European sea bass Dicentrarchus labrax from the Mediterranean were diagnosed with a severe encephalitis. Rickettsia-like organisms (RLOs) were associated with brain lesions in routine paraffin sections. These were found to share common antigens with the Piscirickettsia salmonis typestrain, LF-89, by indirect fluorescent antibody test (IFAT) and by immunohistochemistry (IHC). In addition, we compared the DNA sequences of the 16S rDNA and 16S-23S internal transcribed spacer region (ITS) with those published for P. salmonis strains and found that the sea bass piscirickettsialike organism (SBPLO) was another strain of P. salmonis, closely related to the salmonid pathogens. Furthermore, we showed that the SBPLO possessed at least 2 ITS regions, 1 of which contained tRNA genes.KEY WORDS: Piscirickettsia salmonis · European sea bass Dicentrarchus labrax · IFAT · Immunohistochemistry · rDNA · Phylogeny Resale or republication not permitted without written consent of the publisherDis Aquat Org 64: [107][108][109][110][111][112][113][114][115][116][117][118][119] 2005 was placed among the Gammaproteobacteria (Fryer et al. 1992). Subsequently, Mauel et al. (1999) used comparisons of 16S rRNA gene sequences and internal transcribed spaces (ITS) sequences to determine the relatedness of other P. salmonis isolates from Chile, Canada and Norway. They established that these strains formed a monophyletic group within the Gammaproteobacteria, although 1 Chilean isolate, EM-90, had diverged sufficiently to allow differentiation from the other isolates based on restriction fragment length polymorphism (RFLP) (Mauel et al. 1996). This group found only 1 ITS sequence in the isolates examined but, following polyacrylamide gel analysis of amplified ITS regions, Casanova et al. (2001) have suggested that P. salmonis may contain at least 2 rRNA (rrn) operons, as is commonly the case for other Gram-negative bacteria (Gürtler & Stanisich 1996, Crosby & Criddle 2003. More recently, have extended the information on regional variation of P. salmonis isolates through their comparison of 16S rDNA and ITS sequences from Scottish and Irish isolates.In this study, routine diagnostic histopathological examination was conducted on European sea bass presenting with clinical signs of nervous disease. In paraffin sections an RLO was seen to be associated with the encephalitic lesions. Serological analyses, indirect fluorescent (IFAT) and immunohistochemistry (IHC) were then used to confirm the tentative diagnosis and to confirm whether or not the organism was antigenically related to the Piscirickettsia salmonis type-strain, LF-89. The DNA sequences of the 16S rDNA and ITS region were then compared with those of published P. salmonis strains to establish whether or not the sea bass piscirickettsia-like organism (SBPLO) might be another strain of P. salmonis and how closely genetically related it was to the salmonid pathogens. Furthermore, it was sought to establish if the sea bass isolate possessed at least 2 ITS regions, 1 of which cont...