Phylogenetic analysis of Piscirickettsia salmonis by 16S, internal transcribed spacer (ITS) and 23S ribosomal DNA sequencing

Piscirickettsia salmonis, the etiologic agent of piscirickettsiosis, is a systemic disease of salmonid fish. Variations in virulence and mortality have been observed during epizootics at different geographical regions and in laboratory experiments with isolates from these different locations. This raises the possibility that biogeographical patterns of genetic variation might be a significant factor with this disease. To assess the genetic variability the 16S ribosomal DNA, the internal transcribed spacer (ITS) and the 23S ribosomal DNA of isolates from 3 different hosts and 3 geographic origins were amplified using the polymerase chain reaction (PCR). Results of this analysis confirm that P. salmonis is a member of the gamma subgroup of the Proteobacteria and show that the isolates form a tight monophyletic cluster with 16S rDNA similarities ranging from 99.7 to 98.5 %. The ITS regions were 309 base pairs (bp), did not contain tRNA genes, and varied between isolates (95.2 to 99.7 % similarity). Two-thirds of the 23S rRNA gene was sequenced from 5 of the isolates, yielding similarities ranging from 97.9 to 99.8 %. Phylogenetic trees were constructed based on the 16S rDNA, ITS and 23S rDNA sequence data and compared. The trees were topologically similar, suggesting that the 3 types of molecules provided similar phylogenetic information. Five of the isolates are closely related (>99.4 % 16S rDNA similarity, 99.1 to 99.7 % ITS and 99.3 to 99.8 % 23S rDNA similarities). The sequence of one Chilean isolate, EM-90, was unique, with 16S rDNA similarities to the other isolates ranging from 98.5 to 98.9 %, the ITS from 95.2 to 96.9 % and the 23S rDNA from 97.6 to 98.5 %.