Ancient Herbal Compounds Show Promise as Light-Activated Weapons Against Drug-Resistant Infections

A new narrative review published in Frontiers in Cellular and Infection Microbiology argues that natural photosensitizers long used in Traditional Chinese Medicine (TCM) could form the basis of a non-antibiotic strategy against pathogenic infections, including multidrug-resistant superbugs. The review, led by Jinju Huang of Guangzhou Medical University and colleagues, synthesizes three decades of literature on five plant-derived compounds—coumarin, curcumin, riboflavin, pheophorbide a (PPa), and hypericin—that can be activated by light to generate reactive oxygen species (ROS), molecules capable of destroying bacteria, viruses, fungi, and parasites through multi-target oxidative damage.

The therapeutic approach, known as antimicrobial photodynamic therapy (aPDT), requires three components: a photosensitizer, light of a specific wavelength, and molecular oxygen. When the photosensitizer absorbs light, it transitions to an excited triplet state and transfers energy to surrounding oxygen, producing singlet oxygen, superoxide radicals, and hydroxyl radicals. These species attack microbial cells at multiple levels simultaneously—peroxidizing lipids to disrupt membranes, oxidizing proteins to inactivate enzymes, and fragmenting DNA to impair replication and repair. Because this oxidative assault bypasses conventional resistance mechanisms, even multidrug-resistant pathogens remain susceptible, and the dense biofilm matrices that typically shield bacteria from antibiotics can be penetrated and disrupted.

The review distinguishes two complementary photochemical pathways. In the Type I pathway, the excited photosensitizer undergoes electron or hydrogen transfer reactions with nearby biomolecules, generating free radicals such as superoxide and hydroxyl radicals that initiate chain reactions damaging lipids, proteins, and nucleic acids. The Type II pathway involves direct energy transfer from the photosensitizer’s triplet state to molecular oxygen, yielding singlet oxygen, which rapidly oxidizes unsaturated lipids and amino acid side chains. The balance between these routes depends on oxygen availability: Type II dominates under normoxic conditions, while hypoxic infection sites favor Type I radical processes, allowing aPDT to maintain efficacy across diverse tissue environments.

Each of the five TCM-derived compounds occupies a distinct position on this mechanistic spectrum, shaped by its photophysical properties. Coumarin, found in herbs such as Dang Gui (Angelica sinensis) and Qianhu (Peucedanum praeruptorum), absorbs at the shortest wavelengths, roughly 320 to 380 nanometers, and produces relatively little singlet oxygen, with a quantum yield below 0.1. Curcumin, from turmeric rhizome, and riboflavin, present in goji berry, are activated by blue light around 420 to 445 nanometers and achieve moderate-to-high singlet oxygen yields of up to 0.60 and 0.54, respectively. PPa, a chlorophyll derivative, and hypericin, from St. John’s Wort, operate in the deeper red therapeutic window between 590 and 670 nanometers, reaching exceptionally high quantum yields of up to 0.80—properties that enable superior tissue penetration for treating infections beyond superficial layers.

These photophysical behaviors trace directly back to molecular architecture. Coumarin’s rigid 1,2-benzopyrone scaffold, a benzene ring fused to an unsaturated lactone, absorbs ultraviolet light and emits blue fluorescence but limits ROS potency. Curcumin’s extended pi-conjugation across an alpha, beta-unsaturated beta-diketone linker, modulated by keto-enol tautomerism and symmetrical methoxy-phenolic groups, strengthens visible-light absorption near 420 nanometers, though the molecule’s instability and poor solubility hinder systemic use. Riboflavin’s isoalloxazine ring provides strong blue-region absorption and efficient singlet oxygen generation, while its hydrophilic ribityl side chain improves water solubility—although its dependence on oxygen availability reduces efficacy in hypoxic infections. PPa’s porphyrin core supports deep-red absorption and high quantum yield, and hypericin’s heavily conjugated polycyclic aromatic system, stabilized by intramolecular hydrogen bonds among perihydroxyl groups, confers high ROS production but raises phototoxicity concerns.

The review documents a broad range of preclinical and early translational results. A coumarin-based fluorescent probe generated ROS under 660-nanometer near-infrared irradiation, eradicating Staphylococcus aureus in mouse models while its near-infrared emission enabled deep-tissue imaging. Modified coumarin photosensitizers achieved a singlet oxygen quantum yield of 0.839 under blue light, supporting dual Type I/II antibacterial activity. Curcumin-mediated PDT inhibited Candida biofilm formation by 86.8 percent in a species-dependent manner, though activity against mature biofilms was limited, and reduced the oral microbial load and SARS-CoV-2 presence in hospitalized COVID-19 patients when applied for oral decontamination. Riboflavin-mediated PDT disrupted Acinetobacter baumannii biofilms and completely inactivated the blood-borne parasite Trypanosoma cruzi in stored human platelet concentrates treated with 405-nanometer violet-blue light, offering a proof of concept for pathogen reduction technology in blood products.

PPa and hypericin, operating in the red-light window, have shown particularly consistent results from in vitro assays to in vivo wound infection models against methicillin-resistant S. aureus (MRSA). PPa-mediated PDT produced greater than one log10 reduction in MRSA viability in vitro and achieved potent wound infection control in animal models, while hypericin’s strong ROS generation ensured rapid microbial killing of Gram-positive, methicillin-sensitive and resistant S. aureus—though it was not effective against Gram-negative E. coli. Hypericin also reduced MRSA virulence by downregulating the sarA regulator and associated virulence genes, exhibited antifungal activity against Candida species, and induced necrosis-like death in Leishmania parasites, with murine cutaneous leishmaniasis models showing improved lesion healing.

Beyond direct pathogen killing, the review highlights a dual functionality that distinguishes these natural compounds from purely synthetic photosensitizers: many possess intrinsic antimicrobial, antiviral, antifungal, and antiparasitic activities independent of light activation. The authors also map traditional TCM concepts onto modern biomedical mechanisms. The principle of “invigorating blood circulation” corresponds to vasodilation, improved microcirculation, and enhanced immune cell trafficking to infection sites, linked to modulation of NF-kB and Akt-mTOR signaling. “Clearing heat” parallels anti-inflammatory activity through suppression of NF-kB and MAPK cascades and reduced pro-inflammatory cytokines such as TNF-alpha and IL-6, while “resolving toxicity” aligns with redox regulation via glutathione and trypanothione pathways. PDT-induced ROS may further act as secondary messengers influencing cytokine release, dendritic cell activation, and adaptive immune responses, extending the therapy’s effects to systemic immunomodulation.

Significant obstacles remain before clinical translation. The intrinsic limitations of natural photosensitizers—poor aqueous solubility, rapid degradation, hydrophobicity, aggregation, and limited bioavailability—restrict systemic delivery. Nanotechnology offers a partial solution: nano-curcumin formulations, including sol-oil nano-curcumin, nanoemulsions, and dendrosomal carriers, enhanced antimicrobial photodynamic therapy against Enterococcus faecalis, Lactobacillus casei, and S. mutans in dental applications, and showed antiviral activity against HPV-16 and antifungal activity against Candida albicans. Analogous strategies have produced coumarin-loaded solid lipid nanoparticles effective against S. aureus, riboflavin-gold nanoparticle systems for bacterial inactivation, silver-pheophorbide-a nanoconjugates active against E. coli, and nanostructured lipid carriers of Hypericum perforatum extract targeting E. coli, P. aeruginosa, and herpes simplex virus 1.

The review’s authors caution that most evidence remains in vitro or animal-based, with clinical validation limited and standardized protocols still lacking. Protocol heterogeneity—diverse light sources, irradiation times, and dosimetry—complicates reproducibility, while oxygen dependence, formulation instability, and the risk of photothermal tissue damage demand careful optimization of wavelength, fluence, and photosensitizer concentration. Most research on these compounds has concentrated on oncology, where established clinical protocols exist, leaving infectious disease applications comparatively underexplored. Future directions identified include stimuli-responsive nanocarriers, co-delivery systems combining photosensitizers with immunomodulators, and artificial intelligence-guided light dosimetry that personalizes photosensitizer selection and irradiation parameters. If harmonized methodologies and rigorous multicenter trials can bridge the gap from bench to bedside, the authors conclude, TCM-derived photosensitizers may offer an integrative path that connects ethnopharmacology with modern photomedicine at a time when antibiotic resistance continues to erode conventional treatment options.

Subject of Research: Traditional Chinese medicine-derived natural photosensitizers for antimicrobial photodynamic therapy against pathogenic infections

Article Title: Traditional Chinese medicine-derived natural photosensitizers for photodynamic therapy against pathogenic infections

Article References: Huang, J., Law, S. K., Leung, A. W. N., & Xu, C. (2026). Traditional Chinese medicine-derived natural photosensitizers for photodynamic therapy against pathogenic infections. Frontiers in Cellular and Infection Microbiology, 16, Article 1932770. https://doi.org/10.3389/fcimb.2026.1932770

Image Credits: AI Generated

DOI: 10.3389/fcimb.2026.1932770

Keywords: photodynamic therapy, Traditional Chinese Medicine, photosensitizers, antimicrobial resistance, curcumin, hypericin, riboflavin, pheophorbide a, coumarin, reactive oxygen species, biofilms, nanotechnology