Confirmation of Piscirickettsia salmonis as a pathogen in European sea bass Dicentrarchus labrax and phylogenetic comparison with salmonid strains

Abstract

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...

Isolation of Tasmanian Rickettsia-like organism (RLO) from farmed salmonids: identification of multiple serotypes and confirmation of pathogenicity

“…This method of differentiation appears to be robust, although an RLO detected in seabass Dicentrarchus labrax and later designated as P. salmonis (SBPLO) has also been shown to possess the same genomic signature (McCarthy et al 2005). Since the SBPLO observation was a one-off in a non-salmonid host species, heavy reliance was placed on rDNA sequences which may or may not be phylogenetically informative, and SBPLO was not isolated in culture, preventing further analyses.…”

“…Antigen-based assays may indeed become useful diagnostic tools in future and protocols may require revision, but before this occurs it is important that stable taxonomic relationships are systematically established. Most crucial would be a formal polyphasic approach comparing TRLO, P. salmonis and, if possible, P. salmonis-like organisms (Chen et al 2000a,b, Mauel et al 2003, McCarthy et al 2005 incorporating methods secondary to 16S rDNA or multilocus sequence analysis such as DNA:DNA hybridisations (Rosselló-Mora & Amann 2001, Stackebrandt et al 2002, Tindall et al 2010 or whole genome sequence comparison (Whitman 2015). The latter is now very realistic given that 2 P. salmonis genomes have already been sequenced and published (Eppinger et al 2013, Yañez et al 2014.…”

“…In fish, many infecting bacteria have been named Rickettsia-like organisms (RLOs) on the basis of their morphology and intracellular nature. The most prominent of these was Piscirickettsia salmonis, a well-described pathogen of both salmonid (Fryer et al 1992) and non-salmonid fish (Arkush et al 2005, McCarthy et al 2005. P. salmonis was initially designated a Rickettsiales-like organism and tentatively placed in the Rickettsiaceae family and Ehrlichieae tribe (Fryer et al 1990).…”

Persistence of Piscirickettsia salmonis and detection of serum antibodies to the bacterium in white seabass Atractoscion nobilis following experimental exposure

“…P. salmonis and P. salmonis-like bacteria have been detected from a variety of marine and freshwater fish (Mauel & Miller 2002). Two of these bacteria from marine fish recently have been confirmed to be P. salmonis by phylogenetic comparisons of the small and large subunit ribosomal (16S and 23S) DNA and the internal transcribed spacer (ITS) (Arkush et al 2005, McCarthy et al 2005. Arkush et al (2005) demonstrated that P. salmonis causes a disease similar to piscirickettsiosis in white seabass Atractoscion nobilis.…”

“…Dis Aquat Org 73: [131][132][133][134][135][136][137][138][139] 2006 with lesser amounts in visceral organs (Comps et al 1996, Grant et al 1996, Skarmeta et al 2000, McCarthy et al 2005. P. salmonis and P. salmonis-like bacteria have been detected from a variety of marine and freshwater fish (Mauel & Miller 2002).…”