Cyclic lipopeptides from <i>Bacillus velezensis</i> differentially affect spore germination and cell membranes in fungal plant pathogens

This study investigated the antifungal and mechanistic activity of two Bacillus velezensis strains against the mycelial and spore morphologies of the plant pathogens Alternaria solani (tomato/potato early blight), Cladosporium cucumerinum (cucurbit scab), and Fusarium sambucinum (potato dry rot). The pathogens were tested against B. velezensis strains SHRb and SHRc and their secondary metabolites to evaluate their ability to suppress fungal growth. The effects of Bacillus cyclic lipopeptides on spore germination and membrane permeability were assessed and compared to the ability of B. velezensis SHRb and SHRc precipitates to alter spore membrane permeability. Confrontation assays and treatments with filtrates demonstrated that both strains inhibited mycelial growth of all three fungi by up to 52 and 79%, respectively. Conversely, the cyclic lipopeptides exhibited varying abilities to inhibit spore germination and increase membrane permeability (up to 7.8-fold over the control). The lipopeptide iturin was consistently the most inhibitory of spore germination for all fungi. Membrane permeability assays indicated that the mode of action of B. velezensis SHRb likely includes membrane disruption, while B. velezensis SHRc may act through alternative antimicrobial mechanisms. Spore germination was differentially sensitive when compared to mycelial growth, implying that specific lipopeptide effects may be distinct depending on morphological state.