Characterization of a novel chromosomal virulence locus involved in expression of a major surface flagellar sheath antigen of the fish pathogen Vibrio anguillarum.

The fish pathogenic bacterium Vibrio anguillarum 775.17B was mutated by the use of transposon Tn5-132. Two hundred independent exconjugants were isolated and screened for a reduction of virulence in experimental infections of rainbow trout (Onchorhynchus mykiss). Two of these exconjugants, VAN20 and VAN70, showed a significant reduction in virulence after both intraperitoneal and immersion infections. The avirulent mutants showed no loss of any previously suggested virulence determinants of V. anguillarum. One of the mutants (VAN70) was further characterized. DNA sequence analysis revealed two open reading frames, the gene into which Tn5-132 had been inserted (virA) and a closely linked upstream gene (virB). A virB mutant of 775.17B, NQ706, was isolated and also shown to be avirulent. The deduced amino acid sequences of virA and virB correspond to proteins with molecular weights of 36,000 and 42,000, respectively. Insertional mutagenesis of the corresponding virA and virB genes of a clinical isolate of V. anguillarum, serotype O1, also resulted in avirulence. In immunoblot experiments, the total cell lysates of VAN70 (virA) and NQ706 (virB) did not respond to a rabbit polyclonal antiserum directed against whole cells of 775.17B (wild type). This suggests that virA and virB are involved in the biosynthesis of a major surface antigen important for the virulence of V. anguillarum. Immunogold electron microscopy showed that a constituent of the flagellar sheath was expressed by 775.17B (wild type) but not by VAN70 (virA) and NQ706 (virB), suggesting that the major surface antigen lacking in VAN70 and NQ706 is located on the outer sheath of the flagellum. Analysis of this major surface antigen revealed it likely to be lipopolysaccharide. Further analysis showed that the flagellum and the major surface antigen were expressed in vivo during fish infections.