Isolation and identification of a symbiotic bacterium fromSteinernema carpocapsae

Xenorhabdus nematophilus sp., an insect-pathogenic bacterium, was newly isolated from Korean entomopathogenic nematode ofSteinernema carpocapsae, which can be used as a useful bioinsecticide. Primary and secondary form variants ofXenorhabdus nematophilus were observed when culturedin vitro. Primary form variants adsorbed bromothymol blue, while secondary form did not. However, many other characters of two variants were very similar. The variants were all rod-shaped and cell size was highly variable ranging from 0.5 by 2.0 μm to 1.0 by 5.0 μm. Both produced highly toxic substances and killed the insect larva within 20–38 hr, indicating that insect pathogenicity ofXenorhabdus is not directly associated with its phase variation. In addition, cell-free culture supernatant ofXenorhabdus was sufficient to kill the insect larva by injecting it into insect hemolymph; however, cell-harboring culture broth was more effective for killing the insect. The use ofXenorhabdus nematophilus may provide a potential alternative toBacillus thuringiensis (Bt) toxins.