Grouping by plasmid profiles of atypical Aeromonas salmonicida isolated from fish, with special reference to salmonid fish

Plasmid profile analyses were performed for 113 strains of atypical Aeromon as salmonicida and the reference strain A. salmonicida subsp, salmonicida A TCC 14174. The atypical A. salmonicida strains comprised 98 strains obtaine d from fish originating from 54 farms and 2 lakes in Norway, 10 strains fro m Canada (2), Denmark (2), Finland (1), Iceland (1) and Sweden (4), the ref erence strains NCMB 1109 and ATCC 15711 (Haemophilus piscium) of A. salmoni cida subsp, achromogenes, and the type cultures A. salmonicida subsp, achro mogenes NCMB 1110, A. salmonicida subsp, masoucida ATCC 27013 and A. salmon icida subsp. smithia CCM 4103. A total of 95 strains of atypical A. salmoni cida were separated into 7 groups (I to VII) based on the plasmid profiles. Eighteen strains of atypical A. salmonicida had no common plasmid profile. The type strain NCMB 1110 and the reference strain NCMB 1109 were included in group IV, and the type strain ATCC 27013 in group V, but the other refe rence and type strains had plasmid profiles different from all the other st rains. An epidemiological link was documented between strains collected fro m different farms/localities in each of groups I, III, V and VII. Physiolog ical and biochemical characterizations were performed for 93 of the strains to investigate phenotypic differences between the plasmid groups. Group VI I strains and 3 strains with no common plasmid profile differed from the ot her groups in being catalase-negative. Differences in phenotypic characteri stics were shown between the plasmid groups. However, significant variation s in reactions for several phenotypic characteristics also occurred within each of the groups I to VII. The present study indicates that plasmid profi ling may give useful epidemiological information during outbreaks of atypic al A. salmonicida infections in fish. Additional comprehensive phenotypic c haracterisation is of limited value since the phenotypic characteristics in each plasmid group are not uniform.