In this study, we showed the lack of defense responses and apoptotic death in the Pacific oyster hemocytes when the hemocytes were in vitro exposed to three species of bacteria. The oyster hemocytes exerted a high phagocytic ability against the live cells of both Escherichia coli (45.9%) and Planococcus citreus (33.7%), whereas the hemocytes produced only a slight phagocytosis of live Vibrio tubiashii cells (0.4%). Moreover, the oyster hemocytes were attracted to live E. coli but not to live V. tubiashii cells and their culture filtrate. Migration to both live V. tubiashii cells and their culture filtrate was significantly lower than that to the control (showing random motility, p<0.05, Dunnett's test). In addition to the above results, the oyster hemocytes exposed to live V. tubiashii showed a high mortality at 71.6% after 6-hr incubation. On the other hand, the oyster hemocytes underwent apoptotic cell death due to the phagocytosis of live P. citreus cells. Apoptosis was also induced in the hemocytes ingesting pasteurized P. citreus cells, whereas the hemocytes phagocytizing autoclave-killed P. citreus did not. The apoptosis in the oyster hemocytes was markedly blocked by treatment with antioxidants, especially catalase and N-acetyl-L-cysteine.
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