Molecular characterization of Portuguese strains of Yersinia ruckeri isolated from fish culture systems

A total of 23 Portuguese strains of Yersinia ruckeri , the causative agent of enteric redmouth disease (ERM), were comparatively studied by means of lipopolysaccharide (LPS) and outer membrane protein (OMP) analysis, plasmid profiling and ribotyping in order to investigate the heterogeneity among isolates and the usefulness of these methods as epidemiological markers. Only two LPS profiles were observed among the isolates studied, corresponding with the serotypes O1 and O3 of Y. ruckeri . A higher heterogeneity was detected analysing the OMP, seven different patterns being observed. Although some isolates carried different small plasmids, all the serotype O1 isolates showed a plasmid band of 50 MDa and all the serotype O3 strains shared in common a extrachromosomic DNA band of 30 MDa. The analysis of the ribopatterns obtained using three different enzymes, separated the strains into 10 ribotypes, indicating genetic heterogeneity among the isolates. The heterogeneity was greater within serotype O1 isolates, six ribotypes being detected, than within serotype O3 in which only three ribotypes were found. Although OMP analysis and ribotyping can be useful for the differentiation of strains on the basis of farm and/or season of isolation, we consider that ribotyping is the best candidate for epidemiological studies because it is easier to perform and offers a slightly better discriminative power.