TheexbDgene ofPhotorhabdus temperatais required for full virulence in insects and symbiosis with the nematodeHeterorhabditis.

Photorhabdusare bacteria found colonizing the gut of a specialized stage of the nematodeHeterorhabditis, called the infective juvenile (IJ). The IJ is a free-living stage of the nematode that seeks out and infects insect larvae. Once inside the insect the IJ releasePhotorhabdusinto the haemolymph where the bacteria rapidly proliferate, killing the insect within 48–72 h. The nematodes grow and reproduce in the insect cadaver by feeding on thePhotorhabdusbiomass. In this study we usePhotorhabdus temperataK122 to show that genes involved in iron acquisition play a key role during the course of the tripartite bacteria–nematode–insect interaction. We show that a strain carrying a mutation in a gene with homology toexbD, encoding a component of the TonB complex, is unable to grow well in conditions where iron is not freely available. In addition, this mutant, BMM417, requires a longer time to kill the insect larvae than the wild-type bacteria and this defect in pathogenicity is complemented by the co-injection of iron. Moreover, the increase in LT₅₀ observed with BMM417 is correlated with a significantly slowerin vivogrowth rate suggesting that iron is limiting in the insect. We also show that BMM417 is unable to support the growth and development of theHeterorhabditisnematode. Addition of exogenous iron to the growth media restores nematode growth and development on BMM417, suggesting that aspects of iron metaboism inPhotorhabdusare important during the symbiosis with the nematode. [ABSTRACT FROM AUTHOR]