BIOCHEMICAL AND SEROLOGICAL CHARACTERIZATION OF EDWARDSIELLA SPP. AND THEIR IMPACT UPON POPULATIONS OF ANADROMOUS FISHES

Reports of Edwardsiellosis in Atlantic salmon, Salmo salar, and striped bass, Morone saxatilis, prompted an investigation of the susceptibility of these anadromous fishes to infection by Edwardsiella spp. The relationships of the antigenic type and biochemical characteristics of the bacteria to disease production were studied. Thirty-two isolates of E. tarda obtained from various vertebrate hosts and 83 isolates of E. ictaluri from fish were studied. Biochemical characteristics were homogeneous within each species, and did not provide a basis for further intraspecific differentiation. The E. tarda isolates were divided into 3 major protein types based on SDS-polyacrylamide gel electrophoresis of soluble cell sonicates. Type A was heterogeneous with respect to lipopolysaccharide (LPS) type and host species. Types B and C consisted of one closely related LPS type and all isolates in this group were obtained from fish. Microagglutination assay with heat stable cellular antigen was used to demonstrate the antigenic homogeneity of the type B and C isolates. Immunoblot assay confirmed that antigenic specificity was due to the LPS, rather than the protein components of the bacteria. The E. ictaluri isolates, in contrast to E. tarda, were homogeneous with respect to protein and LPS components. The antigenic homogeneity of these isolates was demonstrated by microagglutination assay with heat stable cellular antigen. Type B isolates were chosen for virulence studies. Type B E. tarda from chinook salmon, Oncorhynchus tshawytscha, and striped bass had an LD50 of 1.0 x 10(\u274) cells/fish and 3.8 x 10(\u275) cells/fish, respectively, when administered by intraperitoneal injection to Atlantic salmon held at 18(DEGREES)C. Furthermore, experiments with dip exposures, using 6 type B E. tarda isolates, demonstrated that four of the isolates were moderately virulent in Atlantic salmon held at 18(DEGREES)C, and highly virulent in striped bass held at 23.5(DEGREES)C. No mortality was produced when striped bass, held at 24(DEGREES)C, were challenged by dip exposure with 3 E. ictaluri isolates