Osmotic stress, glucose transport capacity and consequences for glutamate overproduction in Corynebacterium...

Glucose uptake by Corynebacterium glutamicum is predominantly assured by a mannose phosphotransferase system (PTS) with a high affinity for glucose (Km=0.35 mM). Mutants selected for their resistance to 2-deoxyglucose (2DG) and lacking detectable PEP-dependent glucose-transporting activity, retained the capacity to grow on media in which glucose was the only carbon and energy source, albeit at significantly diminished rates, due to the presence of a low affinity (Ks=11 mM) non-PTS uptake system. During growth in media of different osmolarity, specific rates of glucose consumption and of growth of wild type cells were diminished. Cell samples from these cultures were shown to possess similar PTS activities when measured under standard conditions. However, when cells were resuspended in buffer solutions of different osmolarity measurable PTS activity was shown to be dependent upon osmolarity. This inhibition effect was sufficient to account for the decreased rates of both sugar uptake and growth observed in fermentation media of high osmolarity. The secondary glucose transporter was, however, not influenced by medium osmolarity. During industrial fermentation conditions with accumulation of glutamic acid and the corresponding increase in medium osmolarity, similar inhibition of the sugar transport capacity was observed. This phenomenon provokes a major process constraint since the decrease in specific rates leads to an increasing proportion of sugar catabolised for maintenance requirements with an associated decrease in product yields.