Abstract
An infectious disease caused by a β-hemolytic Streptococcus has been observed in several freshwater and marine fishes, especially in ayu(Plecoglossus altivelis)cultured in commercial farms.
In a previous paper, the biochemical characteristics of 5 strains isolated from diseased ayu and amago(Oncorhynchus rhodurus var. macrostomus)were reported. In the present paper, physiological characteristics and pathogenicity of the isolates from ayu are described. 1)Effects of temperature, sodium chloride and pH on the growth of the organism:It grew at temperature20-37°C(optimum range 25-30°C, at NaCl 0-3%(optimum range 0-1%)and at pH 6-8(optimum7). 2)Survival in waters:It perished within a day in freshwater, and within 3 days in sea water.But it survived over 8 days in 0.01%yeast extract added distilled water. 3)Pathogenicity by intraperitoneal injection:It showed high pathogenicity to ayu and amago, and weak pathogenicity to tilapia(Tilapia nilotica), yellow tail(Seriola quinqueradiata) and seabream(Pagrus major).But it was not pathogenic to carp(Cyprinus carpio)or black seabream(Acanthopagrus schlegeli).
