Synergistic effect of hypocrellin B and curcumin on photodynamic inactivation of Staphylococcus aureus.

Antimicrobial photodynamic inactivation (aPDI) serves as a new approach to control the growth of foodborne bacteria. It remains elusive if the photodynamic efficacy of hypocrellin B (HB) can be potentiated by joint action with curcumin. In this study, we measured the survival rate of Staphylococcus aureus strains under the varying photodynamic conditions. According to our data, a maximum of 5-6 log10 decrease of bacterial survival can be achieved under the tested conditions (500 nM, 9 J cm-2 ). Regarding the bactericidal mechanisms, HB-based aPDI disrupted the membrane integrity of staphylococcal cells, probably owing to the stimulated reactive oxygen species (ROS). In addition, aPDI disrupted the enzymatic activities of bacterial antioxidant proteins and caused the leakage of multiple intracellular substances. The HB-mediated photodynamic efficacy was potentiated by the addition of curcumin with a sublethal dose. This dual-photon synergy arose from unique aPDI conditions (100 nM each and 9 J cm-2 ). The synergistic action might be accounted for by the increased type I/type II ratio of ROS, as evidenced by the effect of different quenchers. Finally, the joint use of photosensitizers reduced the microbial contamination of the tested apple while maintaining its quality. In summary, photodynamic inactivation based on dual photons showed synergistic activity in controlling the growth of Staphylococcal aureus, which provided a novel approach to maintain food safety.