Nano-second pulse electric field has both a lethal and a sub-lethal effect on E. coli K12

Summary form only given. Pulsed electric fields (PEF) of nanosecond duration offer an interesting new approach to the bacterial decontamination of liquids. We have designed, fabricated and tested a nanosecond PEF device in our laboratory. The device was operated at a constant electrical field intensity of 100 kV/cm with pulse durations of 32 ns and a repetition rate of 30 pulses per second. E. coli K12 cell suspensions in buffered solution in 0.1 ml cuvettes were exposed to nanosecond pulse trains of up to 5 minutes (9000 pulses). Bacterial inactivation was assessed by means of plating onto both ordinary tryptone soya agar (TSA), and onto TSA supplemented with either 2, 3 or 4% (w/v) NaCl. The latter imposes a stress on treated cells and only those cells that are wholly uninjured are able to grow at the highest concentration of NaCl used here. By conducting counts both 24 and 48 hrs after plating, we were also able to assess the length of time which sub-lethally injured cells required to recover from their injuries. We observed a distribution of both lethal and sub-lethal injury throughout the period of exposure to nanosecond PEF. Our results point to the accumulation of injurious events over time as the mechanism of cell death and suggest that once a threshold of damage is exceeded cells become non-viable. This would appear to be at odds with the previously widely held view that exposure of microbial cells to nanosecond pulses leads to irreversible membrane breakdown. These findings may have implications for the types of liquids that can successfully be treated by this technique