Reduction of Staphylococcus aureus in vitro and in milk by photodynamic inactivation using riboflavin and curcumin as photosensitizers: Cell damage and effects on product quality

Foodborne bacterial pathogens such as Staphylococcus aureus (S. aureus) are a threat due to the progressive resistance of microorganisms to multiple drugs. In this sense, photodynamic therapy (PDT) is an innovative food decontamination technique based on the combination of a photosensitizer compound (PS), light of adequate wavelength, and molecular oxygen. The reactive oxygen species (ROS) produced are responsible for non-specific cell damage and make the development of resistance unlikely. The objective was to determine the cell viability of S. aureus ATCC 25923 submitted to PDT mediated by Riboflavin (RIB) and Curcumin (CUR) in vitro and in fluid milk. The structural damage responsible for the death of the bacteria was investigated. In milk, lighting was tested by two methods: thin layers and constant agitation. The effects of the treatments on the composition, oxidation, and color of the product were also investigated. The blue light emitting diode (LED) inhibits S. aureus even in the absence of PS. CUR and RIB demonstrated synergistic antimicrobial activity, increasing the efficiency of PDT. CUR was more efficient than RIB, inhibiting 7 log CFU/mL at concentrations above 62.50 μg/mL in vitro. PDT caused cell death through oxidation and disruption of the bacterial membrane. Despite the considerable photoinhibition in milk (1.63 log CFU/mL), the physiological environment of this matrix impaired the efficiency of the technique. Both lighting methods reduced the bacteria counts. There was no change in macronutrients composition and protein oxidation. However, light caused lipid peroxidation and PS pigmented the milk. Therefore, PDT is efficient to inhibit bacterial pathogens and can be used as a complement to traditional food preservation technologies.