The Gram-negative bacterium Photorhabdus luminescens symbiotically associates with the entomopathogenic nematode (EPN) Heterorhabditis bacteriophora. EPN invades insect host and releases the symbiotic bacteria into the hemocoel to kill the host by bacterial toxins. Even though several toxins of P. luminescens are under active investigation, the underlying mechanism of its virulence remains unclear. P. luminescens behaves as a pathogen against not only insects but also non-mutualistic nematodes including Caenorhabditis elegans. We revealed a novel molecular mechanism of P. luminescens virulence toward a model host C. elegans;
p38 MAPK pathway and Insulin/IGF-1 signaling pathway in C. elegans are required for defense response against P. luminescens, but Insulin/IGF-1 signaling pathway is inactivated by P. luminescens through the overexpression of insulin-like gene (Sato et al., 2014). In addition to the lethal effect of P. luminescens on C. elegans adult, we also found its growth inhibitory effect on C. elegans larva.To reveal the new virulence mechanism we constructed transposon mutagenized library of P. luminescens and screened for attenuation of growth inhibitory effect on C. elegans larva. We transferred synchronized L1-stage C. elegans on P. luminescens mutants and checked the C. elegans growth after several days. Until now we isolated three virulence-attenuated mutants by screening approximately 700 independent transposon-inserted mutants. We sequenced transposon insertion sites of the two of these mutants and confirmed the two genes pdxB (encodes Erythronate-4-phosphate dehydrogenase) and
plu3602 (unknown functional gene) were required for full pathogenicity toward C. elegans. On the avirulent mutants, C. elegans could develop from L1 to adulthood and produce a lot of brood, and C. elegans lifespans from the adult stage were about 12.7% (on pdxB mutant) and 11.9 % (on
plu3602 mutant) longer than those when grown on the wild type P. luminescens. Further phenotypic and genetic analyses of these mutants would give us a new insight of the P. luminescens virulence mechanism.Sato et al. SpringerPlus, 2014, 3, 274.