Comparison of Powder and Liquid Forms of Antifungal Metabolites Produced by Xenorhabdus szentirmaii, the Symbionts of Entomopathogenic Nematodes, against Gray Mold Botrytis cinerea

Xenorhabdus spp. bacteria are known to produce antifungal compounds that are highly efficacious against important plant pathogens such as Botrytis cinera. Generally, centrifuged and filtered supernatant or growth cultures are used to test effects of secondary metabolites of Xenorhabdus bacteria against different phytopathogens. We hypothesized that turning the bacterial supernatant into powder will increase the antifungal effects of the bioactive metabolite. Therefore, as a first step, we investigated and compared the effects of powder and liquid forms of antifungal metabolites of X. szentirmaii against B. cinerea. The powdered form of the supernatant was obtained using spray drying technology. The different doses of the powdered supernatant and their liquid equivalents were compared via in vitro assays. Our data indicated that the antifungal activity of the liquid Xenorhabdus supernatant was stronger than the powdered form in in vitro assays. We posit that during the pulverization process, some of the antifungal compounds in cell-free supernatants were either degraded or evaporated as the supernatants were subjected to high processing temperatures and pressure of the spray drying process. It is also possible that the powdered form of the supernatant did not dissolve well in PDA, so, the antifungal compound had limited contact with the tested fungal pathogen. Future studies should extract and purify the bioactive compound/s present in the supernatants of these bacteria and test their efficacy in ppm doses as powdered forms of these compounds have longer shelf-life and can be easily formulated compared to liquid supernatants.