The physiological state of an ethylenogenic Escherichia coli immobilized in hollow-fiber bioreactors

Primary metabolites can be efficiently produced in hollow-fiber bioreactors. In contrast, much poorer productivities have been reported for secondary metabolites. Synthesis of the secondary metabolite, ethylene, under oxygen-limited conditions in batch culture or in hollow-fiber bioreactors operating with low aeration was barely detectable. The need for oxygen has been demonstrated in the conversion of the intermediate 2-oxo 4-methylthiobutyrate (KMBA) to ethylene. By selecting direct mode operation and increasing the air flow to immobilized cells, productivities comparable to aerated batch culture could be achieved. The pattern of fermentation products detected by these cultures indicated that the supply of oxygen is the major factor influencing the product yields under the conditions used. When nitrate was employed as an alternative electron acceptor to oxygen, the E. coli strain grew well but only converted small amounts of methionine to intermediates on the ethylenogenic pathway. No ethylene could be detected in these cultures. Growth with methionine as the sole source of nitrogen was only possible when batch cultures were aerated, indicating that oxygen is needed for other aspects of methionine metabolism in addition to the conversion of KMBA to ethylene.