Occurrence of Streptococcus sp., a Pathogen of Cultured Yellowtail, in Muscle of Sardine for Diets

Abstract

The dorsal muscle of sardine(Sardinops melanosticta), frozen as fish diets, was subjected to detection of Streptococcus sp., a pathogen of cultured yellowtail (Seriola quinoueradiata). After the external surface of sardine was disinfected with 70%ethanol, 2-3 g of samples were cut aseptically from the dorsal muscle. The muscle samples were dipped in 70%ethanol and then flamed for disinfecting its surface. Immediately after that, each of the samples were inoculated into nutrient broth, and then incubated for 1-5 days at 25°C. Subsequently, a loopful of the culture was streaked on nutrient agar, and then incubated for 1-2 days at 25°C. Cultures on the plates consisted of various kinds of colonies. On most plates however, pure culture of the objective organisms was observed.
As a result, Streptococcus sp. was isolated from all of the 10 sardines sampled. Such remarkable frequency in its detection may be attributed to the enrichment culture using ordinary broth and prolonged incubation.
The role of the organism in streptococcal infection-of yellowtail and the cause of its isolation from the sardine muscles as feeds remained to be further clarified in the future.