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.