Hemocyte Apoptosis Induced by Entomopathogenic Bacteria, Xenorhabdus and Photorhabdus, in Bombyx mori

Abstract

Entomopathogenic bacteria of Xenorhabdus nematophila, Xenorhabdus sp., and Photorhabdus temperata subsp. temperata are symbionts of entomopathogenic nematodes including Steinernema and Heterorhabditis. These bacteria have potent insecticidal pathogenicity causing hemolymph septicemia. To explain a mechanism of the septicemia, this research raises a hypothesis that the hemolymph septicemia is due to the induction of the programmed cell death (=apoptosis) of the hemocytes by the entomopathogenic bacteria. Injection of the bacteria into the hemocoel of the fifth instar larvae of Bombyx mori led to septicemia (few viable hemocytes) at 12h in all three bacterial treatments. During the pathogenicity, the infected hemocytes exhibited the membrane blebbing, nuclear chromatin condensation and DNA fragmentation, which reflected typical morphological changes of cells undergoing apoptosis. The apoptosis began as early as 4h after bacterial injection and increased with post-injection time. Three pathogenic bacteria, however, differed in the degree of apoptosis-inducing effect on B. mori by the effective bacterial dose and time. These results support the hypothesis that hemolymph septicemia caused by the bacteria is due to their apoptosis-inducing effect.