A Functional Reciprocal Relationship Between the Host Defense Mechanisms of Oysters and the Bio-control Methods for Protection of Bacillary Necrosis of Larval Oysters Caused by Pathogenic Vibrio Species

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In the present study, the author determined and discussed a functional reciprocal relationship between the host defense process of the giant Pacific oyster, Crassostrea gigas, and the bio-control methods for protection of bacillary necrosis of larval oysters caused by pathogenic Vibrio species. In many hatcheries of bivalve molluscs, mass larval mortalities often induced by marine Vibrio species infections have been reported. Infected larvae showed typical cases of bacillary necrosis including anomalous swimming and the detachment of cilia and vela. As a result, it is considered that proteases produced by the pathogenic Vibrio strains play a major role in the pathogenesis of bacillary necrosis. The use of antibiotics has been shown to be an effective method for controlling this disease and improving the survival of larval oysters. However, antibiotic treatment is not recommended because it is can affect bacterial flora in the culture sea water and can also result in the expression of r … More esistant pathogenic bacteria. Therefore, in order to develop altenative methods to antibiotics for preventing bacillary necrosis, the author examined the effects of ovomacroglobulin, which is a glycoprotein in the hen's egg whites and is well known to bind and inhibit strongly the number of proteases, on the larval survival of the giant Paciftc oyster. The author also investigated the use of bacterial strains that have potentiality as bio-control agents for Vibrio infection. The pathogenic Vibrio tubiashii (ATCC19106) was used to infect larvae of the Pacific oyster. V. tubiashii showed strong pathogenicity to oyster larvae, causing 100 % mortality aftar experimental exposure for 24 h at a concentration of 10^5 cfu (colony-forming units)/ml. In contrast, the addition of ovomacroglobulin at a concentration of 10 μg/ml to larval oysters, challenged with V. tubiashii at 10^5 cfu/ml, led to a marked increase in larval survivel of 96.5% at 24 h after infection. Live larvae were actively motile, and their cilia and vela were not necrotized in the ovomacroglobulin-added group. The addition of ovomacroglobulin clearly suppressed the growth of V. tubiashii in gelatin-seawater broth, but the number of viable V. tubiashii 24 h after incubation did not decrease to the initial dose level. Findings obtained in this study indicate that ovomacroglobulin almost completely protect larval oysters from V. tubiashii infection by non-bactericidally inhibiting the growth of V. tubiashii without affecting the survival of the oysters. Less