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Background   Photorhabdus  are Gram negative entomopathogenic bacteria that also have a mutualistic association with nematodes from the family  Heterorhabditis . An essential part of this symbiosis is the ability of the bacterium to colonize the gut of the freeliving form of the nematode called the infective juvenile (IJ). Although the colonization process (also called transmission) has been described phenomonologically very little is known about the underlying molecular mechanisms. Therefore, in this study, we were interested in identifying genes in  Photorhabdus  that are important for IJ colonization.    Results  In this work we genetically tagged  P. luminescens  TT01 with  gfp  and constructed a library containing over 3200 mutants using the suicide vector, pUT-Km2. Using a combination of  in vitro  symbiosis assays and fluorescent microscopy we screened this library for mutants that were affected in their ability to colonize the IJ i.e. with decreased transmission frequencies. In total 8 mutants were identified with transmission frequencies of ≤ 30% compared to wild-type. These mutants were mapped to 6 different genetic loci; the  pbgPE  operon,  galE ,  galU ,  proQ ,  asmA  and  hdfR . The  pbgPE ,  galE  and  galU  mutants were all predicted to be involved in LPS biosynthesis and, in support of this, we have shown that these mutants are avirulent and sensitive to the cationic antimicriobial peptide, polymyxin B. On the other hand the  proQ ,  asmA  and  hdfR  mutants were not affected in virulence and were either as resistant ( proQ ) or slightly more sensitive ( asmA, hdfR ) to polymyxin B than the wild-type (WT).    Conclusions  This is the first report describing the outcome of a comprehensive screen looking for transmission mutants in  Photorhabdus . In total 6 genetic loci were identified and we present evidence that all of these loci are involved in the assembly and/or maintenance of LPS and other factors associated with the cell surface. Interestingly several, but not all, of the transmission mutants identified were also avirulent suggesting that there is a significant, but not complete, genetic overlap between pathogenicity and mutualism. Therefore, this study highlights the importance of the cell surface in mediating the symbiotic and pathogenic interactions of  Photorhabdus .

bmc microbiology

Identification of genes involved in the mutualistic colonization of the nematode Heterorhabditis bacteriophora by the bacterium Photorhabdus luminescens