In 1975, a new vibriosis occurred in eels (Anguilla japonica and A. anguilla) cultured in Tokushima Prefecture in Shikoku Island of Japan. The disease prevailed in brackish water ponds (Cl-2-6‰) during summer and early autumn, when the water temperature of culture ponds ranged from 20°C to 30°C. The causative Vibrio seems to be classified as Vibrio anguillarum B type (V. anguillarum f. anguillicida) suggested by NYBELIN (1935), but this type can no longer be included in V. anguillarum listed in Bergey's Manual of Determinative Bacteriology 8th ed. Assuming that V. anguillarum B type is independent of V. anguillarum, the present isolate is classified as V. anguillicida BRUUN and HEIBERG 1932. On the other hand, in comparison with Vibrios listed in Bergey's Manual the characteristics of the present isolate are closely resemble to those of V. fischeri. Thus, further investigation are necessary before the taxonomic status of the present Vibrio is determined.
In the previous report biochemical characteristics of an isolate from a vibriosis in eels were presented and the taxonomic status of the isolate was discussed, though a definite conclusion could not be drawn. In this paper, physiological characteristics and pathogenicity of the organism are described. The experimental results are summarized as follows. 1) Effects of sodium chloride, temperature and pH on the growth of the organism: It grew in nutrient broth at NaCl 0.1-4% (optimum range 1-2%), and in peptone water at temperatures from 18°C to 39°C (optimum 30-35°C) and at pH 6-10. 2) Pathogenicity to eel (Anguilla japonica) and mouse: The organism injected intramuscularly killed eels at 18, 20 and 26°C, but not at 15°C. It also killed mice. These results coincide with the fact that the vibriosis due to the organism prevails mainly in brackish-water ponds and at the water temperatures above 18°C.
Populations of rainbow trout (Salmo gairdneri) in freshwater ponds in Japan are often attacked by vibriosis. The causative bacterium was identified as Vibrio piscium var. japonicus by HOSHINA in 1957. But the species is now considered as a synonym of V. anguillarum. From 1973 to 1976, the present authors studied the biochemical characteristics of 22 strains which were isolated from diseased rainbow trout cultured in ponds in various districts of Japan. All the strains differ from V. anguillarum in their properties that they show poor growth on standard nutrient agar and fail to decompose arginine, to ferment mannose and mannitol, and to produce indole. Although further studies are needed to determine the exact taxonomic position of the isolates described here, the authors gave a tentative name of Vibrio sp. RT group to the 22 strains for the convenience of future studies.
Observations were made with the scanning electron microscope (SEM) on two groups of microsporidian spores collected from the Ayu, Plecoglossus altivelis, fixed in 10% formalin. One group of spores were obtained from fry kept in salt water after artificial spawning and the other from adults cultured in fresh water. The spores collected were refixed with 2% KMnO4, dried in aceton series, evapolated with carbon and gold, and examined with SEM. The observations clearly demonstrated that the both groups of spores were the same in shape, size and surface structure, and identified as Glugea plecoglossi TAKAHASHI et EGUSA.