Inactivation of Planktonic Escherichia coli by High Intensity Focused Ultrasound pulses

This study was motivated by the desire to develop HIFU-based abscess treatments and addresses inactivation of E. coli in volumes up to 10 mL. Cells were treated with 1-MHz nonlinear pulsed HIFU (10 cycles, 2 kHz repetition frequency, +65/−12.8 MPa focal pressures, up to 65 MPa shock fronts). Surviving fraction [SF] was assessed by coliform assay demonstrating curvilinear inactivation kinetics. Reduction of SF to 50% required 2.5 or 6 min in 5 or 10 ml samples, respectively. On light microscopy, surviving cells from 5 min exposures appeared normal, with minimal debris; after 20 min, debris dominated. TEM images of insonated samples showed some undamaged cells, a few damaged but largely intact cells, and comminuted debris. Cellular damage associated with substantive but incomplete levels of inactivation can be subtle. This study demonstrates the feasibility of using pulsed HIFU for partial disinfection of liquid volumes.