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Summary Yersinia ruckeri is the causative agent of enteric redmouth in fish and one of the major bacterial pathogens causing losses in salmonid aquaculture. Previously typing methods, including restriction enzyme analysis, pulsed‐field gel electrophoresis and multilocus enzyme electrophoresis (MLEE) have indicated a clonal population structure. In this work, we describe a multilocus sequence typing (MLST) scheme for Y. ruckeri based on the internal fragment sequence of six housekeeping genes. This MLST scheme was applied to 103 Y. ruckeri strains from diverse geographic areas and hosts as well as environmental sources. Sequences obtained from this work were deposited and are available in a public database (http://publmst.org/yruckeri/). Thirty different sequence types (ST) were identified, 21 of which were represented by a single isolate, evidencing high genetic diversity. ST2 comprised more than one‐third of the isolates and was most frequently observed among isolates from trout. Two major clonal complexes (CC) were identified by eBURST analysis showing a common evolutionary origin for 94 isolates forming 21 STs into CC1 and for 6 isolates of 6 STs in the CC2. It was also possible to associate some unique ST with isolates from recent outbreaks in vaccinated salmonid fish.

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...

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