VIRULENCE AND SERUM-RESISTANCE IN DIFFERENT CLONAL GROUPS AND SEROTYPES OF YERSINIA-RUCKERI

The virulence in rainbow trout (Oncorhynchus mykiss) of 32 isolates of Yersinia ruckeri, representing a range of biotypes, serotypes, and OMP-types, was examined. Virulence was assayed in fish of average weight 7.7 g by bath challenge for 1 h with approximately 5 x 10(7) cells per ml. Two of the six serotype O1 clonal groups of Y. ruckeri, clones 2 and 5, were virulent, whereas the other four clonal groups, clones 1, 3, 4 and 6, as well as all serotype O2, O5, O6 and O7 isolates examined, were avirulent. Analysis of susceptibility to the bactericidal effect of non-immune rainbow trout serum demonstrated an association between virulence and serum resistance. The virulent serotype O1 clonal groups were serum resistant, whereas the avirulent serotype O1 clonal groups and other serotypes were, with some exceptions, serum sensitive. The fact that some serum resistant isolates were avirulent suggested that other factors may be required for the full expression of virulence. The study also demonstrated that rainbow trout and brook trout (Salvelinus fontinalis) differ in their susceptibility to Y. ruckeri.