Inactivation of Planktonic Escherichia coli by Focused 2-MHz Ultrasound

This study was motivated by the desire to develop a noninvasive means to treat abscesses, and represents first steps toward that goal. Non-thermal, high intensity focused ultrasound (HIFU) was used to inactivate Escherichia coli (~1 × 109 cells/mL) in suspension. Cells were treated in 96-well culture plate wells using 1.95 MHz ultrasound and incident focal acoustic pressures as high as 16 MPa peak positive and 9.9 MPa peak negative (free field measurements). Surviving fraction was assessed by coliform culture, and by alamarBlue® assay. There was no biologically significant heating associated with ultrasound exposure. Bacterial inactivation kinetics were well described by a half-life model, with a half-time of 1.2 min. At the highest exposure levels, a two-log inactivation was typically achieved within ten minutes. The free field-equivalent peak negative acoustic pressure threshold for inactivation was ~7 MPa. At the highest acoustic pressures used, inactivation efficacy was insensitive to reciprocal changes in pulse length and pulse repetition frequency at constant duty factor. Although treated volumes were very small, proof of principal was provided by these experiments.