Characterisation and isolation of bioactive compounds of anti-oomycete bacterial isolates inhibiting the growth of Phytophthora capsici

Some bacteria can antagonize and competitively exclude pathogens by producing antimicrobial substances in their immediate environment. These antimicrobial agents can be grouped into enzymes, protein such as polypeptides and non-protein compounds. Of the various crop pests and pathogens that devastate harvests, oomycetes and fungi are the most broadly distributed groups causing global issues in food security and environmental damage in natural ecosystems. In this study, three bacterial isolates, Bacillus amyloliquefaciens (UQ154), Bacillus velezensis (UQ156) and Acinetobacter sp. (UQ202), previously shown to promote plant growth and anti-oomycete properties against Phytophthora spp., were selected for further investigation. These isolates were subjected to extractions with dichloromethane. The crude extracts were fractionated by preparative silica gel thin-layer chromatography and tested for their anti-oomycete activities against Phytophthora capsici. The results show that nine out of 16 fractions exhibited high (> 50%) P. capsici growth inhibition, comparable to the application of the live bacterial isolates. Using GCMS analyses, the compounds identified from the bacterial fractions were pyrrolopyrazine and various phenolic compounds, including the previously identified diketopiperazine compounds (DKPs) from the crude bacterial extracts. The ability of these bacterial isolates and their bioactive compounds to inhibit plant pathogen growth deserves further investigations, offering a potential roadmap for the development of bio-pesticides.