Role of Secondary Metabolites in Establishment of the Mutualistic Partnership between Xenorhabdus nematophila and the Entomopathogenic Nematode Steinernema carpocapsae

Abstract

ABSTRACTXenorhabdus nematophilaengages in a mutualistic partnership with the nematodeSteinernema carpocapsae, which invades insects, migrates through the gut, and penetrates into the hemocoel (body cavity). We showed previously that during invasion ofManduca sexta, the gut microbeStaphylococcus saprophyticusappeared transiently in the hemocoel, whileEnterococcus faecalisproliferated asX. nematophilabecame dominant.X. nematophilaproduces diverse secondary metabolites, including the major water-soluble antimicrobial xenocoumacin. Here, we study the role ofX. nematophilaantimicrobials in interspecies competition under biologically relevant conditions using strains lacking either xenocoumacin (ΔxcnKLstrain), xenocoumacin and the newly discovered antibiotic F (ΔxcnKL:F strain), or allngrA-derived secondary metabolites (ngrAstrain). Competition experiments were performed in Grace's insect medium, which is based on lepidopteran hemolymph.S. saprophyticuswas eliminated when inoculated into growing cultures of either the ΔxcnKLstrain or ΔxcnKL:F strain but grew in the presence of thengrAstrain, indicating thatngrA-derived antimicrobials, excluding xenocoumacin or antibiotic F, were required to eliminate the competitor. In contrast,S. saprophyticuswas eliminated when coinjected intoM. sextawith either the ΔxcnKLorngrAstrain, indicating thatngrA-derived antimicrobials were not required to eliminate the competitorin vivo.E. faecalisgrowth was facilitated when coinjected with either of the mutant strains. Furthermore, nematode reproduction inM. sextanaturally infected with infective juveniles colonized with thengrAstrain was markedly reduced relative to the level of reproduction when infective juveniles were colonized with the wild-type strain. These findings provide new insights into interspecies competition in a host environment and suggest thatngrA-derived compounds serve as signals forin vivonematode reproduction.