Ecological investigation of Streptococcus iniae in cultured Japanese flounder ( Paralichthys olivaceus) using selective isolation procedures

Streptococcus iniae is an important bacterial pathogen of Japanese flounder ( Paralichthys olivaceus). We investigated the prevalence of S. iniae in Japanese flounder and its cultural environment for two consecutive years in a flounder farm. Direct plating procedure using thallium acetate-oxolinic acid-blood (TAOAB) agar and enrichment culture in a selective broth (SB) followed by subculture on TAOAB agar were employed for detecting S. iniae from samples. Todd–Hewitt broth (1 l) containing 0.5 g thallium acetate, 10 mg colistin sulphate (CS), and 5 mg oxolinic acid (OA) was chosen as a selective broth. On most sampling dates except for those of low water temperature, S. iniae was detected from sediment and water samples of 1-year-old fish tanks. Detection rates of S. iniae for sediment and water samples from 0-year-old fish tanks were low from January, when rearing of fingerling was started in this farm, until the epizootic broke out. Detection rates of S. iniae for samples from 1-year-old fish tanks were always higher than those of 0-year-old fish tanks. S. iniae was detected also from water supply in high temperature months. A total of 15 out of 230 flounder sampled were S. iniae-infected as the pathogen was isolated from their brain and/or kidney. S. iniae was detected frequently from the gill and skin mucus of apparently healthy flounder. Broth enrichment procedure was suited for detection of S. iniae from samples of fish tissue and water supply. Direct plating procedure was suited for sediment samples. For water samples, both procedures should be employed.