OCCURRENCE AND STABILITY OF PLASMIDS IN AEROMONAS-SALMONICIDA SS SALMONICIDA ISOLATED FROM SALMONIDS WITH FURUNCULOSIS

The fish pathogen Aeromonas salmonicida ss salmonicida was first recor ded in Norwegian farmed salmonids in 1985. Plasmid profiling was inves tigated as a possible method for use in epizootiological work. A total of 383 different isolates was investigated over a period of 6 yr. Pla smid profiling of the isolates revealed 1 to 4 large plasmid bands var ying in size from 52 to 105 MDa. Repeated plasmid screening of isolate s (even a single isolate) grown under varying conditions and DNA-DNA h ybridization, suggested that only one large plasmid was present but th at it could occur in up to 4 configurations. An alternative possibilit y was that there were 4 large plasmids, each sharing common sequences, but that the individual plasmids did not always replicate to the stat e where visible bands were produced. Different growth media or length of incubation of culture had no apparent effect on the occurrence of t he different large plasmid bands. The DNA of the large plasmid bands w as susceptible to mechanical stress and UV-light exposure. A group of 4 small plasmids of 3.4, 3.5, 3.6 and 4.2 MDa characterized most of th e isolates whereas only 7.3% of the isolates contained plasmids of int ermediate size (6.5 to 40 MDa). Overall, the plasmid content of A. sal monicida ss salmonicida seemed to be very constant worldwide, and thus plasmids in this organism would generally be of little epizootiologic al value.