Invasion Route and Pathogenic Mechanisms of Vibrio alginolyticus to Silver Sea Bream Sparus sarba

Potential routes of invasion and associated pathogenic mechanisms of a pathogenic strain of Vibrio alginolyticus (Vib-01) to silver sea bream Sparus sarba were evaluated. Inoculation of V. alginolyticus into juvenile silver sea bream via the intramuscular, intraperitoneal, injury–immersion, and simple immersion routes resulted in 14-d LD50 values (concentrations lethal to half the group studied) of 4.85 × 104 colony-forming units [cfu]/fish, 5.01 × 105 cfu/fish, 3.16 × 106 cfu/mL and more than 2.5 × 108 cfu/mL, respectively. These findings suggest that the integrity of the fish integument is important for protecting fish against vibriosis, such that invasion through integumental breaches would be the main invasive mode of this bacterium to silver sea bream. Phagocytic activities of splenic and pronephric macrophages were significantly increased at the early stages of infection but declined significantly as infection progressed (after 24 or 36 h). The spleen and head kidney of fish inoculated with Vib-01 bacteria exhibited high bacterial counts accompanied by a decline in the phagocytic potential of the macrophages taken from both organs. Sea bream infected with V. alginolyticus exhibited significant decreases in red blood cell count, hematocrit, hemoglobin, serum glucose, and protein. Total leukocyte count increased at the early stages of infection but declined as infection became advanced. The destruction of critical components of both the circulatory and immune systems by V. alginolyticus could be the main cause of mortality in vibriosis. Extracellular products (ECPs) of the present V. alginolyticus isolate were toxic not only to silver sea bream but also to cultured fibroblast cells derived from sea bream fin tissue. The 14-day LD50 of ECPs to sea bream was 0.92 μg per gram of fish weight. Our data suggest an involvement of the ECPs, especially their hydrolytic and hemolytic components, in the invasive and proliferative processes of the bacteria in silver sea bream.