Culture, Description and Identification of Edwardsiella ictaluri   John P. Hawke   Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803     Edwardsiella ictaluri was originally described by biochemical and genetic analysis of 26 cultures from diseased pond reared channel catfish between 1976 and 1979.  The specimens were submitted for diagnosis to the Southeastern Cooperative Disease Laboratory at Auburn University. The disease was described and named Enteric Septicemia of Catfish  (ESC) in 1979 and the causative agent was described as a new species, Edwardsiella ictaluri, in 1981. Culture and isolation is done by streaking tissue from internal organs or brain on tryptic soy agar with 5% sheep blood or brain heart infusion agar. Anaerobic incubation of plates or inoculation of fluid thioglycolate medium may enhance isolation.  Optimum growth is between 28 and 30°C, within which the bacterium produces 2 mm whitish-gray colonies after 48 h of incubation.  Selective isolation and culture is achieved on EIM medium where E. ictaluri forms a small pale-green colony.  The original description of E. ictaluri was done by a combination of staining characteristics, cell morphology, biochemical and physiologic tests.  Genetic characterization was accomplished by DNA/DNA hybridization and determination of G+C ratio on strain GA 77-52 and 5 other isolates indicating that the bacterium was most closely related to Edwardsiella tarda  (56-62% DNA homology) but varied by being negative for H2S, indole, Jordan tartrate, gas from glucose at 37°C and motility at 37°C.  The % G+C was 53 mol % as determined by buoyant density centrifugation. The type strain GA77-52 was deposited in the American Type Culture Collection and designated ATCC 33202. The biochemical, biophysical and serological homogeneity is uniform among isolates from different locations.  Analysis of the lipopolysaccharide, outer membrane proteins, plasmids and isozyme profile have added to the description of the organism and substantiated the homogeneity among isolates. For a presumptive identification, the bacterium should be a gram negative rod  (0.75 x 1.25 μm) that is oxidase negative, ferments glucose, is weakly motile in GMD, is K/A with negative H2S in triple sugar iron agar slants and negative for indole production.  Confirmatory identification is made serologically using slide agglutination, indirect fluorescent antibody, enzyme immunoassay or enzyme-linked immunosorbent assay (ELISA). Antibodies to the organism may be detected in channel catfish serum using the indirect ELISA.  Edwardsiella ictaluri may also be identified using the API 20E (identification code 4004000). Modern molecular approaches for identification, characterization and subtyping of E. ictaluri will be discussed.