UV-C LED wavelength effects on inactivation kinetics, DNA damage and membrane integrity in drinking water indicator bacteria.

Ultraviolet (UV) light-emitting diodes (LEDs) have emerged as a promising technology for water disinfection, offering selectable wavelengths that enable more precise targeting of specific cellular components. This study evaluated the inactivation efficiency of water quality indicators with different cell morphologies and envelope organization, Escherichia coli (Gram-negative) and Enterococcus faecium (Gram-positive), including both environmental and culture collection strains, using UV-C LEDs that emit light at 255 nm, 260 nm, 265 nm, 270 nm, and 280 nm. Inactivation kinetics as well as post-exposure repair, under dark and light conditions, were evaluated. Fluorescence microscopy observations and cyclobutane pyrimidine dimer formation were analyzed to elucidate morphological changes and DNA damage. Within the tested range, 265 nm LEDs achieved the highest inactivation rates for E. coli at a given UV fluence, consistent with the DNA absorption maximum in the 260-270 nm region. In contrast, E. faecium showed similar inactivation between 260 and 270 nm. Nevertheless, all tested wavelengths demonstrated high efficacy, achieving up to 6-log inactivation of both culture collection and environmental strains at low UV fluences (below 7 mJ/cm²). E. faecium showed enhanced resilience at lower fluences for all the wavelengths tested. Fluorescence microscopy revealed that both membrane integrity and DNA structure were increasingly affected by higher UV fluences, although signs of DNA damage were more pronounced and detectable even at lower exposure levels. Additionally, none of the strains tested showed considerable photoreactivation or dark repair capability, implying that the induced DNA lesions were largely irreversible under our experimental conditions. This study shows the efficacy of UV-C LEDs for water disinfection across a range of wavelengths, particularly at 265 nm, highlighting the important contribution of DNA damage to bacterial inactivation, and emphasizing their applicability for the water industry.