Inactivation of Escherichia coli, Saccharomyces cerevisiae and Bacillus subtilis by ultrasonic cavitation

In recent years, inactivation (sterilization) of bacteria using ultrasonic waves has attracted significant attention. However, the details of the inactivation mechanisms have not been elucidated. This study aims to clarify the inactivation mechanisms of bacteria and fungi by ultrasonic cavitation. Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) were used as the test bacteria, and Saccharomyces cerevisiae (S. cerevisiae) as the test fungi. Inactivation was attempted by ultrasonic irradiation at frequencies of 20 kHz to 4.4 MHz and an acoustic power density of 0.1 W/mL. Different frequency dependences of the inactivation were confirmed in E. coli, S. cerevisiae, and B. subtilis. The highest inactivation rate in E. coli was observed at 430 kHz. The effect of ultrasonic cavitation on E. coli was examined. We investigated the inactivation rate of E. coli when the sonochemical efficiency was kept constant at different frequencies (200, 430, and 950 kHz) by adjusting the acoustic power. The inactivation rates at different frequencies showed a similar time dependence. In contrast, B. subtilis spores were inactivated after increasing the power density. Based on these data, the mechanism of inactivation is discussed with a focus on cell characteristics.