Biosurfactant-producing Bacillus spp. suppress Fusarium via fungal membrane disruption and promote cucumber growth

The increasing demand for sustainable agricultural practices has led to the exploration of beneficial microorganisms that promote plant growth and enhance resistance to phytopathogens. In this study, we characterized the Kol B9 strain isolated from the rhizosphere of Vinca minor. Morphological, biochemical, and molecular analyses were used to identify the strain as a member of the genus Bacillus, belonging to the Bacillus subtilis group. The strain exhibited strong antagonistic activity against Fusarium culmorum DSM 1094 and Fusarium sambucinum IM 6525, both on solid and in liquid media. This activity coincided with the production of surface-active cyclic lipopeptides and was accompanied by alterations in fungal membrane lipid composition, increased membrane permeability, and inhibition of spore germination and mycelial development. The environmental isolate F. sambucinum IM 6525 was less sensitive to Bacillus activity. In addition, B. subtilis Kol B9 promoted cucumber seedling growth and reduced the adverse effects of Fusarium infection. These findings support the potential application of B. subtilis Kol B9 as a bioinoculant in sustainable agriculture.