EFFECT OF HIGH-VOLTAGE ELECTRIC PULSES ON YEAST-CELLS - FACTORS INFLUENCING THE KILLING EFFICIENCY

The decisive factors determining the killing efficiency of single rect angular electric pulses of 4-28 kV cm(-1) amplitude and 1-300 mu s dur ation in Saccharomyces cerevisiae S6/1 are pulse amplitude and duratio n, cell size and growth phase, post-pulse temperature and medium condu ctivity. In S. cerevisiae, the minimum pulse duration ensuring substan tial killing is about 10 mu s, the minimum amplitude being about 2 kV cm(-1) The critical pulse-induced transmembrane breakthrough voltage i s 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 p ulse duration. Comparison of killing data on S. cerevisiae S6/1 with t hose on the smaller-celled Kluyveramyces lactis showed the killing pul se amplitude to be roughly proportional to cell size except for low pu lse amplitudes, at which smaller cells are much more killing-prone. In exponential S. cerevisiae cells increased pulse amplitude caused a sh arp increase in killing while in stationary cells this effect was much lower and occurred only at pulse amplitudes above 15-20 kV cm(-1). El evated post-pulse temperature lowered the killing rate whereas lowered temperature promoted it, probably by affecting the pore resealing. Lo wering medium conductivity from 66 to 46 mu S m(-1) by suspension wash ing reduced the killing rate by 6-20%. Reproducible killing or electro poration therefore requires standardized cell concentration, and numbe r of cell washings.