The role of bacterial symbionts in suppressing the defence reaction in larval hemolymph of the cotton leafworm Spodoptera littoralis (Biosd.).

摘要

Spodoptera littoralis haemolymph exhibited a decrease in phenoloxidase [catechol oxidase] activity during the first hour of exposure to live or dead Xenorhabdus nematophilus and Photorhabdus luminescens, even in the presence or absence of laminarin and alpha -chymotrypsin. Also, as the bacterial numbers in the haemolymph of the larvae of S. littoralis increased, the suppression of phenoloxidase activity in vivo increased. On the other hand, in the in vitro incubation of the infected haemolymph with X. nematophilus for 30 min with laminarin, the decrease in phenoloxidase activity reached 92% in the infection with the highest dose (1x1010 cells/ml) live bacteria, and reached 100% at the same dose in the infection with live bacteria in the absence of laminarin. A two-fold decrease in enzyme activity was recorded in the case of injection of (1x106 cells/ml) dead X. nematophilus and the absence of laminarin compared with injection of the same dose in the presence of laminarin. The same trend was also observed by the end of incubation of X. nematophilus-treated haemolymph without alpha -chymotrypsin. The decrease in phenoloxidase activity was highly significantly different in injection of dead P. luminescens and the absence of laminarin during incubation. In the case of injection of (1x108 cells/ml) live P. luminescens, a higher degree in the reduction of enzyme activity was recorded in the absence of alpha -chymotrypsin, where it reached to nearly a two-fold decrease. The results of these studies indicated that both X. nematophilus and P. luminescens alive or dead suppress the phenoloxidase activity in the presence or absence of both laminarin and alpha -chymotrypsin but, the suppression in absence of both during the in vitro incubation was highly comparable.