Capacity of cationic and anionic porphyrins to inactivate the potential aquaculture pathogen Vibrio campbellii

Photodynamic Antimicrobial Chemotherapy (PACT) has emerged as a promising method for pathogen eradication and control. PACT uses light excitation of non-toxic photosensitisers to produce singlet oxygen (¹O₂), which in turn damages and eradicates microbial cells. In the present study, a naturally luminescent Vibrio campbellii strain ISO7 (V. campbellii ISO7) was used as a model aquaculture pathogen to test the suitability of two porphyrin compounds, the tetra-cationic TMPyP and the tetra-anionic TPPS₄, for the treatment of aquaculture water. Initial work accurately identified the bacterial strain and confirmed its virulence towards giant tiger prawns Penaeus monodon when injected. The strain killed 100% of injected prawns within 1 to 16 h, while control prawns injected with sterile buffer remained healthy. Koch's postulates were satisfied by identification of re-isolated strains by multiplex PCR and sequencing of housekeeping genes. In separate time-course experiments, the two photosensitisers were diluted in aquaculture water seeded with the indicator bacterium and samples were irradiated for 24 h using 150 W white LED light. Luminescence assays, growth and regrowth experiments demonstrated that the cytotoxicity of generated ¹O₂ was both time- and dose-dependent, and confirmed that light or porphyrins alone were not toxic. Continuous irradiation in the presence of 20 μM cationic porphyrin for 5 h or 1 μM for 24 h achieved complete lethality of the indicator bacterium. Consistent with previous reports, the tested anionic porphyrin did not impact on the survival of the bacterium, causing only a slight decline in the luminescence signal. Photo-bleaching tests demonstrated natural degradation of both porphyrins after continuous irradiation, making them suitable as ‘self-destructive’ photosensitizers for in situ treatment of aquaculture waters, as they do not accumulate in the water.