The global transcription factor Lrp is both essential for and inhibitory to Xenorhabdus nematophila insecticidal activity

Abstract

In the entomopathogenic bacterium Xenorhabdus nematophila, cell-tocell variation in the abundance of the Lrp transcription factor leads to virulence modulation; low Lrp levels are associated with a virulent phenotype and suppression of antimicrobial peptides (AMPs) in Manduca sexta insects, while cells that lack lrp or express high Lrp levels are virulence attenuated and elicit AMP expression. To better understand the basis of these phenotypes, we examined X. nematophila strains expressing fixed Lrp levels. Unlike the lrp-null mutant, the high-lrp strain is fully virulent in Drosophila melanogaster, suggesting that these two strains have distinct underlying causes of virulence attenuation in M. sexta. Indeed, the lrp-null mutant was defective in cytotoxicity against M. sexta hemocytes relative to that in the high-lrp and low-lrp strains. Further, supernatant derived from the lrp-null mutant but not from the high-lrp strain was defective in inhibiting weight gain when fed to 1st instar M. sexta. These data suggest that contributors to the lrp-null mutant virulence attenuation phenotype are the lack of Lrp-dependent cytotoxic and extracellular oral growth inhibitory activities, which may be particularly important for virulence in D. melanogaster. In contrast, the high-Lrp strain was sensitive to the antimicrobial peptide cecropin, had a transient survival defect in M. sexta, and had reduced extracellular levels of insecticidal activity, measured by injection of supernatant into 4th instar M. sexta. Thus, high-lrp strain virulence attenuation may be explained by its hypersensitivity to M. sexta host immunity and its inability to secrete one or more insecticidal factors.