Chemical identification of two entomopathogenic bacterial metabolites and their immunosuppressive activities by inhibiting eicosanoid biosynthesis

Two entomopathogenic bacteria, Xenorhabdus nematophila and Photorhabdus temperata ssp. temperata are symbiotic to nematodes, Steinernema carpocapsae and Heterorhabditis megidis, respectively. Infective juveniles enter hemocoel of target insects and release their symbiotic bacteria, which then suppress insect immune responses to foster nematode and bacterial growth. Both bacteria inhibit eicosanoid biosynthesis by inhibiting catalytic activity of phospholipase A2 (PLA2) of target insects. The bacterial culture broth contains the inhibitory activity against PLA2. This study identified bacterial metabolites by chemical fractionation of bacterial culture broth and subsequent NMR analysis. Six bacterial metabolites were chemically identified: benzylideneacetone (BZA), pro-tyr (PY), acetylated phe-gly-val (Ac-FGV), cyclic PY, indole, and oxindole. All compounds inhibited PLA2 activity in a dose-dependent manner. They also inhibited phenoloxidase activity. However, there was significant variation in their inhibitory activities in both enzymes. Addition of these chemicals to Bacillus thuringiensis significantly enhanced the bacterial pathogenicity against last instars of diamondback moth (Plutella xylostella) and beet armyworm (Spodoptera exigua). We propose these chemicals at synergists to B. thuringiensis insecticide.