The capacity of Escherichia coli to adapt its catabolism to prevailing redox conditions resides mainly in three catabolic branch points involving (i) pyruvate formate-lyase (PFL) and the pyruvate dehydrogenase complex (PDHc), (ii) the exclusively fermentative enzymes and those of the Krebs cycle, and (iii) the alternative terminal cytochrome bd and cytochrome bo oxidases. A quantitative analysis of the relative catabolic fluxes through these pathways is presented for steady-state glucose-limited chemostat cultures with controlled oxygen availability ranging from full aerobiosis to complete anaerobiosis. Remarkably, PFL contributed significantly to the catabolic flux under microaerobic conditions and was found to be active simultaneously with PDHc and cytochrome bd oxidase-dependent respiration. The synthesis of PFL and cytochrome bd oxidase was found to be maximal in the lower microaerobic range but not in a ⌬ArcA mutant, and we conclude that the Arc system is more active with respect to regulation of these two positively regulated operons during microaerobiosis than during anaerobiosis.Escherichia coli possesses distinct catabolic routes that enable it to conserve energy efficiently under wide ranges of redox conditions. In environments that provide the cell with external electron acceptors such as oxygen, nitrate, fumarate, and dimethyl sulfoxide, reoxidation of reducing equivalents generated by the oxidation of the energy source occurs in the respiratory chain. This process can be coupled to the formation of a proton motive force (PMF) and thus constitutes an efficient pathway for energy conservation. In the absence of oxygen or other external electron acceptors, ATP synthesis occurs at the level of substrate phosphorylation. Under such fermentative conditions E. coli, when growing on glucose, excretes specific products such as ethanol, acetate, lactate, succinate, and formate (or CO 2 and H 2 ). The relative rates of formation of these products are governed by the demand for redox neutrality (5, 21).The actual in vivo fluxes of carbon and electrons through the various pathways are determined largely by three major branch points (Fig. 1). The first of these involves the cleavage of pyruvate, which serves as a common substrate for pyruvate formate-lyase (PFL) and the pyruvate dehydrogenase complex (PDHc). Entry into the fermentative pathway depends largely on the activity of PFL, whereas entry into the respiratory pathway is largely governed by the activity of the PDHc. At the second branch point, acetyl-coenzyme A, the product of both of the above reactions, can be converted to either the major fermentation products acetate and ethanol or can subsequently undergo further oxidation in the tricarboxylic acid (TCA) cycle. Finally, E. coli also possesses a branched respiratory chain. The electron flow into respiration can follow alternative routes to oxygen, via a coupled or an uncoupled NADH dehydrogenase (NDH-1 or NDH-2, respectively) to quinone (19,42). Quinol is then oxidized by either the cytochrome bd or th...