Enhanced photodynamic antimicrobial approach: carboxylated porphyrins as potent light-activated photosensitizers to reduce effectively viability of Staphylococcus aureus and Escherichia coli

Antimicrobial photoinactivation (PDI) is an emerging therapeutic strategy capable of efficiently eliminating a wide range of microorganisms. An important advantage of this approach is the absence of resistance development, making PDI a promising alternative for infection control. In this study, tetra-substituted free-base porphyrins (H₂Pors 1 – 3 ) bearing thiocarboxylic acid groups were synthesized, characterized, and evaluated for the first time as photosensitizers in the PDI of Staphylococcus aureus and Escherichia coli . The compounds were tested at low concentrations (1.0 μM for S. aureus and 10.0 μM for E. coli ), under white light irradiation, both in the absence and presence of potassium iodide (KI, 100 mM). All porphyrins exhibited strong antibacterial activity against S. aureus , achieving inactivation to the detection limit within 10–30 min, with H₂Por 2 showing the highest efficacy. In contrast, the porphyrins alone were not effective against E. coli . However, when combined with KI, bacterial inactivation to the detection limit of the method was achieved. Additionally, KI significantly reduced the treatment time required for S. aureus inactivation. Overall, H₂Pors 1 – 3 +KI demonstrate strong potential as photosensitizers for effective PDI, highlighting their potential applicability in biological settings.