Lipopeptides from Bacillus amyloliquefaciens strain 32a as promising biocontrol compounds against the plant pathogen Agrobacterium tumefaciens.

Despite the potential biological importance of lipopeptides from Bacillus amyloliquefaciens as antimicrobial compounds, their effects on Agrobacterium tumefaciens biofilms have not been previously studied. These latter are important virulence factors for the development and re-occurrence of crown gall disease. As part of the development of a new biopesticide acting as anti-biofilm and biocontrol agent, we investigated for the first time the ability of a mixture of lipopeptides produced by B. amyloliquefaciens strain 32a to inhibit the tumor formation on plants and to reduce the formation of biofilms by the phytopathogenic A. tumefaciens strains C58 and B6. The mixture was found to display a strong biosurfactant activity as well as bactericidal activity against planktonic Agrobacterium cells. Moreover, the lipopeptide treatment inhibited biofilm formation in the range of 79.58 ± 0.60-100.00 ± 0.00% and dislodged 43.42 ± 0.91-93.89 ± 2.70% of preformed biofilm. For these assays, fluorescence microscopy did not show any adherent cell in the anti-adhesive assay and only few ones in the cell-dislodging assay. More importantly, lipopeptide-enriched extract inhibits tumor formation on tomato stem when treatments were applied after pathogen adhesion to wounded tissues. By virtue of its ability to inhibit biofilms formed on biotic and abiotic surfaces and to control efficiently tumor development, the 32a lipopeptide mixture may represent an excellent new tool for an efficient biocontrol of crown gall disease.