Morphological Changes of Salmonella typhimurium Caused by Electrical Stimulation in Various Salt Solutions

To better understand what physical changes occur in bacteria subjected to low-voltage, low-current electrical stimulation (ES), morphological changes in Salmonella typhimurium killed by ES were examined using electron microscopy. Cells (10⁷ CFU/ml) were suspended in 0.015 M NaCl, 0.015 M Na₂CO₃, or 0.005 M Na₃PO₄·12H₂O solutions and pulsed electric signals at 10 mA/cm² current, 1 kHz frequency and 50% duty cycle were applied until more than 90% of the cells were dead. In NaCl solutions, cells were swollen before collapsing into amorphous bodies. In Na₂CO₃ solutions, cells showed irregular surfaces and the cytoplasm became less dense at the centers of the cells. In Na₃PO₄ solutions, the cytoplasm was scattered into small aggregates within the cell and many fibrils were formed outside. The results of this research suggest that the mechanisms of destruction of S. typhimurium by ES depend on the salt used an electrolyte.