David A. Pink

Atomic force microscopy and theoretical considerations of surface properties and turgor pressures of bacteria

The properties of viable bacteria were investigated with the Atomic Force Microscope (AFM). By depositing bacteria on aluminum oxide filters, the adhesion of Si3N4 tips to the surfaces of Gram-negative bacterial strains possessing different lipopolysacharides (LPS) (i.e. Pseudomonas aeruginosa PAO1 and its isogenic mutants) was investigated without the use of surface modifying or bonding agents to adhere cells to the filter. Our measurements suggest that adhesion forces for Si3N4 to these bacteria were below our detection limit of 50–100 pN. Turgor pressures were also investigated for a spherical Gram-positive bacterium (Enterococcus hirae) as well as the rod-shaped Gram-negative P. aeruginosa. A simple relationship between bacterial indentation depth and turgor pressure for the spherical bacterium was first derived and gave a turgor pressure for E. hirae in deionized water of 4–6×10 5 Pa. This is the first such measurement for a spherical Gram-positive bacterium. AFM deformations of the cell envelope of P. aeruginosa gave turgor pressures in the range 0.1–0.2 × 10 5 Pa in growth medium and 1.5–4 × 10 5 Pa in distilled water. These pressure ranges compared well with previously published values derived by other means for Gram-negative rods. The imaging of bacteria under growth medium was only possible on aluminum-oxide filters. It is proposed that the 20 nm diameter pores of these filters might facilitate the attachment of bacteria. A Monte-Carlo study was carried out which showed that bacterial adhesion will be both encouraged and stronger if hydrogen bonding takes place between LPS O-sidechains and the inside surface of the filter’s pores.

Structure and functionality of edible fats

Fat-structured food materials are an important component of our diet. The role that fat plays in material functionality, flavor perception, texture and health characteristics is due in large part to its physical properties. An understanding of these physical properties is relevant from scientific, technological and medical perspectives. The physical properties of fat materials, are, in turn, governed by a complex confluence of the various structural levels in a fat material beginning with triglyceride molecules. The formation of nanoscale structural elements by these molecules is the first step in the formation of a fat material as we know it. This review shows how these microstructural elements can be imaged and characterized. It is also shown that the formation of these nanocrystals is affected by the attendant crystallization parameters. Through simulation and a discussion of van der Waals forces, it is shown that these nanoscale elements assemble into colloidal aggregates with fractal character. The influence of microstructure on the mechanical properties of a fat material is explained using a variety of mechanical models. Lastly, this review examines methods by which the properties and characteristics of the various structural levels can be engineered. Shear has been shown to affect the polymorphism and phase transition kinetics of triglyceride crystals. As well, shear has been shown to modify the aggregation of nanocrystals, with consequences for the porosity and diffusivity of oil through the fat crystal network.

Electrostatic Interactions Affect Nanoparticle-Mediated Toxicity to Gram-Negative Bacterium Pseudomonas aeruginosa PAO1

Nanoscale materials can have cytotoxic effects. Here we present the first combined empirical and theoretical investigation of the influence of electrostatic attraction on nanoparticle cytotoxicity. Modeling electrostatic interactions between cells and 13 nm spheres of zinc oxide nanoparticles provided insight into empirically determined variations of the minimum inhibitory concentrations between four differently charged isogenic strains of Pseudomonas aeruginosa PAO1. We conclude that controlling the electrostatic attraction between nanoparticles and their cellular targets may permit the modulation of nanoparticle cytotoxicity.

Computer simulation of the main gel–fluid phase transition of lipid bilayers

Monte Carlo techniques have been applied to a study of two related quasi‐two‐dimensional microscopic interaction models which describe the phase behavior of phospholipid bilayers. The two models are Ising‐like lattice models in which (a) the acyl chains of the phospholipids interact via anisotropic van der Waals forces and (b) the rotational isomerism of the chains is accounted for by two and ten selected conformational states, respectively. Monte Carlo experiments are performed on both models so as to determine whether the static thermodynamic properties of lipid bilayers are most accurately represented by a simple two state gel–fluid concept or whether a more complicated melting process involving intermediate states takes place. To this purpose, the temperature dependence of several static thermodynamic properties has been calculated for both models. This includes the chain cross‐sectional area, the internal and free energies, the coherence length, the lateral compressibility, and the specific heat. Par...

Confocal Raman microspectroscopy as a tool for studying the chemical heterogeneities of biofilms in situ

Aims:  To investigate the use of confocal Raman microspectroscopy (CRM) for the analysis of the structure, composition and development of fully hydrated biofilms.

Quantitative determination of ion distributions in bacterial lipopolysaccharide membranes by grazing-incidence X-ray fluorescence

A model of the outer membrane of Gram-negative bacteria was created by the deposition of a monolayer of purified rough mutant lipopolysaccharides at an air/water interface. The density profiles of monovalent (K+) and divalent (Ca2+) cations normal to the lipopolysaccharides (LPS) monolayers were investigated using grazing-incidence X-ray fluorescence. In the absence of Ca2+, a K+ concentration peak was found in the negatively charged LPS headgroup region. With the addition of CaCl2, Ca2+ ions almost completely displaced K+ ions from the headgroup region. By integrating the experimentally reconstructed excess ion density profiles, we obtained an accurate measurement of the effective charge density of LPS monolayers. The experimental findings were compared to the results of Monte Carlo simulations based on a coarse-grained minimal model of LPS molecules and showed excellent agreement.

On the Architecture of the Gram-Negative Bacterial Murein Sacculus

The peptidoglycan network of the murein sacculus must be porous so that nutrients, waste products, and secreted proteins can pass through. Using Escherichia coli and Pseudomonas aeruginosa as a baseline for gram-negative sacculi, the hole size distribution in the peptidoglycan network has been modeled by computer simulation to deduce the networks properties. By requiring that the distribution of glycan chain lengths predicted by the model be in accord with the distribution observed, we conclude that the holes are slits running essentially perpendicular to the local axis of the glycan chains (i. e., the slits run along the long axis of the cell). This result is in accord with previous permeability measurements of Beveridge and Jack and Demchik and Koch. We outline possible advantages that might accrue to the bacterium via this architecture and suggest ways in which such defect structures might be detected. Certainly, large molecules do penetrate the peptidoglycan layer of gram-negative bacteria, and the small slits that we suggest might be made larger by the bacterium.

Calcium ions induce collapse of charged O-side chains of lipopolysaccharides from Pseudomonas aeruginosa

Lipopolysaccharide (LPS) monolayers deposited on planar, hydrophobic substrates were used as a defined model of outer membranes of Pseudomonas aeruginosa strain dps 89. To investigate the influence of ions on the (out-of-plane) monolayer structure, we measured specular X-ray reflectivity at high energy (22 keV) to ensure transmission through water. Electron density profiles were reconstructed from the reflectivity curves, and they indicate that the presence of Ca2+ ions induces a significant change in the conformation of the charged polysaccharide head groups (O-side chains). Monte Carlo simulations based on a minimal computer model of LPS molecules allow for the modelling of 100 or more molecules over 10−3 s and theoretically explained the tendency found by experiments.

The even–odd effect in liquid crystals. A simple model

We have developed a model of molecules composed of rigid segments and hydrocarbon chains which interact via van der Waals forces, and it has been used to study the even–odd effect in the nematic–isotropic transition in liquid crystals. Because it is mathematically simpler than other models which have examined this effect, analytical results can be obtained in a molecular field approximation. It shows explicitly the reasons for certain peculiarities observed and identifies the causes of the even–odd effect in the transition temperature and rigid segment order parameter. Finally, the transition temperatures calculated agree more closely with experiment than those obtained by other theories.

Protein Specificity of Charged Sequences in Polyanions and Heparins

Long-range electrostatic interactions are generally assigned a subordinate role in the high-affinity binding of proteins by glycosaminoglycans, the most highly charged biopolyelectrolytes. The discovery of high and low sulfation domains in heparan sulfates, however, suggests selectivity via complementarity of their linear sulfation patterns with protein charge patterns. We examined how charge sequences in anionic/nonionic copolymers affect their binding to a protein with prominent charge anisotropy. Experiments and united-atom Monte Carlo simulations, together with Delphi electrostatic modeling for the protein, confirm strongest binding when polyanion sequences allow for optimization of repulsive and attractive electrostatics. Simulations also importantly identified retention of considerable polyion conformational freedom, even for strong binding. The selective affinity for heparins of high and low charge density found for this protein is consistent with nonspecific binding to distinctly different protein charge domains. These findings suggest a more nuanced view of specificity than previously proposed for heparinoid-binding proteins.