Recovery of infective juveniles of the entomopathogenic nematode Steinernema carpocapsae via factors produced by insect cells and symbiotic bacteria

Abstract

Abstract Entomopathogenic nematodes, Steinernema carpocapsae, show 'recovery' from the dauer form as infective juveniles (IJ) up to fourth-stage juveniles when host invasion occurs. This recovery also occurs within an insect cell line culturing system. Here we addressed the factor(s) that induce recovery. When IJ were exposed to cell medium obtained from the cultivation of Sf9 cell lines derived from armyworms (Spodoptera frugiperda), approximately 50% of IJ recovered after 4 h. By 16 h, 90% of the IJ had undergone recovery. Other insect cell lines such as silkworm (Bombyx mori)-derived BmN cells and fruit fly (Drosophila melanogaster)-derived S2 cells also secreted the recovery inducing factor(s). By contrast, mammalian cells (NIH/3T3 and HeLa) had no effect on nematode recovery. Our data also suggest that symbiotic bacteria are involved in IJ recovery; axenic IJ did not recover in the cell-cultured medium. When symbiotic bacteria isolated from IJ were propagated within the cell-cultured medium, the supernatant gained recovery-inducing activity against axenic IJ. From these results, we conclude that IJ recovery in S. carpocapsae is induced by multiple factor(s) secreted from insect cells and symbiont bacteria.