Exogenous indirect photoinactivation of bacterial pathogens and indicators in water with natural and synthetic photosensitizers in simulated sunlight with reduced UVB

Aims To investigate the UVB‐independent and exogenous indirect photoinactivation of eight human health‐relevant bacterial species in the presence of photosensitizers. Methods and Results Eight bacterial species were exposed to simulated sunlight with greatly reduced UVB light intensity in the presence of three synthetic photosensitizers and two natural photosensitizers. Inactivation curves were fit with shoulder log‐linear or first‐order kinetic models, from which the presence of a shoulder and magnitude of inactivation rate constants were compared. Eighty‐four percent reduction in the UVB light intensity roughly matched a 72–95% reduction in the overall bacterial photoinactivation rate constants in sensitizer‐free water. With the UVB light mostly reduced, the exogenous indirect mechanism contribution was evident for most bacteria and photosensitizers tested, although most prominently with the Gram‐positive bacteria. Conclusions Results confirm the importance of UVB light in bacterial photoinactivation and, with the reduction of the UVB light intensity, that the Gram‐positive bacteria are more vulnerable to the exogenous indirect mechanism than Gram‐negative bacteria. Significance and Impact of the Study UVB is the most important range of the sunlight spectrum for bacterial photoinactivation. In aquatic environments where photosensitizers are present and there is high UVB light attenuation, UVA and visible wavelengths can contribute to exogenous indirect photoinactivation.