Antibiofilm effect of ultrasound combined with microbubbles against Staphylococcus epidermidis biofilm

Biofilms are difficult to eradicate due to their resistance to antibiotics and host immune cells. Ultrasound microbubbles have emerged as a new treatment modality with the underlying mechanisms largely unknown. In this study, we exposed 24-h-old Staphylococcus epidermidis biofilms established in OptiCell™ chambers to ultrasound in combination with microbubbles, and investigated the activities of vancomycin and neutrophils against S. epidermidis biofilms after treatment. The antibiofilm mechanims of ultrasound microbubbles were explored in terms of bacterial permeability and biofilm-associated gene expression. After treatment of ultrasound (1 MHz, 0.5 W/cm2, 50% duty cycle) combined with microbubbles in the concentration of 1% and 4% (v/v) for 5 min, bacterial permeability to extracellular fluorescent dyes was enhanced and the expression of icaA was down-regulated while that of agrB and RNAIII up-regulated. Post-treatment biofilms were more sensitive to vancomycin by demonstrating reduced biomass than those exposed to vancomycin alone (P < 0.05). The phagocytosis, oxidative burst activity as well as chemotaxis of neutrophils in response to biofilms were also significantly increased. The bioeffect of ultrasound combined with microbubbles was generally more significant than that of ultrasound alone, and dependent on microbubble concentration. This study demonstrated that ultrasound combined with microbubbles could enhance the activities of antibiotics and neutrophils against biofilms possibly via mechanical and biochemical mechanisms, and may provide an efficient and non-invasive antibiofilm alternative apart from chemical and biological approaches.