Reducing Escherichia coli Viability through Ultrasound Exposure

Ultrasonic waves can induce inertial cavitation in gas bubbles dissolved in liquid. The shock waves created by the rapidly imploding bubbles can stress living cells in solution via mechanical shearing forces. It has been proposed that ultrasound exposure may be an effective way to kill bacterial cells, possibly with applications in equipment or water sterilization. Such efficacy would be a direct result of mechanical lysis, with secondary effects including thermal and free radical challenge. Prior results from our studies have demonstrated that cells die from the inability to recover from a number of biochemical stresses. With the particular apparatus used for these experiments, which is a more modular and therefore more portable version than those used previously, there was a noticeable and reproducible difference in cell viability between controls and ultrasound-exposed cells in any phase of growth. In this investigation, Escherichia coli cells in constant temperature liquid Luria-Bertani (LB) medium were exposed to 1.15 MHz ultrasound waves of varying intensity and duration in the presence of artificial microbubbles. Cell viability was enumerated after plating serial dilutions on LB agar. Ultrasound-exposed cells had a mean percent survival of 13% ± 4.8% compared to control cells (N = 18). The difference in survivability was statistically significant by a paired t-test (p < 0.0001). Experiments investigating the effects of ultrasound on other genera, such as Deinococcus, were also conducted. Future investigations could attempt to determine at what point bacterial cells become resistant to ultrasound, and whether such a point reflects a threshold effect or a gradual susceptibility to killing.