The genus Edwardsiella is of relatively recent origin, yet has come to the forefront in the field of fish disease. The genus consists of two species, E. tarda and E. ictaluri. E. tarda has been incriminated as the etiological agent in the "emphysematous putrefactive disease of catfish."E. ictaluri has been associated with "enteric septicemia" of catfish.

The genus Edwardsiella displays characteristics which have placed it in the family Enterobacteriaceae.  Both species are gram negative motile rods.  Each species ferments glucose with gas, produces lysine and ornithine decarboxylases, and is methyl red positive.  Both species lack phenylalamine deaminase, urease, and arginine dihydrolase.  They do not ferment mannitol or lactose, do not utilize citrate, and are Voges-Proskauer negative. Differences between these two species include the production of hydrogen sulfide and indole by E. tarda and the lack of motility at 37°C by E. ictaluri.

E. tarda and E. ictaluri are both bio-typically homogenous species.  Biochemically, E. tarda isolates from different countries appear identical.

The enzymatic capabilities of the genus Edwardsiella appear to be limited.  There is an apparent lack of extracellular proteinases, lecithinases, lipases, esterases, amylases, and most carbohydrases. A survey of enzymatic activities using the API-ZYM system has revealed almost identical reactions generically.  Both species demonstrated an intense phosphatase activity.

The MIC/MBC values were determined for E. tarda and E. ictaluri against a battery of antibiotics.  Both species demonstrated a characteristic resistance to colistin. E. ictaluri showed a broad, uniform sensitivity.  E. tarda, though also highly-sensitive, was more isolate variable, with some isolates giving a high resistance to a number of antibiotics.