Effect of high-voltage electric pulses on yeast cells: factors influencing the killing efficiency

The decisive factors determining the killing efficiency of single rectangular electric pulses of 4–28 kV cm⁻¹ amplitude and 1–300 μs duration in Saccharomyces cerevisiae S6/1 are pulse amplitude and duration, cell size and growth phase, post-pulse temperature and medium conductivity. In S. cerevisiae, the minimum pulse duration ensuring substantial killing is about 10 μs, the minimum amplitude being about 2 kV cm⁻¹. The critical pulse-induced transmembrane breakthrough voltage is 0.75 V. A pulse-induced increase in membrane permeability for small species such as inorganic ions suffices to cause cell death. A preset killing rate can be achieved by varying pulse amplitude inversely to pulse duration. Comparison of killing data on S. cerevisiae S6/1 with those on the smaller-celled Kluyveramyces lactis showed the killing pulse amplitude to be roughly proportional to cell size except for low pulse amplitudes, at which smaller cells are much more killing-prone. In exponential S. cerevisiae cells increased pulse amplitude caused a sharp increase in killing while in stationary cells this effect was much lower and occurred only at pulse amplitude above 15–20 kV cm⁻¹. Elevated post-pulse temperature lowered the killing rate whereas lowered temperature promoted it, probably by affecting the pore resealing. Lowering medium conductivity from 66 to 46 μS m⁻¹ by suspension washing reduced the killing rate by 6–20%. Reproducible killing or electroporation therefore requires standardized cell concentration, and number of cell washings.