A Closer Look at Dark Toxicity of the Photosensitizer TMP yP in Bacteria

Photodynamic inactivation of bacteria ( PIB ) is based on photosensitizers which absorb light and generate reactive oxygen species ( ROS ), killing cells via oxidation. PIB is evaluated by comparing viability with and without irradiation, where reduction of viability in the presence of the photosensitizer without irradiation is considered as dark toxicity. This effect is controversially discussed for photosensitizers like TMP yP (5,10,15,20‐Tetrakis(1‐methyl‐4‐pyridinio)porphyrin tetra(p‐toluensulfonate). TMP yP shows a high absorption coefficient for blue light and a high yield of ROS production, especially singlet oxygen. Escherichia coli and Bacillus atrophaeus were incubated with TMP yP and irradiated with different light sources at low radiant exposures (μW per cm²), reflecting laboratory conditions of dark toxicity evaluation. Inactivation of E. coli occurs for blue light, while no effect was detectable for wavelengths >450 nm. Being more susceptible toward PIB , growth of B. atrophaeus is even reduced for light with emission >450 nm. Decreasing the light intensities to nW per cm² for B. atrophaeus , application of TMP yP still caused bacterial killing. Toxic effects of TMP yP disappeared after addition of histidine, quenching residual ROS . Our experiments demonstrate that the evaluation of dark toxicity of a powerful photosensitizer like TMP yP requires low light intensities and if necessary additional application of substances quenching any residual ROS .