Development of a calibration function for optimal transformation efficiencies of Escherichia coli C600 and Bacillus subtilis 168

•Current density and conductivity of the bacteria suspension are determined.•Measurement of current density and conductivity are performed before and after electroporation.•A developed calibration function serves to determine a small range for optimum electric field.•Optimum electroporation can be received with much less attempts.

Transformation of Escherichia coli C600 and Bacillus subtilis 168 have been evaluated in dependence on the electric field and current density by changing the conductivity of the bacteria suspension. For both kinds of bacteria, E. coli C600 and B. subtilis 168, the trans-formation efficiency in dependence on the current density showed a clear shift of the transformation maximum according to the conductivity of the bacteria suspension. For higher conductivities, higher current densities are needed to reach the transformation maximum. Based on the measured conductivity of the bacteria suspension before and during the electroporation process a diagram was developed, which can be used to predict the electric field. It has to be applied for maximal transformation efficiencies of E. coli C600 with pDsRed-Express plasmid DNA (3311 base pairs), when using a voltage generator with an exponential decay (decay time: τ = 3 ms). This means, a transformation mixture of unknown composition (except for the kind of cells and nucleic acid) can be measured for its conductivity in the first place and treated with the resulting voltage. Experiments showed, that a similar calibration graphic could be developed for B. subtilis 168 with pBluescript KS II (+) (2961 base pairs).