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Abstract :

Used in all stock farming sectors, antibiotic treatments are paid particular attention in fish farms. Indeed, it is legitimate to question about the contamination by antibiotics and resistant bacteria of the environment which receives effluents of fish farms. The aim of our research was to study the influences of fish farming along a river on the resistance rates, among Aeromonadaceae, against antibiotics used in fish farms. Still rare in fish pathogens, the acquired resistance against florfenicol, an antibiotic of recent use in fish farming, was searched for genetic characterisation. In a general way, the rates of Aeromonas resistant to oxolinic acid, oxytetracycline and the association sulfamethoxazole-trimethoprime, were often increased in the sediments collected immediately down-stream the fish farms and the wastewater treatment plant effluents. This contamination was not always associated with an antibiotic contamination, which was also noticed down-stream some studied fish farms. Florfenicol resistance was rare in the Aeromonas group, as only two resistant clones were isolated, which were shown to diffuse and to be persistent. In an A. bestiarum clone, florfenicol resistance was due to a conjugative plasmid, which carried a floR gene and other genes coding for resistance to tetracycline, sulfamids and streptomycine, close to an ISCR2 element which may be implicated in their mobility. Results showed first the complementarity of chemical and microbiological methods to evaluate the impact of antibiotics use in aquatic environment, and secondly that conditions are fulfilled for the spread of acquired florfenicol resistance, which should therefore be surveyed.