Regulation of Respiratory and Fermentative Modes of Growth of Citrobacter freundii by Oxygen, Nitrate and Glucose

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Abstract
The common brewery contaminant Citrobacter freundii has been used to investigate how oxygen, nitrate and glucose concentrations determine the growth rates, cell yields, the flux of metabolites and the rates of synthesis of tricarboxylic acid cycle enzymes and terminal electron transfer pathways. The growth rate was inhibited for 4 or 8 h when anaerobic batch cultures were either supplemented with nitrate or aerated, respectively. The subsequent exponential growth rates and yield coefficients were greater than in unsupplemented anaerobic cultures, but glucose was only partially oxidized to organic acids even in the most vigorously aerated cultures. The least active enzyme of the tricarboxylic acid cycle was succinate dehydrogenase, and although there were large differences in individual cytochrome concentrations and NADH oxidase activity between aerated and anaerobic cultures, only small differences in 2-oxoglutarate dehydrogenase activity were detected. Similar results were obtained with sulphate-limited continuous cultures, but in a glucose-limited continuous culture, succinate dehydrogenase activity was derepressed 12-fold and the yield coefficient increased 7-5-fold during aerobic growth. It is concluded that the major determinant of the potential tricarboxylic acid cycle activity is catabolite repression of succinate dehydrogenase synthesis rather than oxygen induction of 2-oxoglutarate dehydrogenase. Synthesis of 2-oxoglutarate dehydrogenase was derepressed, possibly by glutamate or by traces of oxygen in the gas used to sparge the culture during anaerobic growth.