Mechanistic Aspects of the Evolution of 3D Cholesterol Crystallites in a Supported Lipid Membrane via a Quartz Crystal Microbalance with Dissipation Monitoring

The irreversible formation of cholesterol monohydrate crystals within biological membranes is the leading cause of various diseases, including atherosclerosis. Understanding the process of cholesterol crystallization is fundamentally important and could also lead to the development of improved therapeutic strategies. This has driven several studies investigating the effect of the environmental parameters on the induction of cholesterol crystallite growth and the structure of the cholesterol crystallites, while the kinetics and mechanistic aspects of the crystallite formation process within lipid membranes remain poorly understood. Herein, we fabricated cholesterol crystallites within a supported lipid bilayer (SLB) by adsorbing a cholesterol-rich bicellar mixture onto a glass and silica surface and investigated the real-time kinetics of cholesterol crystallite nucleation and growth using epifluorescence microscopy and quartz crystal microbalance with dissipation (QCM-D) monitoring. Microscopic imaging showed the evolution of the morphology of cholesterol crystallites from nanorod- and plate-shaped habits during the initial stage to mostly large, micron-sized three-dimensional (3D) plate-shaped crystallites in the end, which was likened to Ostwald ripening. QCM-D kinetics revealed unique signal responses during the later stage of the growth process, characterized by simultaneous positive frequency shifts, nonmonotonous energy dissipation shifts, and significant overtone dependence. Based on the optically observed changes in crystallite morphology, we discussed the physical background of these unique QCM-D signal responses and the mechanistic aspects of Ostwald ripening in this system. Together, our findings revealed mechanistic details of the cholesterol crystallite growth kinetics, which may be useful in biointerfacial sensing and bioanalytical applications.

Use of spherical particles to understand conidial attachment to surfaces using atomic force microscopy

Binding of particles and spores to surfaces is a natural phenomenon which is a prerequisite for biofilm formation. Perpendicular force measurements were carried out using atomic force microscopy cantilevers modified with a polystyrene or glass sphere. The attachment of the spheres was tested against glass, PVAc, p(γ-MPSco-MMA), p(γ-MPS-co-LMA), PMMAsc, and silicon surfaces. The polystyrene spheres demonstrated less varied force and strength of attachment measurement to the surfaces than the glass spheres. The force of attachment of the polystyrene spheres was also influenced by mobility of the co-polymer surfaces. Surface wettability did not affect the force of polystyrene or glass sphere attachment. The force measurements of the non-biological spheres were similar to those seen in biological systems with fungal conidia, and this was due to their size, shape, and binding energies. The use of non-biological systems may present an insight into understanding the fundamentals of more complex biological processes.

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The attachment of fungal spores to surfaces is not well understood

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Force measurements of non-biological spheres were similar to those of biological systems

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Non-biological systems may be used to represent biological systems

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The results were due to the size of the spheres/spores and their binding energies

Steady flows above a quartz crystal resonator driven at elevated amplitude

A steady flow of liquid was observed above the surface of a quartz crystal microbalance under conditions where the oscillation amplitude exceeded 10 nm. The streaming flow occurs parallel to the displacement vector and is directed towards the center of the plate. It is expected to have applications in acoustic sensing, in microfluidics, and in micromechanics in a wider sense. The flow is caused by the nonlinear term in the Navier-Stokes equation, which can produce a nonzero time-averaged force from a periodic velocity field. Central to the explanation are the flexural admixtures to the resonator's mode of vibration. Unlike pressure-driven flows, the acoustically driven steady flow attains its maximum velocity at a distance of a few hundred nanometers from the surface. It is therefore efficient in breaking bonds between adsorbed particles and the resonator surface. As a side aspect, the flow pattern amounts to a diagnostic tool, which gives access to the pattern of vibration. In particular, it leads to an estimate of the magnitude of the flexural admixtures to the thickness-shear vibration.

Real-time investigation of the muco-adhesive properties of Lactococcus lactis using a quartz crystal microbalance with dissipation monitoring

This work was devoted to probe, at the entire population level, interactions between mucins and Lactococcus lactis, using QCM-D. Real-time monitoring of adsorption on polystyrene of PGM (Pig Gastric Mucin) and subsequent adhesion of L. lactis was performed for IBB477 and MG1820 strains. Measuring simultaneously shifts in resonance frequency and dissipation on the polystyrene-coated crystal demonstrated a two-phase process for PGM adsorption. XPS analysis confirmed the presence of adsorbed mucin. The Voigt-based model was used to describe the QCM-D outputs. The predicted thickness of the PGM layer was consistent with the AFM experimental value. Adhesion of L. lactis to bare or PGM-coated polystyrene was then monitored, in combination with DAPI cell counting. Positive frequency shifts were caused by adhering bacteria. The presence of adsorbed PGM strongly reduced bacterial adhesion. However, adhesion of IBB477 to the PGM coating was greatly increased in comparison with that of MG1820. Muco-adhesion may be a highly variable and valuable phenotypic trait among L. lactis strains.

Adhesive bond stiffness of Staphylococcus aureus with and without proteins that bind to an adsorbed fibronectin film

Staphylococcus aureus is known to cause biomaterial-associated infections of implants and devices once it has breached the skin and mucosal barriers. Adhesion is the initial step in the development of a biomaterial-associated infection, and strategies to prevent staphylococcal adhesion and thus biomaterial-associated infections require understanding of the adhesive bond. The aim of this study was to compare the adhesive bond stiffnesses of two S. aureus strains with and without fibronectin-binding proteins (FnBPs) adhering to a fibronectin-coated quartz crystal microbalance (QCM) sensor surface on the basis of a coupled- resonance model. Both fibronectin adsorption and staphylococcal adhesion were accompanied by negative frequency shifts, regardless of the absence or presence of FnBPs on the staphylococcal cell surfaces. This is the opposite of the positive frequency shifts often observed for other bacterial strains adhering to bare sensor surfaces. Most likely, adhering staphylococci sink into and deform the adsorbed protein layer, creating stiff binding with the sensor surface due to an increased bacterium-substratum contact area. S. aureus 8325-4 possesses FnBPs and yields less negative frequency shifts (Δf) that are further away from the zero-crossing frequency than S. aureus DU5883. This suggests that FnBPs on S. aureus 8325-4 create a stiffer bond to the fibronectin coating than has been observed for S. aureus DU5883. Due to a limited window of observation, as defined by the available resonance frequencies in QCM, we could not determine exact stiffness values.

Acoustic sensing of the bacterium-substratum interface using QCM-D and the influence of extracellular polymeric substances

It is commonly assumed that bacterial presence on a QCM sensor-surface is associated with a negative frequency shift according to conventional mass-loading theory. Here, we demonstrate that bacteria adhering to QCM sensor-surface may yield positive frequency shifts up to 1.9×10(-6) Hz per bacterium according to a coupled-oscillator theory. Furthermore, it is demonstrated that the excretion of extracellular polymeric substances (EPS) by adhering bacteria can change the frequency shift in the negative direction by 1.7×10(-6) Hz per bacterium, according to conventional mass-loading theory. The difference in frequency shifts between an EPS-producing and a non-EPS producing staphylococcal strain correlated with the excretion of 3×10(-14) g EPS per bacterium, representing only a few percent of the weight of a bacterium. Thus an adsorbed molecular mass as low as a few percent of the mass of an adhering bacterium significantly alters the QCM-signal. Since adhesion of many different bacterial strains is accompanied by molecular adsorption of EPS, with potentially opposite effects on the QCM-signal, a combination of the coupled-oscillator and normal mass-loading theory has to be applied for proper interpretation of QCM-frequency shifts in bacterial detection.

Novel analysis of bacterium-substratum bond maturation measured using a quartz crystal microbalance

Studies in flow displacement systems have shown that the reversibility of bacterial adhesion decreases within seconds to minutes after initial contact of a bacterium with a substratum surface. Atomic force microscopy (AFM) has confirmed that the forces mediating bacterial adhesion increase over a similar time span. The interfacial rearrangements between adhering bacteria and substratum surfaces responsible for this bond maturation have never been studied. Quartz crystal microbalance with dissipation (QCM-D) senses the interfacial region in real time and nondisruptively up to 250 nm from the sensor surface. In this paper, QCM-D is combined with real-time observation of bacterial adhesion in a flow displacement system, in order to analyze resident-time-dependent changes in dissipation. Three different Streptococcus salivarius strains showed a nonlinear relation between total dissipation shift (DeltaD) and number of adhering bacteria, whereas inert and rigid silica particles demonstrated a linear relation between DeltaD and the number of adhering particles. This suggests removal of interfacial water due to residence time dependent deformation of the nonrigid bacterium-substratum interface during bond maturation. Dissipation could be described by an exponentially decaying function, which combined with adhesion data allowed extraction of the dissipation shifts per bacterium upon initial contact (DeltaD(0)), after bond maturation (DeltaD(infinity)), as well as a characteristic time constant (tau(bm)). All bacterial strains showed significant bond maturation within one minute after their arrival at the substratum surface, which was not observed for silica particles. Dissipation analysis at the level of individually adhering bacteria would have been impossible without the simultaneous real-time analysis of bacterial adhesion numbers.

Influence of cell surface appendages on the bacterium-substratum interface measured real-time using QCM-D

Quartz crystal microbalance with dissipation (QCM-D) utilizes an oscillating quartz crystal to register adsorption of rigid masses through a decrease in its resonance frequency f. In addition, QCM-D has the ability to measure the dissipative nature of nonrigid masses adhering to the crystal surface in the form of oscillation amplitude decay time. Although QCM has been applied to register bacterial adhesion to the crystal surface, full interpretation of the frequency change and dissipation signal has hitherto been impossible due to the complex interactions within the distance of 250 nm between the substratum and the bacterial cell surface. Here, we study adhesion of a series of Streptococcus salivarius mutants, possessing various surface appendages of known lengths, as a function of time using QCM-D. In addition, the number of bacteria adhering to the crystal surface was determined. The results show that adhesion of a "bald" bacterium, completely devoid of surface appendages, is registered as a frequency decrease. Adhesion of bacteria possessing surface appendages yields either a much smaller decrease or an increase in frequency, despite the fact they adhere in higher numbers. Furthermore, the magnitude of frequency and dissipation shifts was found to be influenced by the distance at which the cell body was held from the sensor surface by its surface appendages.