Occurrence and stability of plasmids iring,n Aeromonas salmonicida as salmonicida isolated from salmonids with furunculosis

The fish pathogen Aerornonas salmonicida ss salmonjcida was first recorded in Norwegian farmed salmonids In 1985. Plasmid profiling was investigated a s a possible method for use in epizootiological work. A total of 383 different isolates was investigated over a period of 6 yr. Plasmid profiling of the isolates revealed l to 4 large plasmid bands varying in size from 52 to 105 MDa. Repeated plasmid screening of isolates (even a single isolate) grown under varying conditions and DNA-DNA hybridization, suggested that only one large plasmid was present but that it could occur in up to 4 configurations. An alternative poss~bi l~ty was that there were 4 large plasmids, each sharing common sequences, but that the individual p lasm~ds did not always replicate to the stage where visible bands were produced. Different growth m e d ~ a or length of Incubation of culture had no apparent effect on the occurrence of the different large plasm~d bands The D N A of the large plasmid bands was susceptible to mechanical stress and UV-l~ght exposure A group of 4 small plasmids of 3.4, 3.5, 3 . 6 and 4.2 MDa characterized most of the isolates whereas only 7 .3 'Yo of the ~sola tes contained plasmids of intermediate size (6.5 to 40 MDa). Overall, the plasmid content of A. salrnonicida ss salmonicida seemed to be very constant worldcvide, and thus plaslnids In this organlsm would generally be of little epizootiological value.