The Oral Microbiome in the Biological and Clinical Response to Photobiomodulation During Cancer Supportive Care: A Critical Mechanistic and Translational Review

Cancer therapy can disrupt the oral microbial ecosystem through mucosal injury, salivary dysfunction, immune suppression, antimicrobial exposure, dietary change, and altered oral hygiene. Photobiomodulation therapy (PBMT) is guideline-supported for prevention of oral mucositis in selected oncologic settings, but whether the oral microbiome contributes to PBMT response remains uncertain. This critical narrative review reframes the question from whether PBMT changes the oral microbiome to what role, if any, microbial ecology plays in the biological and clinical response to PBMT during cancer supportive care. Evidence is organized as direct clinical evidence, indirect clinical evidence, experimental microbial evidence, and mechanistic hypothesis. Direct clinical evidence is sparse and internally mixed: a four-patient uncontrolled pilot reported concurrent cytokine and microbiome shifts after PBMT, whereas a 2023 head-and-neck cancer case series reported clinical improvement in mucositis without significant pre-post changes in salivary bacterial community structure. The strongest current rationale therefore supports a host-first model in which PBMT modifies epithelial integrity, inflammatory and redox signaling, tissue repair, and possibly salivary physiology, thereby changing the ecological conditions experienced by oral microorganisms. Direct microbial photobiological effects remain plausible but are highly dependent on wavelength, irradiance, radiant exposure, oxygen availability, organism, and biofilm state, and should not be conflated with antimicrobial photodynamic therapy. We propose four non-equivalent roles for microbiome change: mediator, modifier, biomarker, or consequence of treatment response. Oral mucositis is the most informative clinical model, but causal interpretation requires longitudinal sampling, functional microbial assays, complete PBMT dosimetry, validated clinical and patient-reported outcomes, and rigorous control of antimicrobials, periodontal status, oral hygiene, diet, feeding support, and anticancer treatment. Microbiome modulation should currently be regarded as an emerging mechanistic hypothesis and research endpoint rather than an established mechanism of PBMT efficacy.