Action spectrum of Bacillus subtilis spores for validation of polychromatic ultraviolet (UV) systems

In Germany, UV treatment systems for municipal drinking water must be validated and certified prior to implementation. This applies to systems using both low- and medium-pressure mercury discharge lamps (LP and MP lamps). To evaluate germicidal efficacy of the systems and define operational ranges, full-scale biodosimetry is carried out by an accredited laboratory using a non-pathogenic surrogate microbes. In Germany, Bacillus subtilis spores are currently the standard surrogate. Due to the introduction of new DIN (Deutsches Institut für Normung - German Institute for Standardization) standards for medium-pressure and LED-based UV systems, and the obsolescence of the current action spectrum for Bacillus subtilis spores, a reassessment using state-of-the-art techniques was carried out. This study determined the action spectrum of Bacillus subtilis spores at 19 monochromatic wavelengths ranging from 220 nm to 320 nm using a tunable laser-based system. The data revealed high-resolution sensitivity variation between 260 nm and 285 nm (≈2.5 nm intervals). Compared to existing data, spores exhibited significantly higher sensitivity in the 260-280 nm range, with negligible inactivation observed at 290 nm and above.