The Global Transcription Factor Lrp Controls Virulence Modulation in Xenorhabdus nematophila

Abstract

The bacterium Xenorhabdus nematophila engages in phenotypic variation with respect to pathogenicity against insect larvae, yielding both virulent and attenuated subpopulations of cells from an isogenic culture. The global regulatory protein Lrp is necessary for X. nematophila virulence and immunosuppression in insects, as well as colonization of the mutualistic host nematode Steinernema carpocapsae, and mediates expression of numerous genes implicated in each of these phenotypes. Given the central role of Lrp in X. nematophila host associations, as well as its involvement in regulating phenotypic variation pathways in other bacteria, we assessed its function in virulence modulation. We discovered that expression of lrp varies within an isogenic population, in a manner that correlates with modulation of virulence. Unexpectedly, although Lrp is necessary for optimal virulence and immunosuppression, cells expressing high levels of lrp were attenuated in these processes relative to those with low to intermediate lrp expression. Furthermore, fixed expression of lrp at high and low levels resulted in attenuated and normal virulence and immunosuppression, respectively, and eliminated population variability of these phenotypes. These data suggest that fluctuating lrp expression levels are sufficient to drive phenotypic variation in X. nematophila. IMPORTANCEMany bacteria use cell-to-cell phenotypic variation, characterized by distinct phenotypic subpopulations within an isogenic population, to cope with environmental change. Pathogenic bacteria utilize this strategy to vary antigen or virulence factor expression. Our work establishes that the global transcription factor Lrp regulates phenotypic variation in the insect pathogen Xenorhabdus nematophila, leading to attenuation of virulence and immunosuppression in insect hosts. Unexpectedly, we found an inverse correlation between Lrp expression levels and virulence: high levels of expression of Lrp-dependent putative virulence genes are detrimental for virulence but may have an adaptive advantage in other aspects of the life cycle. Investigation of X. nematophila phenotypic variation facilitates dissection of this phenomenon in the context of a naturally occurring symbiosis.

Ready or Not: Microbial Adaptive Responses in Dynamic Symbiosis Environments

“…A role for Lrp in VMO was initially suggested by the finding that Lrp protein levels differ from cell to cell in wild-type populations and that these levels correlate with immunosuppressive and virulence phenotypes after injection into insects. Surprisingly, this correlation is inverse: X. nematophila cells expressing high levels of Lrp exhibit attenuated immune suppression and virulence, whereas those expressing low levels of Lrp are immunosuppressive and virulent (68) (Fig. 2E).…”

“…This phenomenon is marked by the spontaneous and reversible switch between virulent and virulence-attenuated phenotypes (as assessed by the ability to kill Manduca sexta insects upon injection) (68,70). A role for Lrp in VMO was initially suggested by the finding that Lrp protein levels differ from cell to cell in wild-type populations and that these levels correlate with immunosuppressive and virulence phenotypes after injection into insects.…”

“…The link between Lrp levels and virulence phenotypes has been substantiated using X. nematophila cells engineered to constitutively express high and low levels of Lrp. These cells exhibit attenuated and virulent phenotypes, respectively (68). More-recent studies have explored the biological role of the virulence-attenuated high Lrp cell type, revealing that high-Lrp expressers are significantly better than low-Lrp expressers in supporting nematode reproduction and are slightly better at initiating colonization of IJs (Fig.…”

“…A more complicated role for Lrp in the X. nematophila life cycle was recently reported, based on the discovery that it controls a phenotypic variation phenomenon known as virulence modulation (VMO) (68,70). This phenomenon is marked by the spontaneous and reversible switch between virulent and virulence-attenuated phenotypes (as assessed by the ability to kill Manduca sexta insects upon injection) (68,70).…”

“…These regulators contribute to nutrient adaptation by sensing various environmental signals and eliciting an adaptive global change in gene expression (66). In X. nematophila, Lrp is a global regulator that controls numerous genes with predicted roles in host interactions, motility, nutrient adaptation, and the production of small molecules (67,68). An lrp mutant has defects in each stage of X. nematophila host interactions: it is defective in immune suppression and killing of M. sexta insects, supporting reproduction of and colonizing nematodes, and transitioning between nutrient-limiting and nutrient-rich media (67,69).…”