Histopathological findings after sea bass (Dicentrarhus labrax L.) exposure to extracellular products of Photobacterium damsela subsp. piscicida produced in vivo

Groups of sea bass (Dicentrarhus labrax L.) were surgically implanted in the abdominal cavity with dialysis tubing of different pore size containing either live Photobacterium damsela subsp. piscicida cells or sterile phosphatebuffered saline, in order to grow the pathogen in vivo. During the course of the experiment mortalities, not due to microbial infection, occurred, that prompted the use of histology in order to identify the lesions caused by molecules released from the dialysis bags during in vivo growth of the pathogen. Collected tissues from moribund fish were processed for light microscopy. Fish implanted with the 2 kD molecular weight cut-off (MWCO) bags suffered very minor histological changes whereas fish with 12 kD MWCO membranes showed severe lesions varying upon the time post operation. Moreover, inflammatory cells appeared in all tissues from fish implanted with 12 kD MWCO especially 48 h after infection. Spleen, liver, intestine and gills showed necrotic changes appearing 60 h post infection. Since no bacteria were isolated after microbiological sampling of tissue, the inflammatorynecrotic changes observed in the tissues were attributed to the toxicity of extracellular products.