Macrolactin a is the key antibacterial substance of Bacillus amyloliquefaciens D2WM against the pathogen Dickeya chrysanthemi

B. amyloliquefaciens D2WM, a plant growth-promoting rhizobacteria from the Camellia oleifera rhizosphere, exhibits strong antimicrobial activity and has the capacity to promote plant growth, and this study aimed to determine its antibacterial mechanism against the soft rot disease pathogen D. chrysanthemi HBEU-9. The metabolites from D2WM were also identified and purified. The crude extract (CE) of D2WM effectively inhibited the mycelial growth of five fungal species and the growth of five bacterial species. In particular, CE was well able to inhibit walnut blight caused by Xanthomonas arboricola pv. Juglandis, which is an effect that has rarely been reported. Compared with the control, the addition of 0.2%-0.3% CE to the growth medium notably decreased the cell biomass of HBEU-9 and significantly increased the membrane permeability, soluble proteins and reducing sugars of HBEU-9. SEM and TEM analyses revealed that CE could destroy the cell wall and distort cells and decreased the levels of cell-degrading enzymes (Cx, PG and PMG) by 23.4%, 53.8% and 27.5%, respectively. Importantly, CE retained nearly 80% activity after either storage for 7 months at 4 degrees C or exposure to 121 degrees C for 30 min. In addition, the most essential antibacterial compound, macrolactin A, was purified and identified from D2WM cultures through column chromatography, semi-preparative HPLC and HPLC-MS. Taken together, our results highlight the potential of B. amyloliquefaciens D2WM as a biocontrol agent against bacterial diseases and the utility of macrolactin A as an antimicrobial compound.