Effect of focused ultrasound-induced heat and cavitation on methicillin-resistant Staphycoccus aureus viability and the role of bacterial structural features in susceptibility to cavitation

Methicillin-resistant Staphylococcus aureus (MRSA) is a gram-positive sphere-shaped bacterium and a common cause of abscesses. Focused ultrasound-induced cavitation can kill bacteria and potentially treat abscesses non-invasively by reducing bacterial populations. As an initial study to evaluate this approach, 10 mL MRSA suspensions were used as a simplified abscess model and treated in polycarbonate vials coupled to a focused ultrasound transducer (H-161, Sonic Concepts, Bothell, WA, F# 0.9). The pulse parameters were 1.2 MHz frequency with 20 μs pulse duration at 2% duty cycle, and 40-min treatments were conducted. The coupling water was heated to 37 °C, with input power adjusted to raise and maintain the sample temperature to 50 °C throughout the treatment. The cavitation dose was determined from PCD broadband noise using a co-axially aligned imaging probe (P4-2), while thermal dose (CEM50) was determined from thermocouple temperature measurements. Control experiments involving heating to 50 °C without ultrasound were also conducted. Results showed logarithmic relationship between MRSA inactivation and thermal dose, with cavitation contributing insignificantly to the effect. To understand this result in contrast to statistically significant inactivation of E. coli (gram-negative rod-shaped) by cavitation, we are investigating the effect of bacterial structural features on susceptibility to focused ultrasound-induced cavitation.