ThepbgPEoperon inPhotorhabdus luminescensIs Required for Pathogenicity and Symbiosis

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Abstract
Photorhabdusis a genus of gram-negativeEnterobacteriaceaethat is pathogenic to insect larvae while also maintaining a mutualistic relationship with nematodes from the familyHeterorhabditis, where the bacteria occupy the gut of the infective juvenile (IJ) stage of the nematode. In this study we describe the identification and characterization of a mutation in thepbgE1gene ofPhotorhabdus luminescensTT01, predicted to be the fifth gene in thepbgPEoperon. We show that this mutant, BMM305, is strongly attenuated in virulence against larvae of the greater wax moth,Galleria mellonella, and we report that BMM305 is more sensitive to the cationic antimicrobial peptide, polymyxin B, and growth in mildly acidic pH than the parental strain ofP. luminescens. Moreover, we also show that the lipopolysaccharide (LPS) present on the surface of BMM305 does not appear to contain any O antigen. Complementation studies reveal that the increased sensitivity to polymyxin B and growth in mildly acidic pH can be rescued by the intransexpression ofpbgE1, while the defects in O-antigen assembly and pathogenicity require the intransexpression ofpbgE1and the downstream genespbgE2andpbgE3. Finally, we show that BMM305 is defective in symbiosis as this mutant is unable to colonize the gut of the IJ stage of the nematode. Therefore, we conclude that thepbgPEoperon is required for both pathogenicity and symbiosis inP. luminescens.