Development of quantitative high-throughput screening methods for identification of antifungal biocontrol strains

Large screens of bacterial strain collections to identify potential biocontrol agents are often time consuming, costly, and fail to provide quantitative results. In this study, we present two quantitative and high-throughput methods to assess the inhibitory capacity of bacterial biocontrol candidates against fungal phytopathogens. One method measures the inhibitory effect of bacterial culture supernatant components on the fungal growth, while the other accounts for direct interaction between growing bacteria and the fungus by co-cultivating the two organisms. The antagonistic supernatant method quantifies the culture components antifungal activity by calculating the cumulative impact of supernatant addition relative to a non-treated fungal control, while the antagonistic co-cultivation method identifies the minimal bacterial cell concentration required to inhibit fungal growth by co-inoculating fungal spores with bacterial culture dilution series. Thereby, both methods provide quantitative measures of biocontrol efficiency and allow prominent fungal inhibitors to be distinguished from less effective strains. The combination of the two methods shed light on the type of inhibition mechanisms and provide the basis for further mode of action studies. We demonstrate the efficacy of the methods using Bacillus spp. with different levels of antifungal activities as model antagonists and quantify their inhibitory potency against classic plant pathogens. ImportanceFungal phytopathogens are responsible for tremendous agricultural losses on annual basis. While microbial biocontrol agents represent a promising solution to the problem, there is a growing need for high-throughput methods to evaluate and quantify inhibitory properties of new potential biocontrol agents for agricultural application. In this study, we present two high-throughput and quantitative fungal inhibition methods that are suitable for commercial biocontrol screening.