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

Xenorhabdus nematophila engages 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 saprophyticus appeared transiently in the hemocoel, whileEnterococcus faecalis proliferated asX. nematophila became dominant.X. nematophila produces diverse secondary metabolites, including the major water-soluble antimicrobial xenocoumacin. Here, we study the role ofX. nematophila antimicrobials in interspecies competition under biologically relevant conditions using strains lacking either xenocoumacin (ΔxcnKL strain), xenocoumacin and the newly discovered antibiotic F (ΔxcnKL :F strain), or allngrA -derived secondary metabolites (ngrA strain). Competition experiments were performed in Grace's insect medium, which is based on lepidopteran hemolymph.S. saprophyticus was eliminated when inoculated into growing cultures of either the ΔxcnKL strain or ΔxcnKL :F strain but grew in the presence of thengrA strain, indicating thatngrA -derived antimicrobials, excluding xenocoumacin or antibiotic F, were required to eliminate the competitor. In contrast,S. saprophyticus was eliminated when coinjected intoM. sexta with either the ΔxcnKL orngrA strain, indicating thatngrA -derived antimicrobials were not required to eliminate the competitorin vivo .E. faecalis growth was facilitated when coinjected with either of the mutant strains. Furthermore, nematode reproduction inM. sexta naturally infected with infective juveniles colonized with thengrA strain 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 vivo nematode reproduction.