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The aim was to understand how interaction of the central carbon and the secondary carnitine metabolisms is affected under salt stress and its effect on the production of L‐carnitine by Escherichia coli. The biotransformation of crotonobetaine into L‐carnitine by resting cells of E. coli O44 K74 was improved by salt stress, a yield of nearly twofold that for the control being obtained with 0.5 M NaCl. Crotonobetaine and the L‐carnitine formed acted as an osmoprotectant during cell growth and biotransformation in the presence of NaCl. The enzyme activities involved in the biotransformation process (crotonobetaine hydration reaction and crotonobetaine reduction reaction), in the synthesis of acetyl‐CoA/acetate (pyruvate dehydrogenase, acetyl‐CoA synthetase [ACS] and ATP/acetate phosphotransferase) and in the distribution of metabolites for the tricarboxylic acid cycle (isocitrate dehydrogenase [ICDH]) and glyoxylate …