Phenotypic Heterogeneity of the Insect Pathogen Photorhabdus luminescens: Insights into the Fate of Secondary Cells

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Abstract
Photorhabdus luminescens are Gram-negative bacteria that live in symbiosis with soil nematodes and are simultaneously highly pathogenic towards insects. The bacteria exist in two phenotypically different forms, designated as primary (1°) and secondary (2°) cells. Yet unknown environmental stimuli as well as global stress conditions induce phenotypic switching of up to 50% of 1° to 2° cells. An important difference between the two phenotypic forms is that 2° cells are unable to live in symbiosis with nematodes and are therefore believed to remain in the soil after a successful infection cycle. In this work, we performed a transcriptomic analysis to highlight and better understand the role of 2° cells and their putative ability to adapt to a life in soil. We could confirm that the major phenotypic differences between the two cell forms are mediated at transcriptional level as the corresponding genes were down-regulated in 2° cells. Furthermore, 2° cells seem to be adapted to another environment as we found several differentially expressed genes involved in the cells9 metabolism, motility and chemotaxis as well as stress resistance, which are either up- or down-regulated in 2° cells. As in contrast to 1° cells 2° cells chemotactically responded to different attractants including plant root exudates there is evidence for the rhizosphere as an alternative environment for the 2° cells. Since P. luminescens is biotechnologically used as bio-insecticide, investigation of a putative interaction of 2° cells with plants is also of great interest for agriculture. Importance The biological function and the fate of P. luminescens 2° cells was unclear. Here we performed comparative transcriptomics of P. luminescens 1° and 2° cultures and found several genes that are up- or down-regulated in 2° cells compared to 1° cells not only coding for known phenotypic differences of the two cell forms. Our results suggest that when 1° cells convert to 2° cells, they drastically change their way of life. Thus, 2° cells could easily adapt to an alternative environment such as a free live in the rhizosphere putatively utilizing plant derived compounds as nutrient source. Since 2° cells are not able to re-associate with the nematodes, an alternative lifestyle in the rhizosphere would be conceivable.
Funding Information
  • Deutsche Forschungsgemeinschaft (SPP1617 HE 5247/5-2)

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