Studies on the pathogenesis of streptococcal infection in cultured yellowtails Seriola spp.: effect of the cell free culture on experimental streptococcal infection

Abstract. A previous paper has revealed that experimental streptococcal infection was associated with a rapid growth of the intestinal bacteria, suggesting association of the condition with an exotoxin. The present study was undertaken to see the effect of the exotoxin on the streptococcal infection by administering cell free culture before the bacterial challenge. Cell free culture of Streptococcus sp. strain YT-3 was inoculated intramuscularly at a dosage of 0–5 ml per 100 g body weight 1 h prior to bacterial challenge. Three fish were killed at intervals after challenge for viable counting of bacteria in the viscera and blood. Intramuscular inoculation with either low virulent or virulent bacteria alone at dosages of 10⁶ to 10⁷ cells did not produce the disease in the fish, with almost complete clearance of the bacteria from the viscera and blood within 120 h. When the exotoxin was inoculated intramuscularly prior to the bacterial challenge, however, either low virulent or virulent bacteria at 10⁶ to 10⁷ cells could produce fatal infection with prominent streptococcal clinical signs.