A Comparative Analysis of Different Strains Reveals a Virulent Factor, Cyclic Pro-Phe, Using a Differential Expression Profile Analysis of Non-Ribosomal Peptide Synthetases.

Entomopathogenic bacteria, classified into the genus , exhibit a dual lifestyle as mutualistic symbionts to nematodes and as pathogens to a broad range of insects. Bacterial virulence depends on toxin proteins that induce toxemia and various immunosuppressive secondary metabolites that cause septicemia. Particularly, the immunosuppressive properties of bacteria determine the variability of their insecticidal activities. This study explored the role of peptide metabolites in virulence and its variation among six bacterial strains across three species: , , and . Initially, their virulence significantly varied against a susceptible lepidopteran host, , but showed less variation against a tolerant coleopteran host, , with high median lethal bacterial doses. In , virulence was strongly correlated with bacterial growth rate and inhibitory activity against phospholipase A. Secondly, the six strains differed in the compositions of their secreted secondary metabolites, analyzed by GC-MS following ethyl acetate extraction. Notably, there was significant variation in the production of di- or tetra-peptides. Highly virulent strains commonly produced the cyclic Pro-Phe (cPF). Thirdly, the expression of non-ribosomal peptide synthetase () genes varied greatly among the strains. NRPS genes were minimally expressed in the tolerant and highly expressed in the susceptible . In , specific genes were markedly expressed in the virulent strains. Finally, cPF demonstrated potent immunosuppressive activity against the cellular and humoral responses of . The addition of cPF significantly enhanced the virulence against the tolerant . These findings suggest that immunosuppression is necessary for the pathogenicity of bacteria, wherein NRPS products play a critical role in suppressing immune-associated factors in target insects.