ThepbgPEoperon inPhotorhabdus luminescensIs Required for Pathogenicity and Symbiosis

Abstract

Photorhabdus is a genus of gram-negative Enterobacteriaceae that is pathogenic to insect larvae while also maintaining a mutualistic relationship with nematodes from the family Heterorhabditis, 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 the pbgE1 gene of Photorhabdus luminescens TT01, predicted to be the fifth gene in the pbgPE operon. 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 of P. 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 in trans expression of pbgE1, while the defects in O-antigen assembly and pathogenicity require the in trans expression of pbgE1 and the downstream genes pbgE2 and pbgE3. 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 the pbgPE operon is required for both pathogenicity and symbiosis in P. luminescens.Photorhabdus is a genus of gram-negative, bioluminescent insect pathogenic bacteria that also has a mutualistic relationship with entomophagous nematodes from the family Heterorhabditis (8). The bacteria colonize the gut of the infective juvenile (IJ) stage of the nematode, a free-living, infectious stage that lives in the soil and actively seeks out insect larvae. The IJ enters the larva and regurgitates the bacteria into the hemolymph, where the bacteria multiply and kill the insect within 24 to 48 h of infection. The bacteria also produce a wide range of extracellular hydrolytic enzymes that function to convert the internal organs and tissues of the insect into a nutrient soup that can support nematode growth and development. After two to three generations within the insect, the nematodes develop into IJs, the bacteria recolonize the intestinal tract, and the IJs emerge from the cadaver in search of a new host (for a recent review, see reference 19).It has been suggested that pathogenicity and symbiosis share common molecular mechanisms and that the outcome of a bacterium-host interaction is the result of a negotiation between the organisms involved in the interaction (28). The Photorhabdus-nematode-insect tripartite system provides a useful model system in which this question can be directly addressed. DNA sequencing has revealed that Photorhabdus contains many genes predicted to encode proteins with homology to virulence factors characterized in other bacterial pathogens (16,20). Recent work has identified the Tc toxin, a large protein complex that is orally active against insect larvae, a...

Genomic basis of endosymbiont-conferred protection against an insect parasitoid

“…This two-component system controls expression of the pbgPE operon in Photorhabdus luminescens and is associated with virulence against insects; pbgPE mutants are lacking an O-antigen, resulting in increased sensitivity to insect-produced antimicrobial peptides and acidic pH (Derzelle et al 2004;Bennett and Clarke 2005).…”

Intraspecies and Temperature-Dependent Variations in Susceptibility of Yersinia pestis to the Bactericidal Action of Serum and to Polymyxin B

“…We used PMB as a representative of this class of molecule for evaluating the resistance levels of different strains of Y. pestis to CAMP. While the actual relatedness of PMB to insect CAMPs is not entirely clear, it has been shown that representatives of both pathogenic and symbiotic bacteria isolated from insects are resistant to PMB (11,23,29), suggesting PMB has a spectrum of activity similar to that of insect CAMPs. Secondly, though insect CAMPs are rather heterogeneous in their structures and mechanisms of action, some insect immune peptides, such as the cecropins, have a spectrum of antibacterial activity similar to that of PMB (14).…”

Whole-Genome Comparison betweenPhotorhabdusStrains To Identify Genomic Regions Involved in the Specificity of Nematode Interaction

“…A recently described species, Photorhabdus asymbiotica, was never found to be associated with nematodes but was isolated from human infections (3,12,15,22). Although a few studies have identified several Photorhabdus genes that are required for normal growth and development of the nematode (5,6,9,19,29), we have little molecular and functional data about the first step of nematode colonization and nematode specificity.…”