The appeal of light-based killing is that it is fundamentally resistance-free. Antibiotics work by targeting specific biochemical pathways that microbes can mutate around; light works by flooding microbes with reactive oxygen species or heat, physical insults that are far harder to evade. The four modalities in the review differ sharply in how they generate those insults. Photodynamic therapy, or aPDT, relies on a photochemical triad: an exogenous, non-toxic photosensitizer such as methylene blue or toluidine blue, light of a matching wavelength, typically red light between 635 and 660 nanometers, and molecular oxygen. Once applied to the tissue and allowed to incubate for five to ten minutes, the photosensitizer absorbs the light and is promoted to an excited triplet state. From there it can follow two destructive pathways. In the type I pathway, it reacts directly with nearby biomolecules, triggering a cascade of reactive oxygen species including superoxide, hydroxyl radicals, and hydrogen peroxide. In the type II pathway, it transfers energy to ground-state oxygen, converting it into highly reactive singlet oxygen. Bacterial membrane oxidation is the most damaging consequence, ripping apart the structures microbes need to survive.

Antimicrobial blue light takes a different route: instead of applying a chemical sensitizer, it exploits the microbes’ own pigments. Wavelengths between 405 and 470 nanometers excite endogenous chromophores, chiefly protoporphyrin IX, the penultimate precursor in heme synthesis, which absorbs most strongly around 405 to 410 nanometers in its Soret band. This is where the biology of periodontal disease creates a remarkable vulnerability. Anaerobic periodontal pathogens such as Porphyromonas gingivalis cannot synthesize heme from scratch; they must scavenge it from host cells. After extracting the iron, they are left with free protoporphyrin IX, which accumulates on their outer membranes and even gives black-pigmented bacteria their characteristic color. Those pigments make the very bacteria that drive periodontitis intrinsically photosensitive to blue light, and they also explain why these species glow red under violet-blue excitation, a property already used in autofluorescence-based plaque diagnostics. In other words, the same wavelength that reveals the pathogens can kill them.

The third strategy, antimicrobial selective photothermolysis, borrows a concept from dermatology and pushes it deeper. Rather than vaporizing tissue like Er:YAG or carbon dioxide lasers, near-infrared lasers at 810, 980, or 1,064 nanometers operate in a spectral window where human tissue is largely transparent but bacterial porphyrins still absorb. If the pulse is shorter than the target’s thermal relaxation time, heat builds up selectively inside the pathogen, destroying it even when bacteria have invaded host cells. Nd:YAG lasers, with pulses of roughly 100 microseconds and peak powers near 1,000 watts, satisfy both physical requirements, whereas continuous-wave diode lasers heat tissue non-selectively. Modeled kill depths for this approach reach two to three millimeters, dwarfing the 90 to 150 micrometers of effective light penetration for blue light and the roughly 23 micrometers of biofilm kill depth measured for aPDT in vitro. Photothermal therapy, the fourth modality, goes further still: exogenous nanoparticles convert continuous near-infrared irradiation into sustained localized heat, penetrating the extracellular polymeric matrix of biofilms to denature microbial proteins and nucleic acids. Above 50 degrees Celsius microbes are destroyed; below 45 degrees the heat instead modulates immune responses and can promote tissue regeneration, opening a rare combination of antimicrobial and regenerative effects.

So why is the clinic not transformed? The review’s central finding is a persistent gap between killing microbes in the laboratory and improving outcomes in patients. In endodontics, the evidence for aPDT looks spectacular on paper: systematic reviews report bacterial load reductions of 91.3 to 100 percent when photosensitizers are introduced into root canals and activated by lasers. Yet when researchers examined the clinical trials closely, only one of four showed any correlation between bacterial reduction and improved treatment outcomes. The mechanistic explanation is sobering. Enterococcus faecalis, detected in 24 to 77 percent of persistent root canal infections, invades dentinal tubules to depths of several hundred micrometers, while aPDT kills only to about 23 micrometers in biofilm. The smear layer retains photosensitizer, and oxygen, essential to the type II reaction, is depleted inside a sealed canal. Reflecting this mismatch, neither the American Association of Endodontists nor the European Society of Endodontology recommends aPDT for endodontic disinfection.

The periodontal story follows the same pattern. Across a meta-analysis of 26 randomized controlled trials, adding aPDT to scaling and root planing produced statistically significant improvements in probing depth, a mean reduction of 0.37 millimeters, and clinical attachment gain of 0.33 millimeters. But both figures fall well below the thresholds of roughly one millimeter and half a millimeter that clinicians consider meaningful. The 2024 Cochrane review rated the certainty of evidence very low, and the European Federation of Periodontology’s S3 guideline explicitly recommends against routine aPDT for stage I to III periodontitis. A 2025 meta-analysis of 26 trials in peri-implantitis found probing pocket depth reductions of 0.8 to 0.9 millimeters, but with low to moderate certainty and high heterogeneity, and the federation’s 2023 guideline again advises against aPDT for non-surgical peri-implantitis therapy or implant surface decontamination. The physical culprits are familiar: oxygen-dependent chemistry performs poorly in the anaerobic niches of deep periodontal pockets, and the photosensitizer struggles to diffuse into mature biofilm.

Selective photothermolysis fares little better despite its deeper reach. Tested only as pulsed Nd:YAG or continuous-wave diode adjuncts to conventional debridement, it was consistently inferior or equivalent to irrigation or scaling alone, with a 2026 umbrella review of nine systematic reviews rating the evidence low to critically low. Notably, the review authors point out that pooled analyses never report pulse parameters or selectivity endpoints, so those trials effectively tested laser-assisted irrigation rather than true selective photothermolysis. Even Nd:YAG-specific meta-analyses show only modest probing depth reduction without attachment gain, so the disappointing result cannot be blamed on continuous-wave settings alone. Nd:YAG also carries a documented risk of dentinal microcracks, and the review stresses that selectivity, the field’s defining theoretical advantage, has never actually been measured in a clinical setting.

There is one genuine clinical success: oral candidiasis. Three separate meta-analyses, spanning hundreds of denture stomatitis patients, found aPDT superior to nystatin in reducing palatal Candida and equivalent to fluconazole, without inducing the resistance that now complicates antifungal therapy, with fluconazole resistance exceeding 50 percent among oral Candida isolates from ICU patients. Low-cost natural photosensitizers such as riboflavin, curcumin, and hypericin have shown antifungal activity as well. Blue light, meanwhile, has demonstrated striking selectivity in vitro: at 415 nanometers, researchers killed 5.42 log units of Candida albicans while losing only 0.11 log units of keratinocyte viability at the equivalent dose, roughly a 42-fold margin, and 405-nanometer LEDs disinfect dual-species biofilms on denture resin. In periodontal patients, a single clinical trial found that twice-daily 455-nanometer exposure over four days reduced Porphyromonas gingivalis by 25 percent and Prevotella intermedia by 56 percent in plaque. Prototype devices, including 405-nanometer mouthguards and wireless blue-light-emitting zirconia implants, achieved substantial bacterial reductions in proof-of-concept studies, though no systematic review or randomized trial exists yet.

Photothermal therapy remains the least mature but perhaps most intriguing direction. A 2025 scoping review on nanomaterial-based phototherapy for caries found only nine publications, none clinical, yet laboratory results are dramatic: silver-doped Prussian blue hydrogels achieved more than 99 percent antibacterial efficacy with remineralization-promoting effects in rat models, and a thermosensitive injectable hydrogel combining mild 45-degree photothermal treatment with photodynamic action killed periodontal pathogens while inducing new bone formation in rats. For deep caries, aPDT reduced bacteria in five of six reviewed studies but showed no improvement in adhesive bond strength compared with conventional disinfectants, and evidence for blue light on carious dentin is limited to a single non-significant study. The review’s authors describe the entire field as mechanism-rich but outcome-poor. Shallow penetration depths, oxygen dependence in anaerobic niches, and narrow thermal safety margins constrain the physical side; nanoparticle compatibility, clearance, and regulatory status constrain the other. What is needed, they argue, are multi-center trials built on standardized protocols that test the hypothesis that actually matters: whether microbial reduction can be translated into durable clinical benefit. Until then, the pills remain, and light waits in the wings.

Subject of Research: Antimicrobial light-based interventions, including photodynamic therapy, blue light, selective photothermolysis, and photothermal therapy, for managing oral infections

Article Title: Antimicrobial light-based interventions for management of oral infections: a mini review

Article References: Antimicrobial light-based interventions for management of oral infections: a mini review. (n.d.). https://doi.org/10.3389/froh.2026.1981642

Image Credits: AI Generated

DOI: 10.3389/froh.2026.1981642

Keywords: antimicrobial resistance, photodynamic therapy, antimicrobial blue light, selective photothermolysis, photothermal therapy, oral infections, endodontics, periodontitis, peri-implantitis, oral candidiasis, dental caries, biofilm