Interactions of Surfactants with the Bacterial Cell Wall and Inner Membrane: Revealing the Link between Aggregation and Antimicrobial Activity

Abstract

Surfactants with their intrinsic ability to solubilize lipid membranes are widely used as antibacterial agents, and their interactions with the bacterial cell envelope are complicated by their differential aggregation tendencies. We present a combined experimental and molecular dynamics investigation to unravel the molecular basis for the superior antimicrobial activity and faster kill kinetics of shorter-chain fatty acid surfactant, laurate, when compared with the longer-chain surfactants studied in contact time assays with live Escherichia coli (E. coli). From all-atom molecular dynamics simulations, translocation events across peptidoglycan were the highest for laurate followed by sodium dodecyl sulfate, myristate, palmitate, oleate, and stearate. The translocation kinetics were positively correlated with the critical micellar concentration, which determined the free monomer surfactant concentration available for translocation across peptidoglycan. Interestingly, aggregates showed a lower propensity to translocate across the peptidoglycan layer and longer translocation times were observed for oleate, thereby revealing an intrinsic sieving property of the bacterial cell wall. Molecular dynamics simulations with surfactant-incorporated bacterial inner membranes revealed the greatest hydrophobic mismatch and membrane thinning in the presence of laurate when compared with the other surfactants. The enhanced antimicrobial efficacy of laurate over oleate was further verified by experiments with giant unilamellar vesicles, and electroporation molecular dynamics simulations revealed greater inner membrane poration tendency in the presence of laurate when compared with the longer-chain surfactants. Our study provides molecular insights into surfactant translocation across peptidoglycan and chain length-induced structural disruption of the inner membrane, which correlate with contact time kill efficacies observed as a function of chain length with E. coli. The insights gained from our study uncover unexplored barrier properties of the bacterial cell envelope to rationalize the development of antimicrobial formulations and therapeutics.

Structure of the Bacterial Cell Envelope and Interactions with Antimicrobials: Insights from Molecular Dynamics Simulations

“…The kinetics of surfactant (C 12 −C 18 ) aggregation and translocation through peptidoglycan have recently been investigated using all-atom MD simulations. 112 Within the simulation time scale of 500 ns, several translocation events were reported for single-molecular transport of surfactants at different rates, and the shorter-chain laurate showed the highest number of translocation events when compared with other long-chain surfactants. The kinetics of aggregation for laurate were slow on account of it having a relatively higher critical concentration for micellar aggregation, and consequently, the laurate molecules were able to translocate more efficiently when compared with the long-chain stearate and oleate molecules.…”

“…Surfactant (C 12 −C 18 )-incorporated bacterial membranes were studied using all-atom MD simulations in our recent work. 112 The membrane properties were found to be perturbed to a greater extent by short-chain laurate molecules in comparison to long-chain oleate and stearate molecules. Laurate molecules in the bacterial membrane resulted in greater hydrophobic mismatch and membrane thinning, and the laurate-mixed membrane was more prone to poration under electric fields compared to the membranes containing longer-chain surfactants.…”

“…Surfactant (C 12 –C 18 )-incorporated bacterial membranes were studied using all-atom MD simulations in our recent work . The membrane properties were found to be perturbed to a greater extent by short-chain laurate molecules in comparison to long-chain oleate and stearate molecules.…”

“…121 Recently, more realistic models for E. coli and S. aureus peptidoglycan cell walls have been developed at atomistic and coarse-grained scales, and their interactions with small actives have been examined. 112,113,122 The contrasting features between peptidoglycan of Gram-negative and Gram-positive strains emerged from a simulation study using monolayered constructs of peptidoglycan. 122 The area per disaccharide, charge density, electrostatic potential, and pore size were found to be lower in magnitude for the model S. aureus cell wall when compared with those of the E. coli model.…”

Martini-3 Coarse-Grained Models for the Bacterial Lipopolysaccharide Outer Membrane of Escherichia coli

“…Although the structure and barrier properties of the bacterial outer membrane have been extensively investigated − the role of the LPS in stabilizing various membrane proteins and molecular pathways for transport of small molecules through the LPS remains elusive. Molecular dynamics simulations have been widely used to study properties of the bacterial inner membrane due to its simpler bilayer architecture. , However, molecular dynamics simulations of the peptidoglycan cell wall − and the outer membrane containing LPS molecules have been studied to a lesser extent, due to the complex architecture of the different components of the bacterial cell envelope. ,− In order to overcome computational limitations of all-atom descriptions, accurate coarse-grained (CG) representations of the outer membrane are needed to evaluate interactions of antimicrobial molecules and build more realistic models that include outer membrane porins, transporters, and channels present in the outer membrane.…”