Serological and molecular heterogeneity among Yersinia ruckeri strains isolated from farmed Atlantic salmon Salmo salar in Chile

Abstract

We investigated 11 strains of Yersinia ruckeri, the causative agent of enteric redmouth disease (ERM), that had been isolated from Atlantic salmon Salmo salar L. farmed in Chile and previously vaccinated against ERM. Phylogenetic analysis of the 16S rRNA gene sequences confirmed the identification of the salmon isolates as Y. ruckeri. A comparative analysis of the biochemical characteristics was made by means of traditional and commercial miniaturised methods. All studied isolates were motile and Tween 80 positive, and were identified as biotype 1. In addition, drug susceptibility tests determined high sensitivity to sulphamethoxazole/trimethroprim, oxytetracycline, ampicillin and enrofloxacin in all isolates. Serological assays showed the presence of O1a, O1b and O2b serotypes, with a predominance of the O1b serotype in 9 strains. Analysis of the lipopolysaccharide profiles and the correspondent immunoblot confirmed these results. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of the outer membrane proteins revealed that all Chilean strains had profiles with a molecular weight range between 34 and 55 kDa, with 3 distinct groups based on differences in the major bands. Genotyping analyses by enterobacterial repetitive intergenic consensus (ERIC-) and repetitive extragenic palindromic (REP-)PCR techniques clearly indicated intraspecific genetic diversity among Chilean Y. ruckeri strains. KEY WORDS: Yersinia ruckeri · Enteric redmouth disease · ERM · Atlantic salmon · Serology · Outer membrane proteins · Lipopolysaccharide · GenotypingResale or republication not permitted without written consent of the publisher Dis Aquat Org 93: 207-214, 2011 can be grouped into clonal types on the basis of biotype, serotype and outer membrane protein (OMP) profiles (Davies 1991). Strains of serotypes O1a (classic serovar I) and O2b (classic serovar II) cause most epizootic outbreaks, and serotype O1a is predominant in cultured salmonids (Stevenson & Airdrie 1984, Romalde et al. 1993.ERM has been successfully controlled for decades by vaccination with monovalent killed whole cell commercial vaccines. Although formulations of most commercial vaccines are based only on serovar I (Hagerman strain), different degrees of cross-protection among serotypes have been reported (Stevenson & Airdrie 1984). In recent years, reports of ERM vaccine breakdown have emerged in Europe and the USA, which were mostly attributed to biotype 2 strains (Austin et al. 2003, Fouz et al. 2006, Arias et al. 2007, Wheeler et al. 2009).In Northern Europe and Chile, both of which are major production areas of farmed Atlantic salmon, the fry are routinely vaccinated with commercial rainbow trout ERM vaccines (Bravo & Midtlyng 2007, Wheeler et al. 2009 In this study we report the full phenotypic, serological and molecular characterisation of a group of Yersinia ruckeri strains that were isolated from these ERM epizootics in Atlantic salmon farmed in Chile. MATERIALS AND METHODSBacterial isolates. Several episodes of ERM with mo...

Yersinia ruckeri Isolates Recovered from Diseased Atlantic Salmon (Salmo salar) in Scotland Are More Diverse than Those from Rainbow Trout (Oncorhynchus mykiss) and Represent Distinct Subpopulations

“…Yersinia ruckeri is primarily a pathogen of rainbow trout, and very few studies have focused on the comparison of isolates recovered primarily or exclusively from Atlantic salmon (8,9). The present analysis of 109 Y. ruckeri isolates recovered from diseased Atlantic salmon in Scotland over a 14-year period represents the most detailed study of isolates from this species performed to date.…”

“…Clearly, the biotype 2 phenotype has neither emerged independently in Atlantic salmon isolates by mutation of one or more of the flagellar secretion genes, nor has it been acquired from rainbow trout by strain transmission. However, it is also notable that Y. ruckeri strains isolated from Atlantic salmon in Australia and Chile are also members of biotype 1 and not biotype 2 (7,9). It has been postulated that vaccination may be providing the selective pressure that is driving the loss of flagellar motility in Y. ruckeri (20,42).…”

“…This approach allowed six distinct clonal groups to be distinguished among serotype O1 isolates and was able to differentiate between the classical Hagerman and the newly emerged biotype 2 strains (21). Subsequently, OMP analysis has played an important role in various epidemiological analyses of Y. ruckeri (9,15,24,(29)(30)(31).…”

“…Surprisingly, few investigations have focused exclusively, or predominantly, on isolates associated with yersiniosis of this species (9,12). There is concern that the emergence of novel strains may become a problem in Atlantic salmon.…”

Analysis of Yersinia ruckeri strains isolated from trout farms in northwest Germany

“…The disease was initially controlled by vaccines prepared with classical motile Y. ruckeri strains (Johnson & Amend 1983), but after appearance and spreading of non-motile variants, the vaccine failed to control infections in Europe (Austin et al 2003, Wheeler et al 2009) and the USA (Arias et al 2007). With the inclusion of both types of strains into the vaccine preparation, an enhancement of protection was achieved (Tinsley et al 2011), but despite the use of those bivalent vaccines, ERM outbreaks occurred in vaccinated Atlantic salmon in Chile (Bastardo et al 2011, Navas et al 2014 as well as in European rainbow trout stocks. In 4 trout farms from the present study, the stocks were vaccinated with a bivalent vaccine, and in 3 of these farms, ERM outbreaks caused by non-motile strains were observed during several sampling campaigns.…”