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.

CXCR4 is a G-protein coupled receptor which mediates signalling for diverse functions such as cell proliferation and migration, hematopoiesis and plays a role in embryogenesis and development. Signal transduction occurs primarily through transmembrane helices that function in the multicomponent lipid environment of the plasma membrane. Elevated levels of plasma membrane oxysterols occur in cardiovascular and metabolic disorders, physiological stress and inflammatory conditions. We use experimental and simulation approaches to study the impact of oxysterol chemistry and composition on CXCL12-mediated CXCR4 signalling. Experiments on HeLa cells show a pronounced decrease in calcium oscillation response for the tail oxidized sterols in comparison with the ring oxidized sterols with 22(R) hydroxycholesterol showing a near complete loss of signalling followed by 27-hydroxycholesterol and 25-hydroxycholesterol. All-atom molecular dynamics simulations reveal that tail oxidized, 27-hydroxycholesterol, displaces cholesterol and ubiquitously binds to several critical signalling residues, as well as the dimer interface. Enhanced 27-hydroxycholesterol binding alters CXCR4 residue conformations, disrupts the toggle switch and induces secondary structure changes at both N and C termini. Our study provides a molecular view of the observed mitigated CXCR4 signalling in the presence of oxysterols revealing that disruption of cholesterol-protein interactions, important for regulating the active state, is a key factor in the loss of CXCR4 signalling. Additionally, a signalling class switching from Gαito Gαsas revealed by increased CREB and ERK phosphorylation is observed in the experiments.

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