Biochemical and Serological Characteristics, Drug Resistance and Plasmid Profiles of Spanish Isolates of Aeromonas salmonicida

Abstract

We conducted a comparative study of Aeromonas salmonicida strains isolated in the Northwest of Spain from salmon and trout (Oncorhynchus mykiss, Salmo salar and S. trutta) cultured in fresh and sea waters with reference strains from different countries. Regardless of the geographic origin and source of isolation, all strains were biochemically and serologically similar, belonging to A. salmonicida subsp. salmonicida. All the isolates exhibited a uniform plasmid pattern. The majority of them (93%) carried three small plasmids of 3.6, 3.5 and 3.4 Megadaltons (Md). In addition, a fourth plasmid of 4.2 Md was present in 21 of the 32 strains examined. Only in some isolates a large plasmid was detected in the range of 40-55 Md. A high number of the Spanish isolates were resistant to oxolinic acid and flumequine. However no correlation between plasmid content and drug resistances was observed. The phenotypic and genetic homogeneity of the pigmented A. salmonicida strains make very difficult the determination of the origin of furunculosis in a particular geographic area.