Biochemistry and differential mechanistic activity of antimicrobial lipopeptides from plant pathogen antagonists from the genus Bacillus

Bacteria of the Bacillus genus produce various antimicrobial compounds that are effective against several plant pathogens. These Bacillus strains are proven plant pathogen antagonists, which produce cyclic lipopeptides (CLP) as a key group of antimicrobial compounds. Fengycins, iturins and surfactins are the three main families of CLP, which share a similar structure of an oligopeptide ring attached to a lipid tail. The proposed direct mechanisms of activity for CLP are their effects on the membranes of bacterial and fungal pathogens. These typically result in changes in the cell membrane structure, leading to pore formation, increased permeability and/or other membrane effects. The exact mechanisms of interaction with plant pathogen biomembranes and resulting outcomes are dependent on the family of CLP present. The differential activity of CLP against plant pathogenic fungi, oomycetes and bacteria indicates that specific determinants are required for their activity. Strains of Bacillus produce varying amounts of CLP, as well as other antimicrobial metabolites, which can influence their spectrum of activity, in a synergistic, additive, or negative manner. Further understanding of the overall variability and spectrum of activity of these antimicrobials is essential to evaluate the short- and long-term efficacy, and therefore sustainability of Bacillus spp. as biocontrol agents.