A phase variation has been reported in an entomopathogenic bacterium, Xenorhabdusnematophila. Compared with a wild-type primary form, a secondary form usually loses severalphysiological and biochemical characters. This study showed that the phase variation of X.
nematophila caused a significant alteration in its immunosuppressive activity and subsequententomopathogenicity. A secondary form of X. nematophila was detected in laboratory coloniesand exhibited significant differences in dye absorption and entomopathogenicity. In addition,the secondary form was different in its production of eicosanoid-biosynthesis inhibitors (EBIs)compared with the primary form of X. nematophila. Production of oxindole and phydroxypropionicacid was significantly reduced in the culture broth of the secondary form ofX. nematophila. The reduced EBI production resulted in significant suppression in theinhibitory effects on cellular nodule formation and phenoloxidase activity. Culture broth ofthe primary form of X. nematophila enhanced the pathogenicity of Bacillus thuringiensis ( Bt)significantly more than the culture broth of the secondary form. Furthermore, this studydeveloped a highly efficient “Dual Bt-Plus” to control both lepidopteran insect pests Plutellaxylostella and Spodoptera exigua, by mixing two effective Bt strains along with the addition ofpotent bacterial metabolites or 100-fold concentrated X. nematophila culture broth.