Synergistic antibacterial activity of combined blue and red light photodynamic therapy

Jaouhra Cherif; Anis Raddaoui; Nada Souissi; Nakkach Mohamed; Jaouhra Cherif; Anis Raddaoui; Nada Souissi; Nakkach Mohamed

doi:10.3934/biophy.2025028

AIMS Biophysics

2025, Volume 12, Issue 4: 572-585. doi: 10.3934/biophy.2025028

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Research article

Synergistic antibacterial activity of combined blue and red light photodynamic therapy

1.
Laboratory of Biophysics and Medical Technologies, LR13ES07, Higher Institute of Medical Technologies of Tunis, University of Tunis El Manar, Tunis, Tunisia
2.
Laboratory Ward, National Bone Marrow Transplant Center, LR18ES39, Faculty of Medicine of Tunis, University of Tunis El Manar, Tunis, Tunisia
3.
Bacteriology Laboratory, Tunisian Institute of Veterinary Research, University of Tunis El Manar, Tunis, Tunisia
4.
Laboratoire de Spectroscopie Atomique, Moléculaire et Applications (LSAMA), Faculty of Science, Tunis El Manar University, Tunis, Tunisia
5.
Higher Institute of Medical Technologies of Tunis, Tunis El Manar University, Tunis, Tunisia

Received: 29 August 2025 Revised: 14 November 2025 Accepted: 28 November 2025 Published: 10 December 2025

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.
- multidrug-resistant bacteria,
- blue light,
- red light,
- photoinactivation,
- photosensitizer,
- synergistic effect
Citation: Jaouhra Cherif, Anis Raddaoui, Nada Souissi, Nakkach Mohamed. Synergistic antibacterial activity of combined blue and red light photodynamic therapy[J]. AIMS Biophysics, 2025, 12(4): 572-585. doi: 10.3934/biophy.2025028

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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.

Conflict of interest

The authors declare that they have no conflicts of interest.

Author contributions

JC designed and planned the study, set up the methodology, and performed the experiments. MN developed the light sources device. AR and NS provided resources and validation. JC, AR and MN wrote and reviewed the manuscript. All authors approved the final version.

Ethical approval

As the bacterial strains were analyzed anonymously, the study was exempted from Human Research Committee approval according to the regulations of the Local Medical Ethical Committee of the National Bone Marrow Transplant Center.

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