Synergistic antibacterial activity of combined blue and red light photodynamic therapy

Abstract

Due to increasing antibiotic resistance and a lack of new antibiotics, alternative treatments are urgently needed. This study investigates photodynamic therapy (PDT), which uses light-activated photosensitizers to produce reactive oxygen species that effectively inactivate bacteria. We evaluated the antibacterial efficacy of PDT against two pathogens that are resistant to current antibiotics, namely Staphylococcus epidermidis (Gram-positive) and Acinetobacter baumannii (Gram-negative), by testing various illumination protocols. The results showed that combining blue light (468 nm) and red light (632 nm) with methylene blue (MB) produced a synergistic effect in bacterial inactivation compared with protocols using either blue or red light individually in combination with methylene blue (MB). Specifically, after just 30 minutes of exposure, S. epidermidis showed a 3.3 log reduction (99.95%), while A. baumannii showed a 3.1 log reduction (99.92%) after 60 minutes. Overall, S. epidermidis was more sensitive to all tested protocols than A. baumannii. We also examined the effects of this protocol on antibiotic susceptibility. For most antibiotics tested, there was no change in the size of the inhibition zones. However, for linezolid, we observed a significant increase in the inhibition zone's diameter, indicating a possible enhanced susceptibility to this antibiotic.