S. aureus biofilm disruption using ultrasound and microbubbles: Influence of radiation force, bubble dynamics and biofilm growth conditions

<i>Staphylococcus aureus</i> is a human pathogen and a major cause of bloodstream infections, which can readily form biofilms on implanted medical devices. Here, we utilise a combination of lipid-shelled microbubbles (MBs) and ultrasound (US) to physically disperse the biofilm from the growth surface. The effects of two peak negative pressures (PNPs) and the direction of the acoustic radiation force (ARF) were evaluated. At 1.1 MHz, a clinically relevant frequency, and low PNP of 360 kPa, no significant biofilm dispersal occurred regardless of ultrasound (US) orientation. In contrast, at a high PNP of 2500 kPa, directing the ultrasound beam upward (US↑) pushed microbubbles (MBs) toward the biofilm, resulting in near-complete dispersal of the biofilm (94 ± 2 %) within the focal zone. Reversing direction to US↓, which pushes MBs away from the biofilm, reduced biofilm dispersal to 81 ± 3 %. Pre-treatment of the biofilm growth surface with fibrinogen or human plasma significantly altered the biofilm morphology and thickness, but did not affect the efficiency of ultrasound and microbubbles (US + MB)-mediated dispersal. Furthermore, multiple consecutive US + MB treatments could be applied to treat larger areas of biofilm without requiring MB replenishment between treatments. High-speed imaging was used to observe MB behaviour (e.g. translation and destruction) during US exposure. We revealed that the near instantaneous destruction of smaller MBs (∼1 μm) at high pressure did not induce significant biofilm dispersal and hypothesise that the translational motion of larger MBs (>10 μm) across the surface of the biofilm was the dominant mechanism behind biofilm dispersal.