Phospholipid bilayer responses to ultrasound-induced microbubble cavitation phenomena

Using liposomal bilayers as a model for bacterial cell membranes, this study investigates the extents to which high-frequency (1.0 and 3.3 MHz) ultrasound-induced microbubble cavitation phenomena are effective for bacterial inactivation. Forster Resonance Energy Transfer (FRET), acoustic scattering and a mathematical model were used to quantify the proximity of microbubbles to liposomes, establish if liposomes affect microbubble cavitation and propose bilayer alterations' mechanisms. Addition of a positive charge on micmbubbles increased their proximity to liposomes through electrostatic attraction, however, dampened microbubble's oscillation compared to freely-floating microbubbles. Alleviation of energy transfer due to phospholipid mixing between microbubbles and liposomes established that close-proximity is necessary for bilayer alterations. Approximately 19% mixing of phospholipids was observed at 3.3 MHz due to microstreaming from stable cavitation, while inertial cavitation at 1.0 MHz increased this mixing to 50%. This method can damage bacterial membrane; but, whether bacterial defense mechanisms can attenuate this effect remains to be understood.