| Summary: | Background: In a recent study we showed that blue light inactivates methicillin-resistant Staphylococcus aureus (MRSA) by perturbing, depolarizing, and disrupting its cell membrane. Purpose: The current study presents visual evidence that the observed biochemical changes also result in cell metabolic changes and structural alteration of the cell membrane. Methods: Cultures of MRSA were treated with 450 nm pulsed blue light (PBL) at 3 mW/cm2 irradiance, using a sub lethal dose of 2.7 J/cm2 radiant exposure three times at 30-min intervals. Following 24 h incubation at 37 °C, irradiated colonies and control non-irradiated colonies were processed for light and transmission electron microscopy. Results: The images obtained revealed three major effects of PBL; (1) disruption of MRSA cell membrane, (2) alteration of membrane structure, and (3) disruption of cell replication. Conclusion: These signs of bacterial inactivation at a dose deliberately selected to be sub-lethal supports our previous finding that rapid depolarization of bacterial cell membrane and disruption of cellular function comprise another mechanism underlying photo-inactivation of bacteria. Further, it affirms the potency of PBL. © 2021 The Authors
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