Effects of Histotripsy on Bacterial Viability in Suspension

Abstract

This dissertation explores the bactericidal effects of histotripsy, a cavitation-based focused ultrasound therapy, on two bacteria in suspension namely Escherichia coli and Staphylococcus aureus, for potential application in abscess treatment. Abscesses are localized encapsulated collections of pus and bacteria that present a clinical challenge. Histotripsy can non-invasively reduce the bacterial burden in the abscess by rupturing bacterial cell walls via intense cavitation. To understand histotripsy process on bacteria in suspension, two regimens of histotripsy (shock-scattering histotripsy and boiling histotripsy) were applied in vitro on bacterial suspensions at frequencies ranging from 0.81 MHz to 3.25 MHz, at 1% duty cycle, and bacterial viability was quantified by the plate count assay. Cavitation was quantified by passive cavitation detection, plane-wave B-mode imaging and high-speed photography. Results showed that both regimens reduced E. coli counts significantly. Cavitation cloud size in shock-scattering histotripsy quantified by high-speed photography strongly predicted the E. coli inactivation. Subsequently, plane-wave B-mode imaging was applied to estimate cavitation cloud size enabling the prediction of E. coli inactivation rates in future clinical studies. On the other hand, Staphylococcus aureus was resistant to histotripsy treatment. Efforts were made to obtain bactericidal activity by de-clumping S. aureus clusters into individual cells, treating smaller volumes, and combining histotripsy with heating, with negligible response. Finally, an investigation was conducted to determine the effect of bacterial shape and gram status on their resistance to histotripsy treatment.