Immune response of the eastern oyster Crassostrea virginica: Effects of cadmium and the localization and bacteriostasis of introduced Vibrio

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Williams, Heidi Rachel
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The Eastern oyster, Crassostrea virginica, is constantly exposed to microorganisms and pollutants in the surrounding environment. While cadmium (Cd) has been shown to alter specific immune defense mechanisms, the direct effect of Cd on the inactivation and degradation of bacteria in this animal is unknown. Furthermore, the roles of individual tissues in the distribution, inactivation and degradation of live bacteria introduced into the host remain unclear. First, we determined the effect of Cd on the immune responses of C. virginica at the organismal, cellular and molecular level. Following a chronic Cd exposure, oysters were injected in the adductor muscle with 105 live V. campbellii. Quantitative real-time polymerase chain reaction (Q-PCR) was used to quantify the number of intact bacteria, while selective plating was used to quantify the number of injected bacteria that remained culturable within the tissues and fluids of the whole animal at 10, 30 and 60 min after injection. While Cd did not alter the oyster’s ability to inactivate bacteria, Cd exposure induced a significant decrease in the numbers of circulating hemocytes and significant changes in the expression of selected genes as determined by microarray analysis. The gills, digestive gland and hemocytes mounted unique responses to chronic Cd exposure, yet the functional integrity of immune defense at the organismal level was maintained under these conditions. Second, we determined the relative contribution of individual tissues to the antibacterial response of C. virginica following injections of V. campbellii. Numbers of intact bacteria were used to identify sites of bacterial accumulation, while percentages of culturable bacteria were quantified to determine sites of bacteriostasis within the digestive gland, mantle, gills, adductor, gonadal tissues, labial palps and hemolymph at 10, 30, 60 and 120 min after injection. The gonadal tissues contained the greatest concentration of intact bacteria and the lowest percentage of culturable V. campbellii compared to all other tissues. We suggest that the gonadal tissues, when present, are the main site of bacterial accumulation and bacteriostasis. Overall, this study demonstrates the resilience of oysters to bacterial infections by their ability to inactivate bacteria.
American oyster; Toxicology