Controlled and Localized Nitric Oxide Precursor Delivery From Chitosan Gels to Staphylococcus aureus Biofilms

Extracellular polymeric substances in bacterial biofilms reduce the penetration of antimicrobials and give rise to extreme recalcitrance and treatment challenges for many persistent biofilms and associated infections. Nitric oxide (NO) is a promising alternative to conventional antimicrobials but is challenging to deliver at precise concentrations for significant periods in a convenient and nontoxic manner. Here we report a unique NO delivery platform by incorporating the NO precursor isosorbide mononitrate (ISMN) into chitosan gels to facilitate sustained ISMN release and effective delivery. The chitosan gels were characterized with respect to the drug release kinetics, rheological properties, as well as antimicrobial efficacy against biofilms of Staphylococcus aureus in the absence and presence of the antibiotic ciprofloxacin. Chitosan gels loaded with ISMN alone (CS-ISMN) showed comparable antimicrobial effects compared to blank chitosan gel (approximately 2 log₁₀ reduction). However, there was strong synergy of CS-ISMN when combined with ciprofloxacin, that is, ∼4 log₁₀ reduction of bacterial colonies of CS-ISMN-CIP compared to the single treatments. These findings were confirmed by confocal imaging and highlight a potentially effective new way to overcome recalcitrant S aureus biofilms using NO precursors.