ULTRASTRUCTURAL-CHANGES IN STAPHYLOCOCCUS-AUREUS TREATED WITH PULSED ELECTRIC-FIELDS

Early stationary phase cells of Staphylococcus aureus were inoculated into a model food, simulated milk ultrafiltrate (SMUF) and subjected t o 16, 32, and 64 pulses at electric field intensities of 20, 40 and 60 kV/cm at 13 degrees C. In addition temperatures of 20, 25 and 30 degr ees C were also tested with 32 pulses and an electric field of 60 kV/c m. The temperature of the SMUF increased by 1-2 degrees C at the end o f the 64 pulses. Cells subjected to 64 pulses at 20, 40 and 60 kV/cm w ere observed for ultrastructural changes using scanning and transmissi on electron microscopy techniques. The cell surface was rough after tr eatment with electric field when observed by scanning electron microsc opy (SEM). The cell wall was broken, and the cytoplasmic contents were leaking out of the cell after exposure to 64 pulses at 60 kV/cm when observed by transmission electron microscopy (TEM). The breaking of th e cell wall is an indication of electro-mechanical breakdown of the ce ll. The increase in inactivation with an increase in the electric fiel d strength can be related to the increase in the damage to the cells. Cells subjected to 32 pulses at 60 kV/cm and 13, 20 or 25 degrees C we re compared microscopically with the untreated control cells. Cells su bjected to heat treatment (10 min, at 66 degrees C) were compared with electric field-treated and untreated control cells. Although importan t changes were observed in the protoplast, no cell wall breakdown was observed in heat-treated cells when compared to the electric field-tre ated cells. This result indicates a different mechanism of inactivatio n of cells with heat treatment.