Kinetics of bacterial inactivation by 405Â nm and 520Â nm light emitting diodes and the role of endogenous coproporphyrin on bacterial susceptibility

Photodynamic inactivation studies of microbial pathogens have focused on the use of an external photosensitizer or a precursor compound to eliminate bacteria. The present study investigated the inactivation kinetics of six bacterial pathogens by a 405 nm light emitting diode (LED) without the addition of any external compound. The role of endogenous coproporphyrin on the bacterial susceptibility to LEDs was also examined. Pathogens were illuminated with LEDs at 25, 10 and 4 °C for 9 h and the inactivation curves were modeled using six different equations. Endogenous coproporphyrin was quantified using an HPLC system equipped with a fluorescence detector. At a dose of 306 J/cm2, the 405 nm LED brought about 4.0, 2.1 and 1.9 log reductions in the populations of Staphylococcus aureus at 25, 10 and 4 °C, respectively. At all three temperatures, the population of Bacillus cereus and Listeria monocytogenes reduced by approximately 2.3 and 1.9 log respectively. Salmonella Typhimurium and Escherichia coli O157:H7 showed moderate susceptibility to 405 nm LED while Pseudomonas aeruginosa was most resistant. Of the six models tested, Hom model proved most suitable. This study demonstrated that 405 nm LEDs can be useful in the inactivation of bacterial pathogens with the aid of endogenous coproporphyrin alone.