during which no growth occurs, cannot be due to lack of uptake and phosphorylation of glucose and fructose by cellobiose-grown cells as measured by 13C- and 31P-NMR spectroscopy. ATP levels and Δψ, the transmembrane electrical potential, are comparable in growing cells and lag phase cells. Of the parameters measured, the only significant difference between cellobiose-grown cells incubated with monosaccharides versus such cells incubated with cellobiose is that the internal pH (and the pH gradient) are lower in the nongrowing cultures. Assuming that the value of the internal pH (or ΔpH) is critical for growth, the following alternative hypotheses are proposed: either the breakdown and internalization of the disaccharide cellobiose contributes to the formation of the essential pH gradient, or the transport of glucose or fructose via proton symport dissipates the pH gradient and effectively prevents growth of these cells .