Thermo-stability, dose effects and shelf-life of antifungal metabolite-containing supernatants produced by Xenorhabdus szentirmaii

Entomopathogenic nematodes in the genus Steinernema are associated with Xenorhabdus spp. bacteria. When steinernematid colonise an insect host the nematode-bacterium association overcomes the insect immune system and kills the host within 48 h. Xenorhabdus spp. produce secondary metabolites that are antifungal to protect nematode-infected cadavers from fungal colonization. The concentrated, or cell-free metabolites of X. szentirmaii exhibit high toxicity against various fungal plant pathogens and show potential as natural bio-fungicides. In the current study, we determined 1) thermo-stability, 2) dose-response, and 3) shelf-life of antifungal metabolites of X. szentirmaii against Monilinia fructicola (cause of brown rot of peach and other stone fruit) and Glomerella cingulata (cause of antharacnose). Thermo-stability was determined by autoclaving bacterial culture broths (121 A degrees C and 15 psi for 15 min) and measuring fungal growth on in potato dextrose agar (PDA) containing 10% of the supernatants. Autoclaving had no impact on the antifungal activity of the secondary metabolites. Over a test period of 9 months, the activity of both extract types did not decline when stored at 4 or 20 A degrees C. A dose-response study (10, 20, 40, 60, 80 and 100% supernatant-containing metabolite) using both phytopathogens demonstrated that a greater dose of supernatant increased antifungal activity. The antifungal-metabolite containing supernatant of X. szentirmaii has potential as a bio-fungicide. These results demonstrate the metabolite(s) are thermo-stable, they have a long shelf-life and require no stabilizing formulation, even at room temperature.