Staphyloxanthin Photolysis Potentiates Low Concentration Silver Nanoparticles in Eradication of Methicillin-Resistant Staphylococcus aureus

The rise of antibiotic resistant bacteria, e.g., methicillin-resistant Staphylococcus aureus (MRSA), has resulted in a widespread search for alternative treatments not reliant on traditional antibiotics. Silver nanoparticles (AgNPs) have long been known to exhibit antimicrobial activity against a wide variety of bacterial species. However, the clinical application of AgNPs as an alternative to antibiotics has been limited by their toxicity at high concentrations. Here, via blue-light photolysis of staphyloxanthin (STX), a carotenoid pigment in the MRSA membrane, we are able to significantly increase the antimicrobial efficacy of AgNPs. With 4 h of 5 μg/mL AgNP exposure, there is no significant change in MRSA burden compared to the control. In contrast, a 99.99% reduction in MRSA burden is observed for samples exposed to 30 J/cm 2 of pulsed blue light. The underlying mechanism is unveiled as that STX photolysis increases permeability of membrane and facilitates the uptake of AgNPs into the bacterium. This approach reduces the working concentration of AgNPs from >10 μg/mL down to 1 μg/mL, well below the toxic threshold of 10 μg/mL for mammalian cells. This approach has been found effective on stationary-phase MRSA and MRSA biofilms, demonstrating its potential for treating MRSA skin infections.