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The activity of permeabilized cells of Escherichia coli strains during the biotransformation of trimethylamonium compounds, such as crotonobetaine and d-carnitine, into l-carnitine was determined. Permeabilization of cells did not affect the biotransformation of d-carnitine but improved the yield of l-carnitine obtained when crotonobetaine was used as biotransformation substrate (by nearly two-fold). A higher degree of cell activity with respect to control (45% yield) was observed with organic solvents such as acetone, ethanol, isopropyl alcohol and toluene (65–70% yield), detergents such as Triton X-100 and Tween 20 (70–75% yield) and polyethylenimine (80–89% yield). However, maximum cell activity was attained with 2% of the detergents, Triton X-100 and Tween 20, and 0.1% of polyethylenimine, using a temperature of 37°C and 10–30min of treatment time, in which case a yield of 75–89% (100% increase) was …