A polyphasic approach to study the intraspecific diversity of Yersinia ruckeri strains isolated from recent outbreaks in salmonid culture

A polyphasic analysis was carried out on Yersinia ruckeri strains isolated from recently outbreaks in vaccinated fish using a combination of different phenotypic and molecular typing methods in order to study their variability and epidemiological relationships. Eighty strains were subjected to biotyping with conventional tests and API 20E system, serotyping, outer membrane protein (OMP) and lipopolysaccharide (LPS) profiling, and genetic fingerprinting by ERIC-PCR and REP-PCR techniques. The strains showed a high diversity, as evidenced by the formation of different phenotypic groups mainly related to the serotypes, LPS and OMP profiles. The diversity among all isolates, calculated as Simpson's diversity index (Di), varied between 0.35 (REP-PCR) and 0.70 (OMP). The most discriminative values (Di value ≥0.86) were obtained from any combination of three methods including biotype, serotype, API 20E profile, LPS or OMP. With the combination of all typing methods used a Di value of 0.90 was obtained. Association between different groups to the host species was evidenced. Furthermore, it seems that strains with similar characteristics are associated with recent outbreaks occurred in vaccinated fish in certain geographical areas. Our results emphasize the usefulness of using a combination of several different typing methods for epidemiological and bacterial diversity studies.