First insights into the putative role of AidA for the bacteria-host interactions of Photorhabdus luminescens

Bacteria engage in a complex interplay with their host organisms. Photorhabdus luminescens is a Gram-negative insect pathogenic bacterium, that undergoes pathogenic as well as symbiotic interactions with several eukaryotes, including nematodes, insects and plants. The bacteria live in close symbiosis with soil nematodes that infect insect larvae and upon entering they inject their bacterial symbiosis partner into the insect's hemolymph, where the bacteria kill the insects. In the insect, the bacteria undergo a phenotypic switch that splits the bacterial population into two parts. Up to 50% of the cells switch from the so-called primary (1°) to the secondary (2°) cell form. Whereas 1° cells reassociate with the nematodes, this trait is absent from 2° cells. They remain in the soil where they interact with plant roots, protecting them from infection with fungi. Little is known how P. luminescens mediates the biotic interactions to these three different hosts. We recently identified a protein of yet unknown function, AidA, that is assumed to be involved in bacteria-host interactions. AidA is a crucial virulence factor in the plant pathogen Ralstonia solanacearum playing a significant role in pathogenicity. In P. luminescens, aidA was differentially regulated in 1° and 2° cells. Furthermore, SdiA, a LuxR solo in P. luminescens, which is involved in plant recognition and colonization is known regulating the expression of aidA. Here we demonstrate that the addition of several plant root exudates to P. luminescens 2° cells increases the PaidA promotoractivity, indicating a regulatory role of AidA in response to various plants species. Furthermore, the deletion of aidA in 1° cells results in increased swimming motility and reduced biofilm formation, especially under the influence of plant root exudates. Moreover, the 1° ΔaidA mutant exhibited an impaired ability to associate with nematodes, suggesting that AidA is also crucial for nematode symbiosis. Finally, the deletion of aidA in both 1° and 2° cells resulted in the loss of the ability to kill insect larvae since neither the 1° nor the 2° ΔaidA mutant showed any pathogenic effects. In summary, we could demonstrate that AidA plays a crucial role for the interaction of P. luminescens with its three eukaryotic hosts, however, the molecular mechanism of AidA action remains yet unclear.