Why dose alone fails: Irradiance-dependent bacterial survival mechanisms challenge UV system validation

Emerging low-intensity UV technologies, including LEDs, side-emitting optical fibers and UV-emitting glasses, offer transformative potential for continuous biofilm control in complex geometries, but operate at irradiances (10⁻²–10² µW/cm²) far below those used to establish dose-based validation protocols. Whether the Bunsen-Roscoe reciprocity law holds under these conditions remains unresolved, creating uncertainty in system design and regulatory compliance. This study systematically evaluated time-dose reciprocity for surface-bound E. coli MG1655 across five irradiances (0.02–100 µW/cm²) at 265 and 280 nm, coupling inactivation kinetics with transcriptional analysis of DNA repair and stress response pathways. Reciprocity held above 0.1 µW/cm², but failed at lower irradiances where negligible inactivation occurred despite increasing dose. Transcriptional profiling at sublethal exposure (2 mJ/cm²) revealed that low irradiance strongly induced SOS response genes (recA, lexA, umuC; 3.4–5.6 log₂ fold) and oxidative stress regulators (oxyR, soxS; 1.6–3.0 log₂ fold), while nucleotide excision repair genes remained near baseline. This transcriptional pattern is consistent with greater engagement of translesion synthesis than of excision repair, suggesting that survival under prolonged low-irradiance exposure may involve damage tolerance alongside lesion removal. These results establish that deviation from reciprocity is biologically mediated by active repair processes, not only photochemical artifacts. For practitioners, this work identifies the presence of lower operational bounds (∼0.1 µW/cm²) below which dose-based performance predictions become unreliable, and highlights that mutagenic survival pathways in sublethal exposure regimes may promote stress-tolerant subpopulations. These considerations should inform both system design and validation protocols for next-generation UV technologies.