During the period from late September through early October 1973, an epizootic occurred among mullet populations in a bay in Kochi Prefecture. Dying fish displayed erratic swimming and a more or less whirling motion at the surface. Affected fish had large malodorous abscesses with hemorrhagic borders in the body surface (Plate 1).
The etiological agent was isolated from the kidney of diseased mullets. These organisms were gram-negative, motile peritrichously flagellated rods, not encapsulated, and measuring about 0.8×0.4μ(Plate 2). The bacteria grew slowly on nutrient agar requiring 48 hr of incubation at 25°C to form the typical small (0.5mm in diameter), circular, transparent, slightly raised colonies. They gave negative oxidase reaction, positive catalase reaction, utilized glucose fermentatively and reduced nitrate to nitrite. They did not utilize citrate and malonate as a sole carbon source. No action on d-tartrate and mucate were exhibited. The organisms were the production of hydrogen sulfide in TSI agar and indole in tryptone water, positive to methyl red test, negative to VP reaction, decarboxylation of lysine and ornithine, and utilization of fructose, galactose, mannose and glycerol (Table 1). They were sensitive to streptomysin, kanamycin, chloramphenicol, furazolidon and nalidixic acid.
On the basis of the above characteristics, the organism was jdentified as Edwardsiella tarda.