Ultrasound-Responsive Nanoparticles Enable Hydrophobic Antibiotic Release and Deep Penetration for Biofilm Treatment

Localized delivery of antibiotics is a promising strategy that leads to transformative treatment pathways of bacterial biofilms and increases the effectiveness of their administration in contrast to traditional delivery methods requiring high antibiotic doses. Hydrophobic antibiotics have poor activity against bacterial biofilms due to their limited penetration and are particularly challenging to deliver. Nanoparticles are ideal drug delivery agents to achieve spatially controlled delivery, but commonly their designs are either soft or porous, which limits temporally triggered release, with the result that most of the antibiotic does not reach deeply into the biofilm. In this study, we present designs of nonporous silica nanoparticles that encapsulate a lipophilic antibiotic, rifampicin, with noncovalent interactions and enable controlled release triggered by Low-Frequency Ultrasound (LFUS). Staphylococcus aureus biofilms treated with the nonporous, core@shell, rifampicin-encapsulated nanoparticles, RIF⊂PhSiO₂@SiO₂, combined with LFUS, achieved 90% biofilm eradication, compared to 20% without ultrasound; treatment with free rifampicin and LFUS resulted only in a 10% reduction. Nanoparticle penetration into biofilm layers was visualized using fluorescent nanoparticles prepared with coencapsulation of the Nile red fluorophore, RIF+NR⊂PhSiO₂@SiO₂. Confocal fluorescence imaging of the biofilms demonstrated penetration of the nanoparticles throughout all the layers of the biofilm upon LFUS application, in sharp contrast to their presence in only the top few biofilm layers without LFUS. Scanning Electron Microscopy of the biofilms confirmed the presence of nanoparticles and the dual role of LFUS in promoting penetration and facilitating drug release by disrupting molecular interactions within the nanoparticle. This work introduces a design paradigm for nonporous nanoparticle agents combined with ultrasound, enabling both temporal and spatial control of drug release in bacterial biofilms. This will open transformative therapeutic approaches for effective localized delivery of drugs that have previously been challenging to deliver.