Photodynamic inactivation of S. aureus with a water-soluble curcumin salt and an application to cheese decontamination

In this study, the optimal parameters for the photodynamic inactivation (PDI) of Staphylococcus aureus in bacterial suspensions and in cheese were assessed using a water-soluble curcumin salt as the photosensitizer (PS). The in vitro study aimed at finding the optimal concentration and light dose to promote S. aureus photokilling. Four main groups were proposed: CONTROL (L−C−), LIGHT (L+C−), CUR (L−C+) and PDI (L+C+). A fixed light dose (LED, 450 ± 10 nm, 10 J cm −2 ) was applied using four different PS concentrations (0.75, 1.0, 1.5 and 3.0 mg mL −1 ). The dose also varied from 10–100 J cm −2 for a fixed concentration. High inactivation rates were observed for all light doses, with a maximum reduction of 7.58 log 10 at 100 J cm −2 ( p ≪ 0.05). Saturation of the PDI effect was observed after a 10 minute illumination time, as well as a slight decrease in the S. aureus population for increasing illumination times in the L+C− group. As an application, the concentration showing the best decontamination performance in vitro (0.75 mg mL –1 ) was applied to decontaminate cheese in loco. PDI in two types of coalho cheese, a rennet-coagulated cheese commonly consumed in Brazil, was investigated. The results showed no significant inactivation in unpasteurized cheese, but a 4.34 log 10 reduction for t > 5 min in pasteurized specimens. In conclusion, the present PDI-catalyzed curcumin photosensitizer inactivated S. aureus at statistically significant levels in vitro, in pasteurized cheese, but not in unpasteurized specimens.