Minimization of treatment time for in vitro 1.1 MHz destruction of Pseudomonas aeruginosa biofilms by high-intensity focused ultrasound

Medical implants are prone to colonization by bacterial biofilms. Normally, surgery is required to replace the infected implant. One promising noninvasive modality is to destroy biofilms with high-intensity focused ultrasound. In our study, Pseudomonas aeruginosa biofilms were grown on implant-mimicking graphite disks in a flow chamber for 3 days prior to exposing them to ultrasound pulses. Exposure time at each treatment location was varied between 5, 15 and 30 s. Burst period was varied between 1, 3, 6 and 12 milliseconds (ms). The pulses were 20 cycles in duration at 1.1 MHz from a spherically focused transducer (f/1, 63 mm focal length), creating peak compressional and rarefactional pressures at the graphite disk surface of 30 and 13 MPa, respectively. P. aeruginosa were tagged with green fluorescent protein, and killed cells were visualized using propidium iodide before determining the extent of biofilm destruction. The exposure-induced temperature rise was measured to be less than 0.2 °C at the focus, namely the interface between graphite disk and water. Then, the temperature rise was measured at the focus between the graphite disk and a tissue-mimicking phantom to evaluate therapy safety. Two thresholds, of bacteria destruction increase and of complete bacteria removal, respectively, were identified to divide our eight exposure conditions. Results indicated that 30-s exposure and 6-ms pulse period were sufficient to destroy the biofilms. However, the 15-s exposure and 3-ms pulse period were viewed as optimum when considering exposure time, efficacy, and safety.