Cationic photosensitizers and potassium iodide: an innovative approach for enhanced photodynamic inactivation of pathogenic bacteria in aquaculture

Aquaculture is a rapidly growing industry, accounting for more than half of the worldwide fish production. However, this sector is often affected by bacterial infections, threatening farmed animals and consumer safety. This study explored the potential of Photodynamic Inactivation (PDI) as an innovative antimicrobial approach for the inactivation of Gram-negative bacteria relevant to the aquaculture sector: Vibrio parahaemolyticus, Vibrio anguillarum, and Escherichia coli. The experiments were conducted in the presence of the cationic photosensitizers (PSs) 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP) and methylene blue (MB). For V. parahaemolyticus, the protocol was extended to the neutral and anionic PSs curcumin (CUR), chlorin e₆ trimethyl ester (Ce₆Me), and 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin (TPPS₄). The assays were performed in the absence or presence of potassium iodide, well-known to improve PDI effect, while its potential for aquaculture application is yet to be studied. These evaluations were performed in phosphate-buffered saline (PBS) and in artificial seawater (ASW) to simulate aquaculture water disinfection. The cationic PSs TMPyP and MB were the most efficient in the bacterial inactivation. Although higher concentrations of PS were required to achieve effective bacterial inactivation in ASW compared to PBS, the application of KI enhanced the effectiveness of all PSs, reducing the treatment time at least by threefold. The bacterial inactivation profiles revealed higher susceptibility of Vibrionaceae bacteria which may be linked to the differential PDI impact on the outer cell membranes of these bacteria. These findings indicate that treatments with PSs, particularly TMPyP and MB, in combination with KI constitute a potential approach for bacteria control in the aquaculture sector.