Vibrio anguillarum resistance to rainbow trout (Oncorhynchus mykiss) serum: Role of O-antigen structure of lipopolysaccharide

The sensitivity of Vibrio anguillarum to the bactericidal effect of rainbow trout serum was investigated with different strains of serogroups O1 and O 2a, which are the most frequently found serogroups in clinical outbreaks of vibriosis. All of the V. anguillarum strains were able to activate complem ent in rainbow trout serum, but smooth strains of V. anguillarum serogroup O1 were resistant to complement-mediated killing in the absence of specific antibodies. In the case of V. anguillarum serogroup O2a strains, 80% of th e analyzed strains were resistant to rainbow trout serum even when specific antibodies were present. Analysis of the lipopolysaccharide structures of the tested V. anguillarum strains showed a positive correlation between the O-antigen size of the lipopolysaccharide and resistance to serum killing, The classical complement pathway was responsible for the antibody-dependent serum killing of susceptible V. anguillarum strains. When serum-resistant V. anguillarum serogroup O2a strains were grown in glucose-enriched Lennox L broth, they produced lipopolysaccharide molecules with fewer high-molecul ar-weight O-antigen units than did strains grown in broth without the addit ion of glucose. Strains grown in glucose-enriched medium became sensitive t o rainbow trout serum killing, indicating that the high-molecular-weight O- antigen side chains prevented the activated complement from damaging the ba cterium.