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

The fish pathogen Aeromonas salmonicida ss salmonicida was first recorded in Norwegian farmed salmonids in 1985. Plasmid profiling was investigated as 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 1 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 possibility was that there were 4 large plasmids, each sharing common sequences, but that the individual plasmids did not always replicate to the state where visible bands were produced. Different growth media or length of incubation of culture had no apparent effect on the occurrence of the different large plasmid bands. The DNA of the large plasmid bands was 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 the isolates whereas only 7.3% of the isolates contained plasmids of intermediate size (6.5 to 40 MDa). Overall, the plasmid content of A. salmonicida ss salmonicida seemed to be very constant worldwide, and thus plasmids in this organism would generally be of little epizootiological value.