PLASMID AND SEROLOGICAL DIFFERENCES BETWEEN EDWARDSIELLA-ICTALURI STRAINS

Several studies have shown that isolates of Edwardsiella ictaluri obta ined from infected channel catfish in the southeastern United States h arbor two cryptic plasmids, designated pCL1 (5.7 kb) and pCL2 (4.9 kb) . These isolates appear to be serologically homogeneous. To extend the se studies, we focused our analyses on two isolates of nonictalurid or igin. Plasmid analyses of a danio isolate showed that it harbored plas mids which were similar if not identical to pCL1 and pCL2. This strain was also serologically indistinguishable from those isolated from cha nnel catfish. In contrast, a green knife fish (GNF) isolate harbored f our plasmids with relative mobilities of 6.0, 5.7, 4.1, and 3.1 kb. So uthern blot analyses indicated that only the 5.7- and 4.1-kb plasmids strongly hybridized under high-stringency conditions to probes specifi c for pCL1 and pCL2, respectively. The GNF isolate showed minimal reac tivity when reacted with polyclonal antiserum prepared against a chann el catfish isolate. However, polyclonal antiserum to the GNF isolate s trongly reacted with the GNF isolate in both surface fluorescence and agglutination reactions. Sodium dodecyl sulfate-polyacrylamide gel ele ctrophoresis analyses of cell lysates showed that the protein banding patterns of the strains compared were similar. However, Western blots of proteinase K-digested cell extracts showed that O antigen of the GN F isolate was antigenically distinct from the 0 antigen of the other i solates. These studies indicate that there are different serotypes of E. ictaluri and suggest that plasmid and serological analyses of futur e isolates of E. ictaluri can be used to determine whether structurall y distinct strains are emerging in major channel catfish aquaculture a reas.