First insights into the role of second messengers for host adaptation of Photorhabdus luminescens

The entomopathogenic bacterium Photorhabdus luminescens is a symbiont of nematodes and plants and is likewise highly pathogenic against insects. The bacteria exist in two phenotypic different variants called primary (1°) and secondary (2°) cells. The 1° cells live in close symbiosis with the nematodes, whereas 2° cells cannot interact with them anymore. However, the 2° cells are believed to be free-living cells in the soil specifically interacting with plants. Both cell forms can infect and kill insects. Since P. luminescens colonizes different eukaryotic hosts, the cells need to precisely adapt to the different interaction partners regulating symbiosis and pathogenicity. In the last years the role of second messengers for regulation of various phenotypes and host adaptation in bacteria came more and more into focus. Here, we investigated the role of second messengers for the host adaptation of P. luminescens. In general, we identified 2',3'- and 3',5'-cyclic nucleotide monophosphates (cNMPs) in both variants of the bacteria. We found that the concentrations of these second messengers altered when they were exposed to signaling molecules derived from nematodes, insects or plants. Moreover, the concentration of cNMPs differed between 1° and 2° cells, while the general amount of 2',3'-cNMPs was significantly higher compared to 3',5'-cNMPs under all tested conditions and in both cell variants. This indicates a central role of the tested second messengers for phenotypic switching of P. luminescens as well as for host adaptation. As described for various other bacteria, formation of biofilms is regulated by cyclic di-GMP. However, P. luminescens forms biofilms, but cyclic di-GMP was not found to be involved in regulating motility and sessility. This is in accordance with the uncommon finding that P. luminescens does not have any proteins containing a PilZ- and GGDEF-domain responsible for cyclic di-GMP synthesis or binding in other bacteria. Overall, our studies suggest a central role of several second messengers in adaptation to different eukaryotic hosts of this biotechnological relevant bacterium.