Porphyrin-Mediated Antimicrobial Photodynamic Therapy against Salmonella enterica strains

AIMS: This study aimed to optimize biofilm cultivation conditions for fifteen Salmonella enterica strains and screen seven porphyrin and phthalocyanine compounds to identify the most effective photosensitizer for antimicrobial photodynamic therapy (aPDT) against planktonic cells and biofilms. METHODS AND RESULTS: Cultivation media and incubation times were optimized, selecting one representative per serovar. Using a light fluence of 13.19 J/cm2, compound 5,10,15,20-Tetrakis(1-methyl-4-pyridinio) porphyrin tetra (p-toluenesulfonate) (C7) emerged as the most efficacious photosensitizer. C7-mediated aPDT achieved a minimum inhibitory concentration (MIC) at 6.25 µmol l-1, reaching a 3.00 log10 reduction (99.9% lethality) at 2 h post-irradiation. At 1.56 µmol l-1 (1/4 MIC), immediate reductions of 2.87 log10 for strains S4 (S. Enteritidis S4) and 2.31 log10 (S. Typhimurium SB3) were observed, though phenotypic regrowth occurred within 24 h. At 1.56 µmol l-1, C7 prevented biofilm development, reducing exopolysaccharides and altering morphotypes. Against mature biofilms, strain SB3 showed marked recalcitrance compared to strain S4. CONCLUSIONS: C7-mediated aPDT represents an effective strategy for planktonic eradication and biofilm prevention in Salmonella enterica. However, eliminating established mature biofilms remains a major challenge due to strain-dependent variations in macrocolony architecture and extracellular matrix fortification.