Insect phenoloxidase is considered an important mediator in defense reaction against pathogens and parasites. It is present in the hemolymph of most insects as an inactive proenzyme, called prophenoloxidase. In Acheta domesticus (L.) the phenoloxidase activity was studied in the presence of Bacillus subtilis. The bacteria induced the activation of the prophenoloxidase and increased the level of phenoloxidase activity when injected into the hemocoel of A. domesticus nymphs. The level of increase was comparable to those observed in nymphs injected with laminarin, an activator of the proenzyme. Incubating B. subtilis in serum with phenoloxidase activated in vitro and then injecting into the insects accelerated the removal of the bacteria from the hemolymph in vivo. The concentration of bacteria was lower than those observed in nymphs injected with Mes buffer alone. In control insects, injection of soybean trypsin inhibitor lowered phenoloxidase activity and the removal of bacteria from the hemolymph. These results indicate that B. subtilis is sensitive to the prophenoloxidase cascade of A. domesticus. It is suggested that activated phenoloxidase could bind to the surface of bacteria and increase the adhesion of hemocytes to bacteria, and thus accelerating their removal by nodule formation.
Hemolynph; phenoloxidase; hemocyte; defense reaction
