A defined medium was developed which, by means of a specific fed-batch mode, allows growth of the recombinant Escherichia coli strain TG1 (pBB210) up to a cell density of 60 g dry weight/l. Apart from glucose and aqueous ammonia fed as carbon and nitrogen sources, it was necessary to supply other nutrients or O2-enriched air. Aqueous ammonia also served for pH control. The pO2 level was kept at 20% saturation via closed-loop controls operating the two output variables of stirrer speed and glucose feeding rate. This fed-batch method prevented significant accumulation of acetate and other metabolic by-products. The recombinant E. coli expressed interferon alpha 1 more efficiently at a lower specific growth rate (muPr approximately 0.15 h-1) than at the maximum specific growth rate (mu max = 0.45 h-1). Therefore, fermentation in the batch phase at mu max was only allowed to continue up to a medium cell density. In the succeeding fed-batch phase, the specific growth rate was reduced to muPr by increasing the stirrer speed according to an empirically developed time scale.
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