Characterization of plasmids in bacterial fish pathogens

Abstract

Plasmid profiles of representative fish pathogens, Aeromonas salmonicida, Aeromonas hydrophila, Vibrio anguillarum, Pasteurella piscicida, Yersinia ruckeri, Edwardsiella tarda, and Renibacterium salmoninarum, were determined by agarose gel electrophoresis with four different plasmid detection methods. A combination of two methods was required to detect the plasmids present in these strains and to calculate precisely the molecular weights of the plasmids. Of 38 strains, 28 harbored one or more plasmids, with the majority of strains demonstrating multiplasmid banding. Similarity in plasmid banding between strains was six plasmid bands having molecular weights of 8.6 x 106, 8.4 x 106, 8.1 x 106, 3.6 x 106, 3.5 x 106, and 3.4 x 106. Four P. piscicida isolates shared three plasmid bands having molecular weights of 37 x 106, 15 x 106, and 5 x 106, and five A. hydrophila strains harbored a common plasmid having a molecular weight of ca. 20 x 106 to 30 x 106. The highest-molecular-weight plasmids (145 x 106 and 130 x 106) were detected in V. anguillarum. From curing experiments, it was found that in A. hydrophila strain 79-62, a loss of resistance to tetracycline was associated with loss of plasmid content in all susceptible derivatives, suggesting plasmid-mediated tetracycline resistance. Cell surface characteristics and metabolic properties were also mediated in cured derivatives of A. hydrophila strain 79-62.