Piscirickettsia salmonis was grown in established insect, frog, and fish tissue culture cells. The yield of P. salmonis in Sf21 cells was up to 100 times that obtained in CHSE-214 cells, and virulence for Atlantic salmon was retained. The ceiling temperature for growth of P. salmonis in Sf21 cells was 24°C

Salmonid rickettsial septicemia, caused by Piscirickettsia salmonis, causes major mortalities in Chilean salmonid aquaculture and is an increasing problem in Atlantic salmon in Ireland and Scotland. Analysis of 16S-to-23S internal transcribed sequences and 16S ribosomal DNA (rDNA) shows that Irish isolates of P. salmonis form two new groups of the organism while Scottish isolates cluster together with Norwegian and Canadian isolates from Atlantic salmon.Salmonid rickettsial septicemia (SRS), or piscirickettsiosis, is a systemic disease of cultured salmonids initially recognized in coho salmon in Chile (1), where it causes significant losses in aquaculture (4). Subsequently, disease has been recorded in Canada (3), Norway (19), Ireland (21), and Scotland (A. N. Grant, A. G. Brown, D. I. Cox, T. H. Birkbeck, and A. A. Griffen, Letter, Vet. Rec. 423:138, 1996), but the impact in the Northern Hemisphere (0.6 to 18% mortalities) has not yet reached the levels experienced in Chile (30 to 90% mortalities) (1). Coho salmon appear to be more susceptible to SRS than Atlantic salmon, which has altered the balance of species cultured in Chile (18). Rickettsia-like organisms have been observed globally in a wide range of marine organisms (7), but difficulty of culture means their gram-negative classification and usual intracellular location within host cells are often all that is known of them. The etiologic agent of SRS was isolated (8) and later named Piscirickettsiosis salmonis as a new genus and species (9). The 16S ribosomal DNA (rDNA) sequence of the P. salmonis type strain, LF-89, indicated taxonomic placement within the ␥ subdivision of the Proteobacter group, rather than alongside true rickettsiae in the ␣ subdivision (9). Ensuing 16S rDNA analysis of isolates from Chile, Canada, and Norway by PCR and restriction fragment length polymorphism revealed a homogeneous group with the presence of one outlier, Chilean strain EM-90 (16), which was substantiated from the 16S rDNA sequence (17). More subtle differences between strains are illuminated by analyzing the 16S-to-23S intergenic transcribed spacer (ITS), which possesses a higher level of sequence diversity (17), meaning that ITS sequence data can be used to infer evolutionary and phylogenetic relationships not evident from 16S rDNA. Both 16S rDNA and ITS data can be complicated by the presence of multiple rDNA operons, documented in P. salmonis strains LF-89 and EM-90 (4). However, the P. salmonis-specific ITS primers described by Marshall et al. (14) are reported to amplify only one ITS region, allowing strain-to-strain comparisons (4). Of the isolates studied by Mauel et al. (17)-four Chilean isolates, one Canadian isolate, and one Norwegian isolate-ITS data analysis indicates phylogenetic diversity between isolates from different geographic regions. Here we apply 16S and ITS sequence data analysis to the widest range of SRS isolates yet to be studied, representing the first report of sequences from Irish and Scottish isolates.Infected fish samples. Samples wer...