Participation of oxygen and role of exogenous and endogenous sensitizers in the photoinactivation of Escherichia coli by photosynthetically active radiation, UV-A and UV-B.

We studied the mechanisms by which photosynthetically active radiation (PAR) and ultraviolet (UV-A and UV-B) radiation damage Escherichia coli suspended in water. The roles played by oxygen and exogenous and endogenous sensitizers were analyzed by monitoring changes in the physiological state of irradiated cells. Impairment of the cellular functions was more severe in the case of UV radiations. Radiation caused cellular damage in the absence of oxygen. PAR, UV-A, and UV-B radiation induced photobiological and photodynamic reactions mediated by endogenous sensitizers, which significantly shortened the T90 (time needed to reduce a cellular parameter by 90%) based on the growth ability of the cells. In addition, when exogenous sensitizers were present, the photodynamic reactions also had a negative effect on the operation of the electron transport chains. The presence of oxygen might enhance photoinactivation, affecting both the growth ability and the electron transport chains. Endogenous sensitizers were responsible for the noxious action of oxygen. The presence of dissolved organic material played a protective role against the oxygen by absorbing the incident radiation, thereby reducing the energy that reached the endogenous sensitizers.