The effect of multiple, microsecond electrical pulses on bacteria

Pulsed electric fields are widely used for bacterial decontamination of water and liquid food [1]. We have explored the effect of microsecond pulses of 13 kV/cm and 15 kV/cm electric field amplitude on the viability of Escherichia coli. Varied was the pulse duration from 4 \is to 32 μs, and the number of 4 μs pulses from one to eight, and finally the effect of the separation between pulses was studied. The pulse generator utilizes IGBTs as closing and opening switches. The maximum voltage was 1.5 kV, at a maximum current of 160 A. The load was a cuvette with plane 1 cm2 aluminum electrodes, 1 mm apart and filled with a solution (LB Broth), which contained E. coli bacteria at a concentration of approximately 3×l0⁵ cells/ml. The viability of the E. coli after electric field application was measured by using manual counting for E, coli colonies in cultured agars. The bacterial decontamination rate was found to increase with pulse duration up to 8 us and then to decrease again. This maximum in decontamination rate is assumed to be due to dipole formation in the rod shaped bacteria. It causes a reorientation of the E. coli into the direction of the electric field and consequently a higher possibility for irreversible damage. The experimental results with varying temporal separation between two subsequent pulses show that independent of field amplitude (in the relatively narrow range of 13 kV/cm to 15 kV/cm) the decontamination rate decreases logarithmically with increasing separation. The decay time constant was found to be approximately 10 ms. This effect is assumed to be due to the randomization of the axes-directions of the rod-shaped bacteria in the broth after exposed to an electric pulse.