The Global Transcription Factor Lrp Is both Essential for and Inhibitory to Xenorhabdus nematophila Insecticidal Activity

Abstract

In the entomopathogenic bacterium <i>Xenorhabdus nematophila</i>, cell-to-cell 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 <i>Manduca sexta</i> insects, while cells that lack <i>lrp</i> or express high Lrp levels are virulence attenuated and elicit AMP expression. To better understand the basis of these phenotypes, we examined <i>X. nematophila</i> strains expressing fixed Lrp levels. Unlike the <i>lrp</i>-null mutant, the high-<i>lrp</i> strain is fully virulent in <i>Drosophila melanogaster</i>, suggesting that these two strains have distinct underlying causes of virulence attenuation in <i>M. sexta</i> Indeed, the <i>lrp</i>-null mutant was defective in cytotoxicity against <i>M. sexta</i> hemocytes relative to that in the high-<i>lrp</i> and low-<i>lrp</i> strains. Further, supernatant derived from the <i>lrp</i>-null mutant but not from the high-<i>lrp</i> strain was defective in inhibiting weight gain when fed to 1st instar <i>M. sexta</i> These data suggest that contributors to the <i>lrp</i>-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 <i>D. melanogaster</i> In contrast, the high-Lrp strain was sensitive to the antimicrobial peptide cecropin, had a transient survival defect in <i>M. sexta</i>, and had reduced extracellular levels of insecticidal activity, measured by injection of supernatant into 4th instar <i>M. sexta</i> Thus, high-<i>lrp</i> strain virulence attenuation may be explained by its hypersensitivity to <i>M. sexta</i> host immunity and its inability to secrete one or more insecticidal factors.<b>IMPORTANCE</b> Adaptation of a bacterial pathogen to host environments can be achieved through the coordinated regulation of virulence factors that can optimize success under prevailing conditions. In the insect pathogen <i>Xenorhabdus nematophila</i>, the global transcription factor Lrp is necessary for virulence when injected into <i>Manduca sexta</i> or <i>Drosophila melanogaster</i> insect hosts. However, high levels of Lrp, either naturally occurring or artificially induced, cause attenuation of <i>X. nematophila</i> virulence in <i>M. sexta</i> but not <i>D. melanogaster</i> Here, we present evidence suggesting that the underlying cause of high-Lrp-dependent virulence attenuation in <i>M. sexta</i> is hypersensitivity to host immune responses and decreased insecticidal activity and that high-Lrp virulence phenotypes are insect host specific. This knowledge suggests that <i>X. nematophila</i> faces varied challenges depending on the type of insect host it infects and that its success in these environments depends on Lrp-dependent control of a multifactorial virulence repertoire.