Blue/violet laser inactivates methicillin-resistant Staphylococcus aureus by altering its transmembrane potential

• Endogenous photosensitizers in MRSA are detected through fluorescence emission.
• Blue light absorbed by photosensitizers generates free radicals.
• Free radicals affect MRSA membrane integrity and reduces transmembrane potential.
• Reduction in transmembrane potential correlates with suppression of MRSA growth.

The resistance of methicillin-resistant Staphylococcus aureus to antibiotics presents serious clinical problems that prompted the need for finding alternative or combination therapies. One such therapy is irradiation with blue light. To determine the alterations in metabolic processes implicated in the observed antimicrobial effects of blue light, we investigated the changes in membrane potential and the presence of free-radical-producing photo-acceptor molecules. Bacterial cultures irradiated with one or two doses of 405 nm laser light (each consisting of 121 J/cm2) were imaged with spectrally resolved laser-scanning microscopes to detect endogenous fluorescent species as well as the voltage sensitive dye 3,3′-Diethyloxacarbocyanine iodide. The endogenous fluorescence indicated the presence of photosensitizers (i.e., porphyrins, NADH, FAD) in the cells, while the exogenous signal allowed us to monitor rapid changes in transmembrane potential following treatment with light. The changes were drastic within the first 5 min after irradiation with the first dose and continued slowly after the second irradiation. These results suggest that the early antimicrobial activity of blue light results from alteration of membrane integrity with a consequent decrease in membrane polarization and rapid alteration of vital cellular functions. The observation of an early antimicrobial activity of light is very encouraging, as it suggests that treatment does not necessarily have to be administered over a long period of time.