Abstract
Non-trophic interactions are increasingly recognised as a key parameter of predator-prey interactions. In soil, predation by bacterivorous nematodes is a major selective pressure shaping soil bacterial communities, and many bacteria have evolved defence mechanisms such as toxicity. In this study, we show that extracellular secondary metabolites produced by the model soil bacterium Pseudomonas fluorescens CHA0 function as a complex defence strategy against bacterivorous nematodes. Using a collection of functional mutants lacking genes for the biosynthesis of one or several extracellular metabolites, we evaluated the impact of bacterial secondary metabolites on the survival and chemotactic behaviour of the nematode Caenorhabditis elegans. Additionally, we followed up the stress status of the nematodes by measuring the activation of the abnormal DAuer Formation (DAF) stress cascade. All studied secondary metabolites contributed to the toxicity of the bacteria, with hydrogen cyanide efficiently repelling the nematodes, and both hydrogen cyanide and 2,4-DAPG functioning as nematicides. Moreover, these metabolites elicited the DAF stress response cascade of C. elegans, showing that they affect nematode physiology already at sublethal concentrations. The results suggest that bacterial secondary metabolites responsible for the suppression of plant pathogens strongly inhibit bacterivorous nematodes and thus likely contribute to the resistance of bacteria against predators in soil.