Abstract

Osmotic stress constitutes a major bacterial stress factor in the soil and during industrial fermentation. In this paper, we quantified the metabolic response, in terms of metabolic flux redistribution, of a lysine-overproducing strain of Corynebacterium glutamicum grown under continuous culture, to gradually increasing osmolality. Oxygen and carbon dioxide evolution rates, and the changes in concentration of extracellular, as well as intracellular, metabolites were measured throughout the osmotic gradient. The metabolic fluxes were estimated from these measurements and from the mass balance constraints at each metabolite-node of the assumed metabolic reaction network. Our results show that formation rates of compatible solutes - trehalose first and proline at a later stage of the gradient - increased with osmotic stress to equilibrate the external osmotic pressure. Estimated flux distributions indicate that the observed increase in the glucose specific uptake rate with osmotic stress is channeled through the main energy generating pathways - glycolysis and the tricarboxylic acid cycle - while the flux through the pentose phosphate pathway remains constant throughout the gradient. This results in a significant increase in the net specific ATP production rate, which may possibly be used to support the higher energy requirements required for cellular maintenance at high osmolalities. Finally, nodal analysis confirmed that the PEP/pyruvate node is essentially rigid and that the glucose-6-phosphate, oxaloacetate and ?-ketoglutarate nodes are flexible and therefore adaptable to changes in osmotic pressure in C. glutamicum.