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 o f enteric redmouth disease (ERM), were comparatively studied by means of li popolysaccharide (LPS) and outer membrane protein (OMP) analysis, plasmid p rofiling and ribotyping in order to investigate the heterogeneity among iso lates and the usefulness of these methods as epidemiological markers. Only two LPS profiles were observed among the isolates studied, corresponding wi th the serotypes O1 and O3 of Y. ruckeri. A higher heterogeneity was detect ed analysing the OMP, seven different patterns being observed. Although som e isolates carried different small plasmids, all the serotype O1 isolates s howed a plasmid band of 50 MDa and all the serotype O3 strains shared in co mmon a extrachromosomic DNA band of 30 MDa. The analysis of the ribopattern s obtained using three different enzymes, separated the strains into 10 rib otypes, indicating genetic heterogeneity among the isolates. The heterogene ity 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 strain s on the basis of farm and/or season of isolation, we consider that ribotyp ing is the best candidate for epidemiological studies because it is easier to perform and offers a slightly better discriminative power.