Characterization of the Bacterial Properties that Impair Respiration in the Atlantic Blue Crab, Callinectes sapidus
Johnson, Nathaniel Garver
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In the blue crab Callinectes sapidus, injection with the bacterial pathogen Vibrio campbellii causes a decrease in oxygen consumption. The physical obstruction of hemolymph flow through the gill vasculature, caused by aggregations of bacteria and hemocytes, appears to underlie the decrease in aerobic function associated with bacterial infection. We sought to elucidate the bacterial properties sufficient to induce a decrease in circulating hemocytes, reflecting hemocyte aggregation and decreased respiration. Lipopolysaccharide (LPS), the primary component of the gram-negative bacterial cell wall, is known to interact with crustacean hemocytes. Purified LPS was covalently bound to the surfaces of polystyrene beads, resembling bacteria in size. Injection of these "LPS beads" caused a decrease in circulating hemocytes comparable to that seen with V. campbellii injection, while beads alone failed to do so. These data suggest that in general, gram-negative bacteria could stimulate hemocyte aggregation. To test this hypothesis, the effects of different bacterial species on total hemocyte counts were assessed. Six of the seven gram-negative species tested caused decreases in circulating hemocytes, suggesting an important role for LPS in the induction of this response. However, LPS is not necessary to provoke the immune response, as a gram-positive bacterium, which lacks LPS, caused a decrease in circulating hemocytes. These results imply that a broad range of naturally-occurring bacteria could impair respiration in C. sapidus. To further verify the relationship between reduced circulating hemocyte counts and impaired respiration, we sought to evaluate the effects of selected treatments on oxygen uptake. In contrast to previous reports, injection of V. campbellii did not lead to a decrease in oxygen uptake. Interestingly, ventilatory pauses were observed following bacterial injection in crabs that had been starved for 72 h, but not 2 or 24 h. These results indicate that the immune response against bacteria can affect respiratory physiology and aerobic metabolism in ways that are more complicated than previously assumed.