Resting cells of a wild and a recombinant strain of Escherichia coli were used for D(+)-carnitine racemisation into L(-)carnitine. A scheme of cell reuse and cell regeneration/biotransformation for enlarging the production process and reducing process costs was established. For the recombinant strain high levels Of D(+)-carnitine racemase activity were obtained by growing the cells under anaerobic conditions at 41 degreesC. The biotransformation process depended on both cell and substrate concentrations. The addition of fumarate as an energy providing substrate during the cell resting state improved the biotransformation process at high substrate concentrations. In this way, the use of 500 mmol/l D(+)-carnitine and 200 mmol/l fumarate provided 28.2 and 36.3 g/l L(-)-carnitine (35% and 41% product yield and 3.50 and 1.45 g/l h L(-)-carnitine productivity) for the recombinant and wild strains, respectively. A storage study showed that resting cells retained 70% of their initial biotransformation capacity after 2 months storage at 4 degreesC. Resting cells could be reused in a biotransformation/regeneration system, cutting the overall process time by approximately 70 and 25% for the transformed and wild strain, respectively. (C) 2003 Elsevier Science Ltd. All rights reserved.