Metabolic response of Bacillus stearothermophilus chemostat cultures to a secondary oxygen limitation

Abstract

With variously limited chemostat cultures of Bacillus s teawthermophilus, the glucose consumption rate increased markedly as the concentration of dissolved oxygen (d.o.t.; dissolved oxygen tension) was lowered from 50% to 1 % air saturation. Concomitantly, the specific rate of acetate production increased and lactate, which was not present in the fully aerobic cultures, accumulated in large amounts. Moreover, whereas at a high d.o.t. only an ammonia- limited culture excreted 2-oxoglutarate, all glucose-sufficient cultures excreted this metabolite at a d.o.t. of 1 % air saturation, even more being produced by a K+-limited culture than by the ammonia-limited one. The activities of those enzymes of glycolysis that were measured increased in parallel with the glucose consumption rate, as did the activities of enzymes of the Entner-Douderoff pathway. Similarly, the activities of lactate dehydrogenase and acetate kinase (which were synthesized constitutively also) reflected the corresponding metabolite production rates. Tricarboxylic acid (TCA) cycle activity markedly diminished with a lowering of the available oxygen supply and again (with the exception of aconitase and 2-oxoglutarate dehydrogenase) this was mirrored in decreases in the activities of TCA cycle enzymes. Assessments of energy flux in terms of ATP equivalents suggested that it was energetically more expensive to synthesize biomass at a low d.o.t. than at a high one. However, the presence of enzymes of the methylglyoxal bypass (methylglyoxal synthase and glyoxylase) at high activities in cells grown at a low d.o.t. render assessments of ATP flux rates unreliable.