The in vivo and in vitro toxicity of bacterial cells and their extracellular products (ECPs) from 16 strains of Photobacterium damselae subsp. damselae isolated from 7 epizootic outbreaks were evaluated. On the basis of their 50% lethal dose (LD 50 ) values (about 1 × 10 5 CFU), these strains may be considered as moderately virulent. However, their ECPs were strongly lethal for redbanded seabream Pagrus auriga causing fish death within 2 h post-inoculation (protein concentration ranged between 2.1 and 6.41 µg g -1 fish). The bacterial ECPs tested exhibited several enzymatic activities, such as amylase, lipase, phospholipase, alkaline phosphatase, esterase-lipase, acid phosphatase, and β-glucosaminidase. These ECPs displayed a strong cytotoxic effect on 4 fish and 2 mammalian cell lines, although this activity disappeared when ECPs were heated at 100°C. The virulence of the strains tested could not be related to the hemolytic activity or to the production of the toxin damselysin. Therefore, another unknown type of toxin could play an important role in the virulence mechanisms of this bacterial pathogen.KEY WORDS: Toxicity · ECP · Photobacterium damselae subsp. damselae · Cultured marine fish Resale or republication not permitted without written consent of the publisherDis Aquat Org 92: [31][32][33][34][35][36][37][38][39][40] 2010 brane, has been described (Kreger 1984, Kothary & Kreger 1985, Kreger et al. 1987. In addition, a relationship between the degree of virulence and the hemolytic activity has been demonstrated in P. damselae subsp. damselae strains isolated from fish (Fouz et al. 1993). However, Cutter & Kreger (1990) found that not all the P. damselae subsp. damselae strains presented the damselysin gene (dly), but only those strains showing intense hemolytic activity. A further study demonstrated that the presence of this gene was not correlated to the virulence in mice and fish of 17 P. damselae subsp. damselae strains isolated from different sources (Osorio et al. 2000).The extracellular products (ECPs) are produced by bacterial pathogens to facilitate the uptake of nutrients from the surrounding environment, and/or for the successful penetration and survival of pathogens inside the host (Bakopoulos et al. 2003). However, the role of ECPs in the pathogenesis of Photobacterium damselae subsp. damselae in fish is poorly known, which is a considerable disadvantage for the development of vaccines and vaccine strategies, since it has been suggested that the ECP components are major antigenic compounds of several vaccine formulations (Collado et al. 2000, Bakopoulos et al. 2004.The aim of this study was to determine the toxicity of different Photobacterium damselae subsp. damselae strains isolated from cultured diseased fish. For this purpose, we performed in vivo and in vitro assays using bacterial cultures and their ECPs. In addition, the enzymatic activities of the ECPs and their cytotoxicity in fish and mammalian cell lines were compared. MATERIALS AND METHODSBacterial strains. In this...