Hydrophobic polymeric particles of the plant polyester suberin destroy bacteria by interacting with their membranes

Suberin is a complex plant polyester primarily composed of poly-functional long-chain fatty acids and glycerol, and it is ubiquitously found in specialized plant cell walls. By employing an ionic liquid-based extraction method, isolated suberin retains nearly native levels of its cross-linking component, glycerol (∼5 mg g^-1). These highly esterified polymeric particles exhibit antimicrobial activity. This study aims to further understand the innate antimicrobial properties of suberin particles. Suberin particles were isolated from cork, and their complex polymeric chemistry was comprehensively characterized using a range of analytical methods. Their interaction with model gram-positive and gram-negative bacteria, specifically <i>Staphylococcus aureus</i> and <i>Escherichia coli</i>, was investigated through growth assays and microscopy. Cytotoxicity assays were conducted using keratinocyte cells. Coarse-grained models of suberin polymers were constructed; their action on membrane mimics, as nanoparticle suberin aggregates, was then tested using molecular dynamics simulations. The results demonstrate that suberin particles are non-cytotoxic to keratinocytes but effectively kill both types of bacteria by interacting with their membranes, without causing significant cell lysis (>80 % of the cells died when expose to suberin concentrations ≥200 μg⋅mL^-1). Remarkably, reduced esterification in suberin (>3.5-fold reduction in glycerol amount) enhances its activity against gram-positive bacteria but leads to cell lysis. Simulations identified a suberin nanoparticle-mediated membrane disruption process, involving absorption into the target membranes. A stark contrast was observed for bacterial vs mammalian membrane mimics, with clear disruption of the former but much weaker interaction with the latter. The balance between surface hydrophobicity and hydrophilicity of suberin particles is critical for membrane activity, and, presumably, bactericidal efficacy. These findings raise important questions about the role of suberin-bacteria interactions in soil ecosystem services.