Facilitating flip-flop: Structural tuning of molecule-membrane interactions in living bacteria

Monitoring Molecular Interactions with Cell Membranes Using Time-Dependent Second Harmonic Generation Microscopy

Time-resolved second harmonic generation (SHG) microscopy is used to investigate the physicochemical interactions between positively charged, hydrophobic, drug-like molecules and the plasma membrane of human cells (nonsmall cell lung cancer, H596). In the present study, molecular adsorption and transport of the cationic molecules, malachite green (MG) and malachite green isothiocyanate (MGITC), are studied in real time in living H596 cells and in dead, fixed H596 cells. MGITC is shown to have stronger adsorption and more rapid transport kinetics as compared to MG due to increased dipole-dipole interactions. Additionally, MGITC is found to have faster adsorption and transport kinetics in living H596 cells in comparison to fixed H596 cells, as well as higher dispersity in transport rate, pointing to changes in the nature of the plasma membrane or its integrity. Overall, the findings highlight the importance of electrostatic interactions, chemical functional groups, and cell integrity in molecular translocation dynamics across cell membranes.

Effect of Exogenous Glucose on Molecular Transport through Bacterial Membranes Studied by Second Harmonic Light Scattering

Recent studies revealed that exogenous glucose increases the efficacy of aminoglycosides in eliminating bacterial persister cells. It was speculated that this increased antimicrobial efficacy is induced by glucose-facilitated uptake of the antibiotics. Here, we examine this hypothesis by using second-harmonic light scattering to time-resolve the transport of an antimicrobial quaternary ammonium compound (QAC), malachite green, across the membranes of living Escherichia coli (E. coli) in the presence of glucose. The results show that when the glucose concentration increased from 0 to 100 μM, the QAC transport rate constant across the cytoplasmic membrane of E. coli increased by 3.6 times. Further increase of glucose concentration into the millimolar range, however, does not further enhance the transport rate constant. Conversely, a study of QAC transport across the protein-free membrane of liposomes (constructed from the polar lipid extract of E. coli) indicates that the glucose-induced enhancement in membrane transport in E. coli is mediated by protein transporters. Cell viability experiments show that low concentration of exogenous glucose enhances the QAC efficacy in eliminating E. coli. The loss of viability of the bacterial cells in the presence of the QAC at minimum inhibitory concentration increased dramatically when glucose concentration increased from 0 to 100 μM but increasing the glucose concentration up to 1 mM did not further enhance the antimicrobial efficacy. This behavior coincides with the observed increase in QAC transport rate constant as a function of glucose concentration. These observations reveal a novel antimicrobial strategy in which low concentration glucose enhances the uptake of antimicrobials into the bacteria, thereby improving antimicrobial efficacy.

Second harmonic scattering investigation of bacterial efflux induced by the antibiotic tetracycline

Efflux pumps are a key component in bacteria's ability to gain resistance to antibiotics. In addition to increasing efflux, new research has suggested that the antibiotic, tetracycline, may have larger impacts on bacterial membranes. Using second harmonic scattering, we monitor the transport of two small molecules across the membranes of different Gram-positive bacteria. By comparing our results to a simple kinetic model, we find evidence for changes in influx and efflux across both bacterial species. These changes, however, are probe-dependent, opening new questions about the localization of the drug's effects and the specificity of the efflux pumps involved.

Miltefosine impacts small molecule transport in Gram-positive bacteria

Miltefosine (MLT) is an alkylphosphocholine with clinical success as an anticancer and antiparasitic drug. Although the mechanism of action of MLT is highly debated, the interaction of MLT with the membrane, specifically lipid rafts of eukaryotes, is well-documented. Recent reports suggest MLT impacts the functional membrane microdomains in bacteria - regions of the membrane structurally and functionally similar to lipid rafts. There have been conflicting reports, however, as to whether MLT impacts the overall fluidity of cellular plasma membranes. Here, we apply steady-state fluorescence techniques, generalized polarization of laurdan and anisotropy of diphenylhexatriene, to discern how MLT impacts the global ordering and lipid packing of Staphylococcus aureus membranes. Additionally, we investigate how the transport of a range of small molecules is impacted by MLT for S. aureus and Bacillus subtilis by employing time-resolved second harmonic scattering. Overall, we observe MLT does not have an influence on the overall ordering and packing of S. aureus membranes. Additionally, we show that the transport of small molecules across the membrane can be significantly altered by MLT - although this is not the case for all molecules studied. The results presented here illustrate the potential use of MLT as an adjuvant to assist in the delivery of drug molecules in bacteria.

Probing bacterial membranes with polarization-resolved second harmonic scattering

For antibiotics that target Gram-positive bacterial cell structures, optimizing their interaction with the cytoplasmic membrane is of paramount importance. Recent time-resolved second harmonic scattering (trSHS) experiments with living bacterial cells have shown that some amphiphilic small molecules display signals consistent with organization within the membrane environment. Such organization could arise, for example, from aggregation, solvent interactions, and/or environmental rigidity. To expand our study of this system, we turn to polarization-resolved SHS (pSHS). PSHS has previously been used with model membranes to extract information about the angular distribution of integrated small molecules. Here we apply pSHS, for the first time, to cells, specifically living Staphylococcus aureus. In doing so, we aim to address contributions ascribed to the organization of amphiphilic molecules in bacterial membranes.

Observing mechanosensitive channels in action in living bacteria

Mechanosensitive (MS) channels act to protect the cytoplasmic membrane (CM) of living cells from environmental changes in osmolarity. In this report, we demonstrate the use of time-resolved second-harmonic light scattering (SHS) as a means of experimentally observing the relative state (open versus closed) of MS channels in living bacteria suspended in different buffer solutions. Specifically, the state of the MS channels was selectively controlled by changing the composition of the suspension medium, inducing either a transient or persistent osmotic shock. SHS was then used to monitor transport of the SHG-active cation, malachite green, across the bacterial CM. When MS channels were forced open, malachite green cations were able to cross the CM at a rate at least two orders of magnitude faster compared with when the MS channels were closed. These observations were corroborated using both numerical model simulations and complementary fluorescence experiments, in which the propensity for the CM impermeant cation, propidium, to stain cells was shown to be contingent upon the relative state of the MS channels (i.e., cells with open MS channels fluoresced red, cells with closed MS channels did not). Application of time-resolved SHS to experimentally distinguish MS channels opened via osmotic shock versus chemical activation, as well as a general comparison with the patch-clamp method is discussed.