Unraveling Mechanisms of Direct Bacteria-Oomycete Interactions using Co-Culture System

Beneficial microorganisms and pathogens directly and consistently interact with each other in nature which can shape soil microbial communities and influence plant health. Here, we developed a co-culture system to study the direct interactions between Pseudomonas protegens Pf-5, a plant beneficial bacterium producing multiple antibiotics, and Aphanomyces euteiches, an oomycete causing pea root rot disease. Our study focused on how bacterial antibiotics, motility and hyphal nutrient acquisition influence the pathogen’s survival in the direct interactions. Our results revealed that Pf-5 used multiple antibiotics, including 2,4-diacetylphloroglucinol (DAPG), toxoflavin and a few more, to kill A. euteiches in the co-culture. This result is consistent with our previous report that DAPG is the major antibiotic of Pf-5 to inhibit A. euteiches in dual-culture and advanced our knowledge by identifying additional antibiotics used by bacteria to kill oomycetes. We also found that bacterial motility is critical in pathogen suppression, as a Pf-5 mutant lacking flagella was significantly less effective in moving on hyphae of A. euteiches or killing the pathogen. Furthermore, results showed that P. protegens Pf-5 can efficiently utilize nutrients including different amino acids from hyphae of A. euteiches, suggesting a mycophagous trait of the beneficial bacterium. Overall, these results advanced our understanding of the mechanisms used by beneficial bacteria to kill plant pathogens and provided new insights into the direct microbial interactions that mimic the natural soil ecosystems.