Effectiveness of antimicrobial compounds produced by entomopathogenic nematode symbiotic bacteria to control pests and bacterial plant diseases

Abstract: Entomopathogenic nematode-bacterium symbiotic complexes (EPN-EPB) provide several options for environmentally friendly biological pest control. In the last decades EPN formulations become commercially available for agricultural applications. Relatively less attention was dedicated to the antimicrobial potential of the symbiotic bacteria. The role of the EPB partner is essential for the successful colonization of the insect host cadaver via their production of antibiotics and hydrolytic exoenzymes, and processes the insect’s cadaver into utilisable nutrient for the nematodes. The broad-spectrum antibiotics of peptide nature produced by EPB non-ribosomally keep monoxenic conditions in insect cadavers in soil. These compounds show significant antagonistic activity against numerous plant pathogenic bacteria and fungi in vitro. Potato wilt caused by Ralstonia solanacearum is one of the most challenging plant diseases in several countries, including Hungary. In this project we established some parameters needed prior to experimental field application of antibacterial substances from EPB strains Xenorhabdus budapestensis (EMA) and X. szentirmaii (EMC) in Ralstonia control. We quantitatively determined the (i) optimum inoculum size needed for successful Ralstonia infection; (ii) the minimum phytotoxic concentration and (iii) the minimal inhibitory concentration of EMA cell free conditioned medium (CFCM). The bactericide effect of EMA CFCM against Ralstonia solanacearum test organism was determined under in vitro circumstances. At the light of the results we consider the antibacterial component(s) of EMA CFCM as potential tool(s) of Ralstonia control.