Bacillus-Based Biocontrol Agents Mediate Pathogen Killing by Biodegradable Antimicrobials from Macrolactin Family

The transition to organic farming is one of the most desirable achievements of our time. Rational use of organic farming approaches not only enables a reduction in costs and increased yields but also limits the risks associated with the use of pesticides and chemicals. Despite the widest practical application of numerous biocontrol agents based on Bacillus strains, their metabolome, including the main active substances, often remains unknown. In order to understand the basic principles of the functioning of the Bacillus velezensis K-3618 strain, widely used in organic farming, we studied its spectrum of antimicrobial metabolites in detail. It was shown that the main antimicrobial agents of B. velezensis K-3618 are representatives of the macrolactin family. The identified macrolactin A (MLN A) and its acylated analogs 7-O-malonyl macrolactin A (mal-MLN A) and 7-O-succinyl macrolactin A (suc-MLN A) are active against Gram-positive bacterial pathogens, including multidrug-resistant strains. Among them, suc-MLN A is the most potent antimicrobial, highly active (MIC = 0.1 μg/mL) against the common human pathogen methicillin-resistant Staphylococcus aureus (MRSA). It was revealed that the primary mechanism of action of MLN A-based macrolactins is protein translation inhibition. Acylated macrolactins outperform MLN A in the prokaryotic cell-free system, displaying high efficiency in low micromolar concentrations. We observed that acylated MLN A analogs undergo pathogen-mediated biotransformation into MLN F analogs, having their antimicrobial activity reduced by two orders of magnitude. Hence, both acylation of MLNs and stabilization of the MLN A core are essential for the creation of new synthetic MLNs with improved antimicrobial activity and stability. However, we speculate that these degradability modes are of prime importance for bacterial ecology, and they are highly conserved in Bacillus species from various ecological niches.