When growing aerobically in glucose-sufficient chemostat environments, Klebsiella aerogenes NCTC 418 consumes carbon substrate and oxygen at high rates and excretes into the medium a range of intermediary metabolites in amounts that vary with the nature of the growth limitation (Neijssel and Tempest, 1975). This pattern of behaviour has been rationalized in terms of specific bottlenecks in metabolism imposed by the particular nutrient limitation, and of special requirements for energy and key intermediates needed to scavenge traces of limiting nutrient from the environment (Neijssel and Tempest, 1975). However, in this latter case it is by no means clear how the excess energy that is generated (presumably as ATP) is turned over.
To obtain further insight into the physiological significance of this overflow metabolism, studies were undertaken into the behaviour of K. aerogenes when growing anaerobically on glucose. Here additional constraints are imposed upon the cell's metabolism in that (a) only a small fraction of the energy content of the energy source can be made available, (b) the excess reducing equivalents must be disposed of and (c) fermentation products accumulate in the environment to levels that might exert some influence on growth and metabolism.
It was found that the most efficient conversion of glucose to biomass occurred under glucoselimited conditions and that here only acetate and ethanol (plus CO 2 and formate) were formed. Under all glucose-sufficient conditions D-lactate, 2,3-butanediol and succinate also were excreted and the Yglu values were substantially lowered.
Increasing the input glucose concentration to a glucose-limited culture resulted in a shift in fermentation towards 2,3-butanediol and D-lactate with a decrease in Yglu-This effect could not be ascribed to accumulation of any of the fermentation products, since adding these products to the medium did not effect the changes observed above. However, a negative correlation was seen between Yglu values and the partial CO2-pressure in the culture vessel.
When a glucose-limited culture was suddenly relieved of its growth limitation the specific rate of glucose uptake almost doubled instantaneously without a concomitant increase in growth rate. This extra glucose was fermented to D-lactate, 2,3-butanediol and succinate. This suggests that either energy-spilling reactions were invoked or that glucose was dissimilated without net ATP synthesis. In connection with this, we found that K. aerogenes is able to form D-lactate via the methylglyoxal bypass (Cooper, 1975). Moreover, it could be calculated that, by invoking this bypass, ATP synthesis could be completely uncoupled from glucose dissimilation, thereby allowing the cells to dissociate catabolism from anabolism.