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There is a need for a standard in vitro procedure to infect larval fish with bacteria. Such a procedure could be used as a tool to evaluate fish quality, verify fish sensitivity to a certain bacterial strain or evaluate the pathogenicity of a bacterium towards a given fish species. The availability of a challenge test is a prerequisite in studies dealing with the effect of different medicinal treatments on diseased fish. An oral challenge test would offer the possibility to investigate the routes of infection via the digestive tract, i.e. after feeding contaminated feeds. Different authors have suggested that infection of marine fish was established through the food chain (Sera and Kumata, 1972;Campbell and Buswell, 1983;Muroga et al., 1987). The present study aims to verify this hypothesis and proposes a standard challenge procedure for turbot (Scophthalmus maximus) larvae with a pathogenic Vibrio anguillarum strain.Turbot (32 days old) originating from a commercial hatchery (France Turbot SA, Noirmoutiers, France) were acclimated for 1 week in a holding tank maintained at 19 °C. The water was continuously aerated and circulated over a biofilter. Daily, 10% of the water was replaced with new aerated seawater of the same temperature. The animals were fed twice daily with Artemia at a density of 300 nauplii fish -1. No mortalities were noticed after 3 days acclimation. The fish were then distributed in ten 5 1 aquaria at a density of 20 individuals per aquarium. Each aquarium was continuously aerated with an airstone and maintained at 19 °C. Daily, dead fish and faeces were siphoned off, one-third of the water was changed, salinity and nitrogen levels were checked, and fish behaviour was observed. The experiment involved two groups of fish, challenged and non-treated controls, with five replicates for each.

Samplings were organized in three marine fish hatcheries. Samples were taken at different steps during the production of algae, rotifers and Artemia. There were 233 dominant colony types isolated for further characterization. The isolates were compared to each other and to reference strains of Phorobacrerium, Yibrio and Alreromonas, using cellular fatty acid fingerprints. Fatty acid profiles of the isolates were grouped using principal component analysis. The isolates fell into at least ten major groups, of which two correspond to the genera Vibrio and to Alteromonas, respectively. Some groups seem to be restricted to a particular type of live food or isolation site. Although 43% of the isolates belong to Vibrio, only a minority of them could be readily assigned to known species.live feed with pathological conditions in fish larvae are presently underway.

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