Efficacy of blue LED in microbial inactivation: Effect of photosensitization and process parameters.

Efficacy of blue (462 ± 3 nm) Light emitting diode (LED) illumination to inactivate the foodborne pathogens
like Escherichia coli and Staphylococcus aureus in the presence of exogenous photosensitizer (curcumin) was
studied in vitro. The effect of temperature, concentration of photosensitizer and incubation time with photosensitizer
for microbial inactivation was investigated and sublethal injury of cells was determined. Mechanism of
inactivation by the combination of photosensitizer and blue light was also examined. A maximum reduction of
5.94 ± 0.22 and 5.91 ± 0.20 log CFU/ml was obtained for E. coli and S. aureus, respectively, when treated with
photosensitizer (20 μM) at 13 J/cm2 of blue light. There was no significant change in the inactivation of these
pathogens both at 9 °C and 27 °C in the presence of photosensitizer. Even, the incubation with the photosensitizer
didn't show any significant difference on the inactivation of these food-borne pathogens. Sublethal injury
(> 90% injury) was also observed for the cells treated with photosensitizer and blue light simultaneously.
Confocal laser scanning microscopy analysis revealed that membrane integrity was disturbed due to photodynamic
activity of curcumin in both the bacteria. Further, both cells produced intracellular reactive oxygen
species by the action of photosensitizer and blue light. Scanning electron microscopy of E. coli and S. aureus cells
treated with photosensitizer and blue light showed morphological changes in the cell wall compared to untreated
group. The study indicated that photodynamic inactivation of foodborne pathogens using LED-based
photosensitization can be explored as a potential technique for food safety.