Photodynamic inactivation: a new potential approach against Philasterides dicentrarchi

Philasterides dicentrarchi is a ciliate protozoan responsible for fatal Scuticociliatosis in marine fish (Iglesias et al., 2001). This parasitosis cause high mortality in farmed fish and, consequently, extensive economic losses in aquaculture sector. So far, there are no commercial vaccines to prevent P. dicentrarchi outbreaks and no effective chemotherapeutic agents to treat infected animals are known (Jalenques et al., 2021). More recently, microbial photodynamic inactivation (PDI) has demonstrated to be a promising approach against aquatic protozoa, suggesting its application in aquaculture (Wohllebe et al., 2012). In this study, the potential of PDI approach in the inactivation of P. dicentrarchi was evaluated. Photodynamic experiments were carried out in seawater using two photosensitizers (PS), 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin tetraiodide (Tetra-Py(+)- Me) and Methylene Blue (MB). Ciliates were exposed to several concentrations of each PS (0.1 – 20 µM) and the mortality determined along 60 min of white light irradiation (10 mW.cm-2). The effect of each PS was also evaluated in combination with potassium iodide (KI), a well-known PDI adjuvant. The results obtained indicate that PDI with Tetra-Py(+)-Me (5.0 µM) and MB (0.1 µM) were effective in killing all ciliates after 60 min of treatment. The application of KI (1.0 mM) improved the photodynamic action of Tetra-Py(+)-Me (5.0 µM), allowing to reduce the treatment time to eliminate all ciliates to half (30 min). On the other hand, the combination of MB (0.1 µM) + KI (1.0 mM) had an effect similar to that of this PS alone. The effect of PDI on the cellular morphology of ciliates was also evaluated: ciliates subjected to MB treatment mostly showed plasma membrane vacuolation, cell rounding and membrane disintegration, while extensive cell lysis was observed with Tetra-Py(+)-Me treatment. In conclusion, the results obtained in this study indicate that PDI is an effective approach against infective stages of P. dicentrarchi and suggest its promising application for the control of this parasitosis in aquaculture, particularly with MB.