Biocontrol potential of a novel <i>Bacillus velezensis</i> strain against major soft rot bacteria <i>Pectobacterium</i> and <i>Dickeya</i>

Management of Soft Rot Pectobacteriaceae (SRP) remains a major challenge because effective control options such as bactericides, chemical treatments, or resistant commercial varieties are currently lacking. In a quest for an effective control measure against SRP, we isolated bacteria from soil and potato samples from potato fields across Montana. The bacterial isolates were screened for their effective suppression of major soft rot and blackleg pathogens Pectobacterium brasiliense strain Pb1692 and Dickeya dianthicola strain ME23. We screened more than 3,000 bacterial isolates using inhibition-zone assays on nutrient agar plates. From this collection, we identified a strong antagonist effective against Pb1692 and ME23. This isolate successfully suppressed potato soft rot and blackleg disease in both laboratory and green house evaluations. Genome sequencing identified the bacterial antagonist as Bacillus velezensis strain DN539, which can survive well at 8 oC, a potato postharvest storage temperature. We enriched the B. velezensis DN539 supernatant in bioactive fractions and mass spectrometry analysis identified the bioactive compound as isomers of surfactin. Scanning electron microscopy identified that surfactin enriched fraction resulted in the leakage of the cellular content of Pb1692 and ME23 tested in our study in as little as 10 min, followed by complete degradation of bacterial cells within 1 hr. The surfactin enriched fraction also had antimicrobial effects against other economically important phytobacteria such as Erwinia amylovora, Xanthomonas campestris, and Pseudomonas syringae. These indicate that surfactin synthesized by Bacillus velezensis DN539 has potential to be developed as a biocontrol agent against broad range of phytobacteria.