Efficacy of photodynamic treatment with new methylene blue on planktonic cells and biofilms of Salmonella isolated from vegetal sources in Brazil

Non-typhoid Salmonella enterica (NTS) serovars Enteritidis (S. Enteritidis), Infantis (S. Infantis), and Typhimurium (S. Typhimurium) are among the main pathogens causing foodborne diseases in humans worldwide. Considering their broad presence in several hosts, including plants, and their increasing antimicrobial resistance rates globally, methods to control spoilage and eradicate these pathogens are necessary. Antimicrobial photodynamic treatment (APDT) shows promising results for the control of foodborne pathogens, but only few studies have analyzed the potential of this method to control NTS, especially among isolates from vegetal sources. Therefore, this study evaluated the effect of APDT with new methylene blue N (NMBN) against planktonic cells and biofilms of S. Enteritidis, S. Infantis, and S. Typhimurium isolated from vegetal sources in Brazil. The eight strains were exposed to APDT with 6.25, 12.5, and 25 μM of NMBN and 4.16, 12.5, and 25 J/cm² light fluences (5, 15 and 30 min, respectively) for planktonic cells. Two S. Enteritidis and one S. Infantis strains with consistent biofilm formation on crystal-violet staining assay and confocal microscopy were selected for the anti-biofilm APDT under the same conditions. For planktonic cells, 12.5 or 25 μM NMBN with 15 and 30 min exposures fully eradicated all strains tested. For biofilms, these conditions reduced cell counts for all strains, showing that APDT is effective in controlling biofilms. In conclusion, APDT with NMBN was capable of successfully eradicating planktonic cells of S. Enteritidis, S. Infantis, and S. Typhimurium, and also reduced cell counts of S. Enteritidis and S. Infantis biofilms. These results showed the promising capacity of APDT treatment with NMBN as a method for the control and eradication of NTS in food items.