In this tutorial review we illustrate the origin and dependence on various system parameters of the ionic conductance that exists in discrete nanochannels as well as in nanoporous separation and preconcentration units contained as hybrid configurations, membranes, packed beds, or monoliths in microscale liquid phase analysis systems. A particular complexity arises as external electrical fields are superimposed on internal chemical and electrical potential gradients for tailoring molecular transport. It is demonstrated that the variety of geometries in which the microfluidic/nanofluidic interfaces are realized share common, fundamental features of coupled mass and charge transport, but that phenomena behind the key steps in a particular application can be significantly tuned, depending on the morphology of a material. Thus, the understanding of morphology-related transport in internal and external electrical potential gradients is critical to the performance of a device. This addresses a variety of geometries (slits, channels, filters, membranes, random or regular networks of pores, etc.) and applications, e. g., the gating, sensing, preconcentration, and separation in multifunctional miniaturized devices. Inherently coupled mass and charge transport through ion-permselective (charge-selective) microfluidic/nanofluidic interfaces is analyzed with a stepwise-added complexity and discussed with respect to the morphology of the charge-selective spatial domains. Within this scenario, the electrostatics and electrokinetics in microfluidic and nanofluidic channels, as well as the electrohydrodynamics evolving at microfluidic/nanofluidic interfaces, where microfluidics meets nanofluidics, define the platform of central phenomena.

In reversed-phase liquid chromatography (RPLC), retained analytes can diffuse faster along the hydrophobic surface of the stationary phase than when dissolved in the water (W)–acetonitrile (ACN) mobile phase. We investigate the surface diffusion of four typical aromatic hydrocarbon analytes in RPLC through molecular dynamics simulations in a slit-pore RPLC model consisting of a silica-supported, end-capped, C18 stationary phase and a 70/30 (v/v) W/ACN mobile phase. Our data show that the lateral (surface-parallel) diffusive mobility of the analytes goes through a maximum in the ACN ditch, an ACN-rich border layer around the terminal part of the bonded-phase chains, because the solvent composition there is more conducive to analyte mobility than the W-rich mobile phase. At their lateral mobility maximum, analytes have contacts with 12–15 bonded-phase groups, 5–6 ACN and 1–2 W molecules. The lateral mobility gain from surface diffusion decreases with analyte polarity first and size second (like and unlike retention in RPLC, respectively). The lateral diffusive mobility of analytes at the ACN density maximum in the RPLC system can be approximated by their bulk molecular diffusion coefficient in a W–ACN mixture that matches the local solvent composition at the ACN density maximum. On the basis of data received from analyte-free simulations of the RPLC system with mobile phases between 10/90 and 90/10 (v/v) W/ACN and from simulations of the bulk molecular diffusion coefficients of the analytes over this range of W/ACN ratios, we predict that enhanced surface diffusion persists under gradient elution conditions.

In reversed-phase liquid chromatography (RPLC), analyte molecules retained on the hydrophobic stationary phase can undergo fast surface diffusion within an acetonitrile (ACN)-rich border layer between the stationary phase and the water (W)–ACN mobile phase. We perform molecular dynamics simulations in an RPLC mesopore model employing an endcapped C18 phase to determine retention and diffusive mobility data for four analytes at solvent ratios between 80/20 and 10/90 (v/v) W/ACN. Simulated retention data are validated by experimental retention factors acquired over the full range of studied W/ACN ratios. Our data show that for a given analyte, the lateral mobility gain from surface diffusion increases with the retention factor because both decrease with the elution strength (ACN content) of the mobile phase. A general correlation between analyte retention and surface diffusion is, however, ruled out, as analyte properties influence retention and surface diffusion differently. Complementary simulations of bulk diffusion in W–ACN mixtures show that the lateral mobility of analyte molecules in the ACN ditch can be higher than expected from the local solvent ratio. This occurs only for W-rich mobile phases, when analyte molecules have numerous contacts with bonded-phase groups, and suggests a bonded-phase contribution to surface diffusion through lubrication of retained analytes.

Lactobacillus reuteri LTH2584 exhibits antimicrobial activity that can be attributed neither to bacteriocins nor to the production of reuterin or organic acids. We have purified the active compound, named reutericyclin, to homogeneity and characterized its antimicrobial activity. Reutericyclin exhibited a broad inhibitory spectrum including Lactobacillus spp., Bacillus subtilis, B. cereus, Enterococcus faecalis, Staphylococcus aureus, and Listeria innocua. It did not affect the growth of gram-negative bacteria; however, the growth of lipopolysaccharide mutant strains of Escherichia coli was inhibited. Reutericyclin exhibited a bactericidal mode of action against Lactobacillus sanfranciscensis, Staphylococcus aureus, and B. subtilis and triggered the lysis of cells of L. sanfranciscensis in a dose-dependent manner. Germination of spores of B. subtilis was inhibited, but the spores remained unaffected under conditions that do not permit germination. The fatty acid supply of the growth media had a strong effect on reutericyclin production and its distribution between producer cells and the culture supernatant. Reutericyclin was purified from cell extracts and culture supernatant of L. reuteri LTH2584 cultures grown in mMRS by solvent extraction, gel filtration, RP-C 8 chromatography, and anion-exchange chromatography, followed by rechromatography by reversed-phase high-pressure liquid chromatography. Reutericyclin was characterized as a negatively charged, highly hydrophobic molecule with a molecular mass of 349 Da. Lactic acid bacteria (LAB) are the biological basis for the production of a great multitude of fermented foods. Their metabolic activity during these fermentative processes determines and maintains food quality. Food preservation by lactic fermentations relies mainly on the accumulation of organic acids and the acidification of the substrate. Metabolites such as acetaldehyde, diacetyl, hydrogen peroxide, and carbon dioxide contribute to this preservative effect (15). have identified low-molecular-weight compounds from cultures of Lactobacillus plantarum that contribute to the inhibitory effect of lactic acid. Certain strains of Lactobacillus reuteri produce a unique antagonistic activity, reuterin (1). This antimicrobial activity against a broad range of microorganisms was attributed to monomers, hydrated monomers, and cyclic dimers of ␤-hydroxypropionic aldehyde formed during anaerobic catabolism of glycerol. Furthermore, a great number of strains of LAB produce bacteriocins, ribosomally synthesized peptides that exhibit antagonistic activity against closely related species (32, 54). These compounds have received increasing attention since they have the potential to inhibit food pathogens (24, 51). Furthermore, lactobacilli of intestinal origin exhibit antimicrobial activity that could not be attributed to either bacteriocins or organic acids (10, 49). However, to date, no nonbacteriocin antibiotic of lactobacilli has been purified and characterized on the molecular level.The applications of antagonis...

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