Atomic force microscopy of microbial cells: Application to nanomechanical properties, surface forces and molecular recognition forces

Applying the Atomic Force Microscopy Technique in Medical Sciences-A Narrative Review

Penetrating deep into the cells of the human body in real time has become increasingly possible with the implementation of modern technologies in medicine. Atomic force microscopy (AFM) enables the effective live imaging of cellular and molecular structures of biological samples (such as cells surfaces, components of biological membranes, cell nuclei, actin networks, proteins, and DNA) and provides three-dimensional surface visualization (in X-, Y-, and Z-planes). Furthermore, the AFM technique enables the study of the mechanical, electrical, and magnetic properties of cells and cell organelles and the measurements of interaction forces between biomolecules. The technique has found wide application in cancer research. With the use of AFM, it is not only possible to differentiate between healthy and cancerous cells, but also to distinguish between the stages of cancerous conditions. For many years, AFM has been an important tool for the study of neurodegenerative diseases associated with the deposition of peptide amyloid plaques. In recent years, a significant amount of research has been conducted on the application of AFM in the evaluation of connective tissue cell mechanics. This review aims to provide the spectrum of the most important applications of the AFM technique in medicine to date.

The surface charge of electroactive materials governs cell behaviour through its effect on protein deposition

The precise mechanisms underlying the cellular response to static electric cues remain unclear, limiting the design and development of biomaterials that utilize this parameter to enhance specific biological behaviours. To gather information on this matter we have explored the interaction of collagen type-I, the most abundant mammalian extracellular protein, with poly(vinylidene fluoride) (PVDF), an electroactive polymer with great potential for tissue engineering applications. Our results reveal significant differences in collagen affinity, conformation, and interaction strength depending on the electric charge of the PVDF surface, which subsequently affects the behaviour of mesenchymal stem cells seeded on them. These findings highlight the importance of surface charge in the establishment of the material-protein interface and ultimately in the biological response to the material. STATEMENT OF SIGNIFICANCE: The development of new tissue engineering strategies relies heavily on the understanding of how biomaterials interact with biological tissues. Although several factors drive this process and their driving principles have been identified, the relevance and mechanism by which the surface potential influences cell behaviour is still unknown. In our study, we investigate the interaction between collagen, the most abundant component of the extracellular matrix, and poly(vinylidene fluoride) with varying surface charges. Our findings reveal substantial variations in the binding forces, structure and adhesion of collagen on the different surfaces, which collectively explain the differential cellular responses. By exposing these differences, our research fills a critical knowledge gap and paves the way for innovations in material design for advanced tissue regeneration strategies.

Atomic force microscopy in disease-related studies: Exploring tissue and cell mechanics

Despite significant progress in human medicine, certain diseases remain challenging to promptly diagnose and treat. Hence, the imperative lies in the development of more exhaustive criteria and tools. Tissue and cellular mechanics exhibit distinctive traits in both normal and pathological states, suggesting that "force" represents a promising and distinctive target for disease diagnosis and treatment. Atomic force microscopy (AFM) holds great promise as a prospective clinical medical device due to its capability to concurrently assess surface morphology and mechanical characteristics of biological specimens within a physiological setting. This review presents a comprehensive examination of the operational principles of AFM and diverse mechanical models, focusing on its applications in investigating tissue and cellular mechanics associated with prevalent diseases. The findings from these studies lay a solid groundwork for potential clinical implementations of AFM. RESEARCH HIGHLIGHTS: By examining the surface morphology and assessing tissue and cellular mechanics of biological specimens in a physiological setting, AFM shows promise as a clinical device to diagnose and treat challenging diseases.

Evaluation of extracellular polymeric substances matrix volume, surface roughness and bacterial adhesion property of oral biofilm

Background/purpose

Oral biofilms are highly structured bacterial colonies embedded in a highly hydrated extracellular polymeric substances (EPS) matrix. This study aimed to investigate the characteristics of oral biofilm at different stages of maturation.

Materials and methods

Oral multispecies biofilms were grown anaerobically from plaque bacteria on collagen coated hydroxyapatite discs in brain heart infusion broth for one and three weeks. The volume of live bacteria and EPS matrix of the biofilms were determined by using corresponding fluorescent probes and confocal laser scanning microscopy. Atomic force microscopy (AFM) was used to quantitatively probe and correlate cell surface adhesion force of biofilms. The surface roughness was quantified in terms of the root mean square average of the height deviations. Adhesion was measured from force-distance data for the retraction of the cell from the surface.

Results

The volume of live bacteria and EPS of 3-week-old biofilms was higher than 1-week-old biofilms. The surface roughness value in 1-week-old biofilms was significantly higher than that in 3-week-old biofilms. AFM force-distance curve results showed that the adhesion force at the cell-cell interface was significantly more at-tractive than those at bacterial cells surface of both stages biofilms. Adhesion forces between the AFM tip and the surface of bacterial cell were fairly constant, whereas the cell-cell interface experienced greater adhesion forces in the biofilm's development.

Conclusion

As oral biofilms become mature, EPS volume and cell-cell adhesion forces increase while the surface roughness decreases.

Analytical Models for Measuring the Mechanical Properties of Yeast

The mechanical properties of yeast play an important role in many biological processes, such as cell division and growth, maintenance of internal pressure, and biofilm formation. In addition, the mechanical properties of cells can indicate the degree of damage caused by antifungal drugs, as the mechanical parameters of healthy and damaged cells are different. Over the past decades, atomic force microscopy (AFM) and micromanipulation have become the most widely used methods for evaluating the mechanical characteristics of microorganisms. In this case, the reliability of such an estimate depends on the choice of mathematical model. This review presents various analytical models developed in recent years for studying the mechanical properties of both cells and their individual structures. The main provisions of the applied approaches are described along with their limitations and advantages. Attention is paid to the innovative method of low-invasive nanomechanical mapping with scanning ion-conductance microscopy (SICM), which is currently starting to be successfully used in the discovery of novel drugs acting on the yeast cell wall and plasma membrane.

AFM Force Mapping Elucidates Pilus Deployment and Key Lifestyle-Dependent Surface Properties in Bdellovibrio bacteriovorus

Bdellovibrio bacteriovorus is known for predation of a wide variety of Gram-negative bacteria, making it of interest as an alternative or supplement to chemical antibiotics. However, a fraction of B. bacteriovorus follows a nonpredatory, "host-independent" (HI) life cycle. In this study, live predatory and HI B. bacteriovorus were captured on a surface and examined, in buffer, by collecting force maps using atomic force microscopy (AFM). The approach curves obtained on HI cells are similar to those on other Gram-negative cells, with a short nonlinear region followed by a linear region. In contrast, the approach curves obtained on predatory cells have a large nonlinear region, reflecting the unusual flexibility of the predatory cell. As the AFM tip is retracted, it shows virtually no adhesion to predatory B. bacteriovorus but has multiple adhesion events on HI cells and the 200-500+ nm region immediately surrounding them. Measured pull-off forces, pull-off distances, and effective spring constants are consistent with the multiple stretching events of Type IV pili, both on and especially adjacent to the cells. Exposure of the HI B. bacteriovorus to a pH-neutral 10% cranberry juice solution, which contains type A proanthocyanidins that are known to interfere with the adhesion of multiple types of pili, results in a substantial reduction in adhesion. Type IV pili are required for successful predation by B. bacteriovorus, but pili used in the predation process are located at the non-flagellated pole of the cell and can retract when not in use. Such pili are rarely observed under the conditions of this study, where the predator has not encountered a prey cell. In contrast, HI cells appear to have many pili distributed on and around the whole cell, presumably ready to be utilized for a variety of HI cell activities including attachment to surfaces.

Surface structure and nanomechanical properties of Actinoplanes missouriensis sporangia analyzed via atomic force microscopy

The surface structures of the sporangia produced by Actinoplanes missouriensis were analyzed at high resolution in air and liquid via atomic force microscopy. Results revealed a dynamic change in sporangium surface structure in response to the amount of moisture. Furthermore, the Young's modulus of the sporangium surface (1.95 ± 0.92 GPa) was calculated by analyzing the force-distance curves in air.

Effect of pepper extracts on the viability kinetics, topography and Quantitative NanoMechanics (QNM) of Campylobacter jejuni evaluated with AFM

Campylobacter jejuni is a pathogen bacterium that causes foodborne gastroenteritis in humans. However, phenolic compounds extracted from natural sources such as capsicum pepper plant (Capsicum annuum L. var. aviculare) could inhibit the growth of C. jejuni. Therefore, different extracts were prepared using ultrasonic extraction (USE), conventional extraction (CE) and thermosonic extraction (TSE). C. jejuni was then exposed to chili extracts to examine the antimicrobial effect and their growth/death bacterial kinetics were studied using different mathematical models. Atomic force microscopy was applied to investigate the microstructural and nanomechanical changes in the bacteria. Extracts obtained by TSE had the highest phenolic content (4.59 ± 0.03 mg/g of chili fresh weight [FW]) in comparison to USE (4.12 ± 0.05 mg/g of chili FW) and CE (4.28 ± 0.07 mg/g of chili FW). The inactivation of C. jejuni was more efficient when thermosonic extract was used. The Gompertz model was the most suitable mathematical model to describe the inactivation kinetics of C. jejuni. Roughness and nanomechanical analysis performed by atomic force microscopy (AFM) provided evidence that the chili extracts had significant effects on morphology, surface, and the reduced Young's modulus of C. jejuni. The novelty of this work was integrating growth/death bacterial kinetics of C. jejuni using different mathematical models and chili extracts, and its relationship with the morphological, topographic and nanomechanical changes estimated by AFM.

Opposing Effects of PhoPQ and PmrAB on the Properties of Salmonella enterica serovar Typhimurium: Implications on Resistance to Antimicrobial Peptides

The increasing number of resistant bacteria is a major threat worldwide, leading to the search for new antibiotic agents. One of the leading strategies is the use of antimicrobial peptides (AMPs), cationic and hydrophobic innate immune defense peptides. A major target of AMPs is the bacterial membrane. Notably, accumulating data suggest that AMPs can activate the two-component systems (TCSs) of Gram-negative bacteria. These include PhoP-PhoQ (PhoPQ) and PmrA-PmrB (PmrAB), responsible for remodeling of the bacterial cell surface. To better understand this mechanism, we utilized bacteria deficient either in one system alone or in both and biophysical tools including fluorescence spectroscopy, single-cell atomic force microscopy, electron microscopy, and mass spectrometry (MoskowitzS. M.;Antimicrob. Agents Chemother.2012, 56, 1019−103022106224; ChengH. Y.;J. Biomed. Sci.2010, 17, 6020653976). Our data suggested that the two systems have opposing effects on the properties of Salmonella enterica. The knockout of PhoPQ made the bacteria more susceptible to AMPs by making the surface less rigid, more polarized, and permeable with a slightly more negatively charged cell wall. In addition, the periplasmic space is thinner. In contrast, the knockout of PmrAB did not affect its susceptibility, while it made the bacterial outer layer very rigid, less polarized, and less permeable than the other two mutants, with a negatively charged cell wall similar to the WT. Overall, the data suggest that the coexistence of systems with opposing effects on the biophysical properties of the bacteria contribute to their membrane flexibility, which, on the one hand, is important to accommodate changing environments and, on the other hand, may inhibit the development of meaningful resistance to AMPs.

Surface architecture of Neisseria meningitidis capsule and outer membrane as revealed by Atomic Force Microscopy

An extensive morphological analysis of the Neisseria meningitidis cell envelope, including serogroup B capsule and outer membrane, based on atomic force microscopy (AFM) together with mechanical characterization by force spectroscopic measurements, has been carried out. Three meningococcal strains were used: the encapsulated serogroup B strain B1940, and the isogenic mutants B1940 siaD(+C) (lacking capsule), and B1940 cps (lacking both capsule and lipooligosaccharide outer core). regularly structured AFM experiments with the encapsulated strain B1940 provided unprecedented images of the meningococcal capsule, which seems to be characterized by protrusions ("bumps") with the lateral dimensions of about 30 nm. Measurement of the Young's modulus provided quantitative assessment of the property of the capsule to confer resistance to mechanical stress. Moreover, Raman spectroscopy gave a fingerprint by which it was possible to identify the specific molecular species of the three strains analyzed, and to highlight major differences between them.

Characterization of the nanomechanical properties of the fission yeast (Schizosaccharomyces pombe) cell surface by atomic force microscopy

Variations in cell wall composition and biomechanical properties can contribute to the cellular plasticity required during complex processes such as polarized growth and elongation in microbial cells. This study utilizes atomic force microscopy (AFM) to map the cell surface topography of fission yeast, Schizosaccharomyces pombe, at the pole regions and to characterize the biophysical properties within these regions under physiological, hydrated conditions. High-resolution images acquired from AFM topographic scanning reveal decreased surface roughness at the cell poles. Force extension curves acquired by nanoindentation probing with AFM cantilever tips under low applied force revealed increased cell wall deformation and decreased cellular stiffness (cellular spring constant) at cell poles (17 ± 4 mN/m) relative to the main body of the cell that is not undergoing growth and expansion (44 ± 10 mN/m). These findings suggest that the increased deformation and decreased stiffness at regions of polarized growth at fission yeast cell poles provide the plasticity necessary for cellular extension. This study provides a direct biophysical characterization of the S. pombe cell surface by AFM, and it provides a foundation for future investigation of how the surface topography and local nanomechanical properties vary during different cellular processes.

Investigating virus-host cell interactions: Comparative binding forces between hepatitis C virus-like particles and host cell receptors in 2D and 3D cell culture models

Cell cultures have been successfully used to study hepatitis C virus (HCV) for many years. However, most work has been done using traditional, 2-dimensional (2D) cell cultures (cells grown as a monolayer in growth flasks or dishes). Studies have shown that when cells are grown suspended in an extra-cellular-matrix-like material, they develop into spherical, 'organoid' arrangements of cells (3D growth) that display distinct differences in morphological and functional characteristics compared to 2D cell cultures. In liver organoids, one key difference is the development of clearly differentiated apical and basolateral surfaces separated and maintained by cellular tight junctions. This phenomenon, termed polarity, is vital to normal barrier function of hepatocytes in vivo. It has also been shown that viruses, and virus-like particles, interact very differently with cells derived from 2D as compared to 3D cell cultures, bringing into question the usefulness of 2D cell cultures to study virus-host cell interactions. Here, we investigate differences in cellular architecture as a function of cell culture system, using confocal scanning laser microscopy, and determine differences in binding interactions between HCV virus-like particles (VLPs) and their cognate receptors in the different cell culture systems using atomic force microscopy (AFM). We generated organoid cultures that were polarized, as determined by localization of key apical and basolateral markers. We found that, while uptake of HCV VLPs by both 2D and 3D Huh7 cells was observed by flow cytometry, binding interactions between HCV VLPs and cells were measurable by AFM only on polarized cells. The work presented here adds to the growing body of research suggesting that polarized cell systems are more suitable for the study of HCV infection and dynamics than non-polarized systems.

Probing Internal Pressures and Long-Term Stability of Nanobubbles in Water

Nanobubbles (NBs) in liquid exhibit many intriguing properties such as low buoyancy and high mass transfer efficiency and reactivity as compared to large bulk bubbles. However, it remains elusive why or how bulk NBs are stabilized in water, and particularly, the states of internal pressures of NBs are difficult to measure due to the lack of proper methodologies or instruments. This study employed the injection of high-pressure gases through a hydrophobized ceramic membrane to produce different gaseous NBs (e.g., N₂, O₂, H₂, and CO₂) in water, which is different from cavitation bubbles with potential internal low pressure and noncondensed gases. The results indicate that increasing the injection gas pressure (60-80 psi) and solution temperatures (6-40 °C) both reduced bubble sizes from approximately 400 to 200 nm, which are validated by two independent models developed from the Young-Laplace equation and contact mechanics. Particularly, the colloidal force model can explain the effects of surface tension and surface charge repulsion on bubble sizes and internal pressures. The contact mechanics model incorporates the measurement of the tip-bubble interaction forces by atomic force microscopy to determine the internal pressures and the hardness of NBs (e.g., Young's modulus). Both the colloidal force balance model and our contact mechanics model yielded consistent predictions of the internal pressures of various NBs (120-240 psi). The developed methods and model framework will be useful to unravel properties of NBs and support engineering applications of NBs (e.g., aeration or ozonation). Finally, the bulk NBs under sealed storage could be stable for around a week and progressively reduce in concentrations over the next 30-60 days.

Force-Averaging DLVO Model Predictions of the Adhesion Strengths Quantified for Pathogenic Listeria monocytogenes EGDe Grown under Variable pH Stresses

The roles of the bacterial surface biopolymers of pathogenic Listeria monocytogenes EGDe grown under variable pH conditions in governing their adhesion to a model surface of silicon nitride were investigated using atomic force microscopy under water. Our results indicated that the adhesion forces were the highest for cells cultured in media adjusted to pH 7 followed by 1.39, 1.49, 1.57, and 2.18-fold reductions at pH 6, 8, 9, and 5, respectively. Adhesion energies followed the same trends with 1.35, 1.67, 2.20, and 2.79-fold reductions in energies at pH 6, 8, 9, and 5, respectively, compared to the energy measured at pH 7. Furthermore, the structural properties of the bacterial surface biopolymer brush represented by the biopolymer brush thickness (L_o) and the molecular density (Γ) were determined by fitting a steric model of repulsion to the approach force-distance data. The L_o values followed the same trends as adhesion forces and energies, with thickness being highest at pH 7 followed by 1.82, 2.99, 3.11, and 4.66-fold reductions at pH 6, 8, 9, and 5, respectively. Γ was the highest at pH 5 and was followed by 1.26, 1.27, 1.70, and 2.82-fold reductions at pH 8, 9, 6, and 7, respectively. Our results indicated that bacterial adhesion forces and energies increased linearly with the product of L_o and Γ representing the number of biopolymers per unit length of the bacterial surface. To predict the adhesion forces and energies measured, a force-averaging model of the soft-particle analysis of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used. In addition to the standard parameters accounted for in the soft-particle analysis of the DLVO theory such as surface potential, hydrophobicity, and size, this averaging model incorporates in it structural bacterial parameters such as L_o and Γ as well as a surface coverage factor (ϕ) that represents the fraction of the bacterial surface covered by biopolymers. When the soft-particle analysis of DLVO was considered, repulsive hydrogen bond strengths were predicted at close distances of approach (<0.3 nm). In comparison, the force-averaging model predicted that attractive hydrogen bonds dominate the bacterial adhesion strengths quantified. The highest adhesion quantified for cells grown at pH 7 was related to longer and more spaced biopolymers, higher contents of cellular carbohydrates, and more hydrophilic biopolymers, each of which contributes to higher possibilities for hydrogen bonding formation. These results are significant in designing new strategies that aim at controlling bacterial adhesion to surfaces.

Response of Non-Polar Oil Component on Low Salinity Effect in Carbonate Reservoirs: Adhesion Force Measurement Using Atomic Force Microscopy

While the effect of polar-oil component on oil-brine-carbonate system wettability has been extensively investigated, there has been little quantitative analysis of the effect of non-polar components on system wettability, in particular as a function of pH. In this context, we measured the contact angle of non-polar oil on calcite surface in the presence of 10,000 ppm NaCl at pH values of 6.5, 9.5 and 11. We also measured the adhesion of non-polar oil group (–CH3) and calcite using atomic force microscopy (AFM) under the same conditions of contact angle measurements. Furthermore, to gain a deeper understanding, we performed zeta potential measurements of the non-polar oil-brine and brine-calcite interfaces, and calculated the total disjoining pressure. Our results show that the contact angle decreases from 125° to 78° with an increase in pH from 6.5 to 11. AFM measurements show that the adhesion force decreases with increasing pH. Zeta potential results indicate that an increase in pH would change the zeta potential of the non-polar oil-brine and calcite-brine interfaces towards more negative values, resulting in an increase of electrical double layer forces. The total disjoining pressure and results of AFM adhesion tests predict the same trend, showing that adhesion forces decrease with increasing pH. Our results show that the pH increase during low-salinity waterflooding in carbonate reservoirs would lift off non-polar components, thereby lowering residual oil saturation. This physiochemical process can even occur in reservoirs with low concentration of polar components in crude oils.

Effect of pH change on size and nanomechanical behavior of whey protein microgels

Microgels specific structural and functional features are attracting high research interest in several applications such as bioactives and drug delivery or functional food ingredients. Whey protein microgels (WPM) are obtained by heat treatment of whey protein isolate (WPI) in order to promote intramolecular cross-linking. In the present work, atomic force microscopy (AFM) was used in contact mode and in liquid to investigate WPM particles topography and mechanical properties at the nanoscale at native pH (6.5) and acid pH (5.5 and 3.0). Prior to AFM, WPM particles were captured on a gold substrate via low energy interactions by means of specific monoclonal antibodies. AFM images clearly showed an increase in the size of WPM particles induced by pH decrease. AFM in force spectroscopy mode was employed to monitor the elasticity of WPMs. The obtained effective Young's modulus data showed a significant increase in stiffness at pH 5.5 and pH 3.0, over 15-fold compared to native pH. These findings indicate that the mechanical profile of the WPM network varied with the pH decrease. The WPM topographic and nanomechanical changes induced by acidification were most likely due to substantial changes in the shape and inner structure of WPM particles. Our results suggest that internally cross-linked structures, modified by acidification could display interesting functional properties when used as a food ingredient.

Effect of Wetting and Dewetting Dynamics on Atomic Force Microscopy Measurements

Water bridge dynamics between an atomic force microscopy (AFM) tip and a flat substrate is studied by using a multibody dissipative particle dynamics (MDPD) model. First, the numerical model is validated by comparing the present results of droplet contact angles and liquid bridges with those reported in the literature. Then, the ability of MDPD to capture the meniscus shape and behavior for different operating conditions and geometric parameters is examined for both static and dynamic cases. Hence, several parametric studies and analyses of the AFM tip configuration and its operating conditions are reported. It is found that a critical capillary number of about 0.001 is calculated based on 5% change on the force measurements between the static and dynamic results. It is also demonstrated that the hysteresis behavior in the capillary force exerted on the AFM tip can be successfully predicted by using the MDPD model when the tip approaches or retracts from the substrate. Moreover, there is an excellent agreement in the results of breakup distance for different water bridge volumes between the predictions of the MDPD model and the theory. Also, the hysteresis of capillary force exerted on an AFM tip composed of multibody design is studied. The prediction on the transition of the capillary force vs distance between the AFM tip and the substrate is in good agreement with the experimental results. Therefore, we demonstrate a validated MDPD model which can successfully capture liquid bridge dynamics. This model can be used as a powerful design tool for meniscus manipulation technology, such as dip-pen nanolithography, as well as for studying dynamic, e.g., tapping mode AFM tip, interactions with a liquid bridge.

Differential homotypic and heterotypic interactions of antigen 43 (Ag43) variants in autotransporter-mediated bacterial autoaggregation

Antigen 43 (Ag43) is a cell-surface exposed protein of Escherichia coli secreted by the Type V, subtype a, secretion system (T5aSS) and belonging to the family of self-associating autotransporters (SAATs). These modular proteins, comprising a cleavable N-terminal signal peptide, a surface-exposed central passenger and an outer membrane C-terminal translocator, self-recognise in a Velcro-like handshake mechanism. A phylogenetic network analysis focusing on the passenger revealed for the first time that they actually distribute into four distinct classes, namely C1, C2, C3 and C4. Structural alignment and modelling analyses demonstrated these classes arose from shuffling of two different subdomains within the Ag43 passengers. Functional analyses revealed that homotypic interactions occur for all Ag43 classes but significant differences in the sedimentation kinetics and aggregation state were present when Ag43^C3 was expressed. In contrast, heterotypic interaction occurred in a very limited number of cases. Single cell-force spectroscopy demonstrated the importance of specific as well as nonspecific interactions in mediating Ag43-Ag43 recognition. We propose that structural differences in the subdomains of the Ag43 classes account for different autoaggregation dynamics and propensities to co-interact.

Towards nanoscale electrical measurements in liquid by advanced KPFM techniques: a review

Fundamental mechanisms of energy storage, corrosion, sensing, and multiple biological functionalities are directly coupled to electrical processes and ionic dynamics at solid-liquid interfaces. In many cases, these processes are spatially inhomogeneous taking place at grain boundaries, step edges, point defects, ion channels, etc and possess complex time and voltage dependent dynamics. This necessitates time-resolved and real-space probing of these phenomena. In this review, we discuss the applications of force-sensitive voltage modulated scanning probe microscopy (SPM) for probing electrical phenomena at solid-liquid interfaces. We first describe the working principles behind electrostatic and Kelvin probe force microscopies (EFM & KPFM) at the gas-solid interface, review the state of the art in advanced KPFM methods and developments to (i) overcome limitations of classical KPFM, (ii) expand the information accessible from KPFM, and (iii) extend KPFM operation to liquid environments. We briefly discuss the theoretical framework of electrical double layer (EDL) forces and dynamics, the implications and breakdown of classical EDL models for highly charged interfaces or under high ion concentrations, and describe recent modifications of the classical EDL theory relevant for understanding nanoscale electrical measurements at the solid-liquid interface. We further review the latest achievements in mapping surface charge, dielectric constants, and electrodynamic and electrochemical processes in liquids. Finally, we outline the key challenges and opportunities that exist in the field of nanoscale electrical measurements in liquid as well as providing a roadmap for the future development of liquid KPFM.

Anisotropic surface chemistry properties and adsorption behavior of silicate mineral crystals

Anisotropic surface properties of minerals play an important role in a variety of fields. With a focus on the two most intensively investigated silicate minerals (i.e., phyllosilicate minerals and pegmatite aluminosilicate minerals), this review highlights the research on their anisotropic surface properties based on their crystal structures. Four surface features comprise the anisotropic surface chemistry of minerals: broken bonds, energy, wettability, and charge. Analysis of surface broken bond and energy anisotropy helps to explain the cleavage and growth properties of mineral crystals, and understanding surface wettability and charge anisotropy is critical to the analysis of minerals' solution behavior, such as their flotation performance and rheological properties. In a specific reaction, the anisotropic surface properties of minerals are reflected in the adsorption strengths of reagents on different mineral surfaces. Combined with the knowledge of mineral crushing and grinding, a thorough understanding of the anisotropic surface chemistry properties and the anisotropic adsorption behavior of minerals will lead to the development of effective relational models comprising their crystal structure, surface chemistry properties, and targeted reagent adsorption. Overall, such a comprehensive approach is expected to firmly establish the connection between selective cleavage of mineral crystals for desired surfaces and designing novel reagents selectively adsorbed on the mineral surfaces. As tools to characterize the anisotropic surface chemistry properties of minerals, DLVO theory, atomic force microscopy (AFM), and molecular dynamics (MD) simulations are also reviewed.

Detrimental impact of silica nanoparticles on the nanomechanical properties of Escherichia coli, studied by AFM

Despite great innovative and technological promises, nanoparticles (NPs) can ultimately exert an antibacterial activity by affecting the cell envelope integrity. This envelope, by conferring the cell its rigidity and protection, is intimately related to the mechanical behavior of the bacterial surface. Depending on their size, surface chemistry, shape, NPs can induce damages to the cell morphology and structure among others, and are therefore expected to alter the overall mechanical properties of bacteria. Although Atomic Force Microscopy (AFM) stands as a powerful tool to study biological systems, with high resolution and in near physiological environment, it has rarely been applied to investigate at the same time both morphological and mechanical degradations of bacteria upon NPs treatment. Consequently, this study aims at quantifying the impact of the silica NPs (SiO₂-NPs) on the mechanical properties of E. coli cells after their exposure, and relating it to their toxic activity under a critical diameter. Cell elasticity was calculated by fitting the force curves with the Hertz model, and was correlated with the morphological study. SiO₂-NPs of 100 nm diameter did not trigger any significant change in the Young modulus of E. coli, in agreement with the bacterial intact morphology and membrane structure. On the opposite, the 4 nm diameter SiO₂-NPs did induce a significant decrease in E. coli Young modulus, mainly associated with the disorganization of lipopolysaccharides in the outer membrane and the permeation of the underlying peptidoglycan layer. The subsequent toxic behavior of these NPs is finally confirmed by the presence of membrane residues, due to cell lysis, exhibiting typical adhesion features.

Atomic Force Microscopy: A three-dimensional reconstructive tool of oral microbiota in gingivitis and periodontitis

Aim:

This study aims to ascertain the advantages of Atomic Force Microscopy (AFM) in the morphologic study of microorganisms and their interactions within the subgingival biofilm in patients with gingivitis and periodontitis.

Settings and Design:

Conducted a study on twenty patients, ten patients with severe periodontitis with probing the pocket depth of ≥8 mm, with a clinical attachment loss (CAL) of ≥6 mm CAL and ten patients with gingivitis: ≥5 mm pocket depth, and no attachment loss, was selected for the study.

Materials and Methods:

Bacterial biofilms were collected and slide preparation done. Morphological study was done using AFM. AFM consists of a cantilever-mounted tip, a piezoelectric scanner, a photodetector diode, a laser diode, and a feedback control. The laser beam is reflected from back of the cantilever into the quadrant of the photodetector. AFM works on the principle of interaction between the tip and the sample which causes the cantilever to deflect, thereby changing the position of laser onto the photodetector. Methodology used for studying the bacteria through AFM includes the following: (1) Probe type: Platinum coated silicon nitrate tip. (2) Probe force: 0.11 N/m. (3) Probe geometry: Triangular shaped tip. (4) Probe frequency: 22 KHz. (5) Probe immobilization: Used in Contact mode. AFM Solver Pro-M (NT-MDT) equipped with ETALON probe was used to take images in Nova software.

Results:

The investigation showed various morphological features, such as shape, size, and secretory product-like vesicles of the bacterial species involved in gingivitis and periodontitis. More bacterial surface details were studied by reproducing a three-dimensional reconstruction using AFM.

Conclusions:

The morphological variations of bacteria of different sizes, and shapes, cell wall structures, secretory product-like vesicles flagellated and filamentous microorganisms, polymorphonuclear leukocytes, and bacterial coaggregation analysis were done by AFM. Results of the present study conclude that AFM is a quite a reliable method for studying morphology of bacterial species involving periodontal diseases and is also used to study microbial interactions in biofilm.

Bacterial Adhesion to Ultrafiltration Membranes: Role of Hydrophilicity, Natural Organic Matter, and Cell-Surface Macromolecules

Insight into the mechanisms underlying bacterial adhesion is critical to the formulation of membrane biofouling control strategies. Using AFM-based single-cell force spectroscopy, we investigated the interaction between Pseudomonas fluorescens, a biofilm-forming bacterium, and polysulfone (PSF) ultrafiltration (UF) membranes to unravel the mechanisms underlying early stage membrane biofouling. We show that hydrophilic polydopamine (PDA) coatings decrease bacterial adhesion forces at short bacterium-membrane contact times. Further, we find that adhesion forces are weakened by the presence of natural organic matter (NOM) conditioning films, owing to the hydrophilicity of NOM. Investigation of the effect of adhesion contact time revealed that PDA coatings are less effective at preventing bioadhesion when the contact time is prolonged to 2-5 s, or when the membranes are exposed to bacterial suspensions under stirring. These results therefore challenge the notion that simple hydrophilic surface coatings are effective as a biofouling control strategy. Finally, we present evidence that adhesion to the UF membrane surface is mediated by cell-surface macromolecules (likely to be outer membrane proteins and pili) which, upon contacting the membrane, undergo surface-induced unfolding.

Probing the threshold of membrane damage and cytotoxicity effects induced by silica nanoparticles in Escherichia coli bacteria

The engineering of nanomaterials, because of their specific properties, is increasingly being developed for commercial purposes over the past decades, to enhance diagnosis, cosmetics properties as well as sensing efficiency. However, the understanding of their fate and thus their interactions at the cellular level with bio-organisms remains elusive. Here, we investigate the size- and charge-dependence of the damages induced by silica nanoparticles (SiO₂-NPs) on Gram-negative Escherichia coli bacteria. We show and quantify the existence of a NPs size threshold discriminating toxic and inert SiO₂-NPs with a critical particle diameter (Φ_c) in the range 50nm-80nm. This particular threshold is identified at both the micrometer scale via viability tests through Colony Forming Units (CFU) counting, and the nanometer scale via atomic force microscopy (AFM). At this nanometer scale, AFM emphasizes the interaction between the cell membrane and SiO₂-NPs from both topographic and mechanical points of view. For SiO₂-NPs with Φ>Φ_c no change in E. coli morphology nor its outer membrane (OM) organization is observed unless the NPs are positively charged in which case reorganization and disruption of the OM are detected. Conversely, when Φ<Φ_c, E. coli exhibit unusual spherical shapes, partial collapse, even lysis, and OM reorganization.

Effects of Streptococcus sanguinis Bacteriocin on Deformation, Adhesion Ability, and Young's Modulus of Candida albicans

In order to study the thallus changes on microscopic morphology and mechanical properties of Candida albicans antagonized by Streptococcus sanguinis bacteriocin, the adhesion ability and Young's modulus of thalli and hypha of Candida albicans were measured by the relative measurement method using atomic force microscope's (AFM) tapping model. The results showed that the average adhesion ability and Young's modulus of thalli were 7.35 ± 0.77 nN and 7.33 ± 1.29 Mpa, respectively; the average adhesion ability and Young's modulus of hypha were 9.82 ± 0.39 nN and 4.04 ± 0.76 Mpa, respectively. After being antagonized by Streptococcus sanguinis bacteriocin, the adhesion ability was decreased along with the increasing of deformation in reaction region and Young's modulus followed the same changes. It could be concluded that the adhesion ability of hypha was greater than thalli, Young's modulus of hypha was less than thalli, and adhesion ability and Young's modulus of Candida albicans were decreased significantly after being antagonized by Streptococcus sanguinis bacteriocin.

Genetically-Tunable Mechanical Properties of Bacterial Functional Amyloid Nanofibers

Bacterial biofilms are highly ordered, complex, dynamic material systems including cells, carbohydrates, and proteins. They are known to be resistant against chemical, physical, and biological disturbances. These superior properties make them promising candidates for next generation biomaterials. Here we investigated the morphological and mechanical properties (in terms of Young's modulus) of genetically-engineered bacterial amyloid nanofibers of Escherichia coli (E. coli) by imaging and force spectroscopy conducted via atomic force microscopy (AFM). In particular, we tuned the expression and biochemical properties of the major and minor biofilm proteins of E. coli (CsgA and CsgB, respectively). Using appropriate mutants, amyloid nanofibers constituting biofilm backbones are formed with different combinations of CsgA and CsgB, as well as the optional addition of tagging sequences. AFM imaging and force spectroscopy are used to probe the morphology and measure the Young's moduli of biofilm protein nanofibers as a function of protein composition. The obtained results reveal that genetically-controlled secretion of biofilm protein components may lead to the rational tuning of Young's moduli of biofilms as promising candidates at the bionano interface.

Atomic Force Microscopy Studies of the Interaction of Antimicrobial Peptides with Bacterial Cells

Antimicrobial peptides (AMPs) are promising therapeutic alternatives to conventional antibiotics. Many AMPs are membrane-active but their mode of action in killing bacteria or in inhibiting their growth remains elusive. Recent studies indicate the mechanism of action depends on peptide structure and lipid components of the bacterial cell membrane. Owing to the complexity of working with living cells, most of these studies have been conducted with synthetic membrane systems, which neglect the possible role of bacterial surface structures in these interactions. In recent years, atomic force microscopy has been utilized to study a diverse range of biological systems under non-destructive, physiologically relevant conditions that yield in situ biophysical measurements of living cells. This approach has been applied to the study of AMP interaction with bacterial cells, generating data that describe how the peptides modulate various biophysical behaviours of individual bacteria, including the turgor pressure, cell wall elasticity, bacterial capsule thickness, and organization of bacterial adhesins.

Nanoscale tomographic reconstruction of the subsurface mechanical properties of low-k high-aspect ratio patterns

In this work, intermittent contact resonance atomic force microscopy (ICR-AFM) was performed on high-aspect ratio a-SiOC:H patterned fins (100 nm in height and width from 20 to 90 nm) to map the depth and width dependencies of the material stiffness. The spatial resolution and depth sensitivity of the measurements were assessed from tomographic cross-sections over various regions of interest within the 3D space of the measurements. Furthermore, the depth-dependence of the measured contact stiffness over the scanned area was used to determine the sub-surface variation of the elastic modulus at each point in the scan. This was achieved by iteratively adjusting the local elastic profile until the depth dependence of the resulted contact stiffness matched the depth dependence of the contact stiffness measured by ICR-AFM at that location. The results of this analysis were assembled into nanoscale sub-surface tomographic images of the elastic modulus of the investigated SiOC:H patterns. A new 3D structure-property representation emerged from these tomographic images with direct evidence for the alterations sustained by the structures during processing.

Colonization of Polystyrene Microparticles by Vibrio crassostreae: Light and Electron Microscopic Investigation

Microplastics collected at sea harbor a high diversity of microorganisms, including some Vibrio genus members, raising questions about the role of microplastics as a novel ecological niche for potentially pathogenic microorganisms. In the present study, we investigated the adhesion dynamics of Vibrio crassostreae on polystyrene microparticles (micro-PS) using electronic and fluorescence microscopy techniques. Micro-PS were incubated with bacteria in different media (Zobell culture medium and artificial seawater) with or without natural marine aggregates. The highest percentage of colonized particles (38-100%) was observed in Zobell culture medium, which may be related to nutrient availability for production of pili and exopolysaccharide adhesion structures. A longer bacterial attachment (6 days) was observed on irregular micro-PS compared to smooth particles (<10 h), but complete decolonization of all particles eventually occurred. The presence of natural marine agreggates around micro-PS led to substantial and perennial colonization featuring monospecific biofilms at the surface of the aggregates. These exploratory results suggest that V. crassostreae may be a secondary colonizer of micro-PS, requiring a multispecies community to form a durable adhesion phenotype. Temporal assessment of microbial colonization on microplastics at sea using imaging and omics approaches are further indicated to better understand the microplastics colonization dynamics and species assemblages.

Indentation with atomic force microscope, Saccharomyces cerevisiae cell gains elasticity under ethanol stress

During bioethanol fermentation process, Saccharomyces cerevisiae cell membrane is the first target to be attacked by the accumulated ethanol. In such a prominent position, S. cerevisiae cell membrane could reversely provide protection through changing fluidity or elasticity secondary to remodeled membrane components or structure during the fermentation process. However, there is yet to be a direct observation of the real effect of the membrane compositional change. In this study, atomic force microscope-based strategy was performed to determine Young's modulus of S. cerevisiae to directly clarify ethanol stress-associated changes and roles of S. cerevisiae cell membrane fluidity and elasticity. Cell survival rate decreased while the cell swelling rate and membrane permeability increased as ethanol concentration increased from 0% to 20% v/v. Young's modulus decreased continuously from 3.76MPa to 1.53MPa while ethanol stress increased from 0% to 20% v/v, indicating that ethanol stress induced the S. cerevisiae membrane fluidity and elasticity changes. Combined with the fact that membrane composition varies under ethanol stress, to some extent, this could be considered as a forced defensive act to the ethanol stress by S. cerevisiae cells. On the other hand, the ethanol stress induced loosening of cell membrane also caused S. cerevisiae cell to proactively remodel membrane to make cell membrane more agreeable to the increase of environmental threat. Increased ethanol stress made S. cerevisiae cell membrane more fluidized and elastic, and eventually further facilitated yeast cell's survival.

Influence of csgD and ompR on Nanomechanics, Adhesion Forces, and Curli Properties of E. coli

Curli are bacterial appendages involved in the adhesion of cells to surfaces; their synthesis is regulated by many genes such as csgD and ompR. The expression of the two curli subunits (CsgA and CsgB) in Escherichia coli (E. coli) is regulated by CsgD; at the same time, csgD transcription is under the control of OmpR. Therefore, both genes are involved in the control of curli production. In this work, we elucidated the role of these genes in the nanomechanical and adhesive properties of E. coli MG1655 (a laboratory strain not expressing significant amount of curli) and its curli-producing mutants overexpressing OmpR and CsgD, employing atomic force microscopy (AFM). Nanomechanical analysis revealed that the expression of these genes gave origin to cells with a lower Young's modulus (E) and turgidity (P0), whereas the adhesion forces were unaffected when genes involved in curli formation were expressed. AFM was also employed to study the primary structure of the curli expressed through the freely jointed chain (FJC) model for polymers. CsgD increased the number of curli on the surface more than OmpR did, and the overexpression of both genes did not result in a greater number of curli. Neither of the genes had an impact on the structure (total length of the polymer and number and length of Kuhn segments) of the curli. Our results further suggest that, despite the widely assumed role of curli in cell adhesion, cell adhesion force is also dictated by surface properties because no relation between the number of curli expressed on the surface and cell adhesion was found.

Essential factors of an integrated moving bed biofilm reactor-membrane bioreactor: Adhesion characteristics and microbial community of the biofilm

This work aims at revealing the adhesion characteristics and microbial community of the biofilm in an integrated moving bed biofilm reactor-membrane bioreactor, and further evaluating their variations over time. With multiple methods, the adhesion characteristics and microbial community of the biofilm on the carriers were comprehensively illuminated, which showed their dynamic variation along with the operational time. Results indicated that: (1) the roughness of biofilm on the carriers increased very quickly to a maximum value at the start-up stage, then, decreased to become a flat curve, which indicated a layer of smooth biofilm formed on the surface; (2) the tightly-bound protein and polysaccharide was the most important factor influencing the stability of biofilm; (3) the development of biofilm could be divided into three stages, and Gammaproteobacteria were the most dominant microbial species in class level at the last stage, which occupied the largest ratio (51.48%) among all microbes.

Mechanical properties of the superficial biofilm layer determine the architecture of biofilms

Cells in biofilms sense and interact with their environment through the extracellular matrix. The physicochemical properties of the matrix, particularly at the biofilm-environment interface, determine how cells respond to changing conditions. In this study we describe the application of atomic force microscopy and confocal imaging to probe in situ the mechanical properties of these interfacial regions and to elucidate how key matrix components can contribute to the physical sensing by the cells. We describe how the Young's modulus of microcolonies differs according to the size and morphology of microcolonies, as well as the flow rate. The Young's modulus increased as a function of microcolony diameter, which was correlated with the production of the polysaccharide Psl at later stages of maturation for hemispherical or mushroom shaped microcolonies. The Young's modulus of the periphery of the biofilm colony was however independent of the hydrodynamic shear. The morphology of the microcolonies also influenced interfacial or peripheral stiffness. Microcolonies with a diffuse morphology had a lower Young's modulus than isolated, circular ones and this phenomenon was due to a deficiency of Psl. In this way, changes in the specific polysaccharide components imbue the biofilm with distinct physical properties that may modulate the way in which bacteria perceive or respond to their environment. Further, the physical properties of the polysaccharides are closely linked to the specific architectures formed by the developing biofilm.

Physical Characteristics of the Leaves and Latex of Papaya Plants Infected with the Papaya meleira Virus

Sticky disease, which is caused by Papaya meleira virus (PMeV), is a significant papaya disease in Brazil and Mexico, where it has caused severe economic losses, and it seems to have spread to Central and South America. Studies assessing the pathogen-host interaction at the nano-histological level are needed to better understand the mechanisms that underlie natural resistance. In this study, the topography and mechanical properties of the leaf midribs and latex of healthy and PMeV-infected papaya plants were observed by atomic force microscopy and scanning electron microscopy. Healthy plants displayed a smooth surface with practically no roughness of the leaf midribs and the latex and a higher adhesion force than infected plants. PMeV promotes changes in the leaf midribs and latex, making them more fragile and susceptible to breakage. These changes, which are associated with increased water uptake and internal pressure in laticifers, causes cell disruption that leads to spontaneous exudation of the latex and facilitates the spread of PMeV to other laticifers. These results provide new insights into the papaya-PMeV interaction that could be helpful for controlling papaya sticky disease.

Localization of adhesins on the surface of a pathogenic bacterial envelope through atomic force microscopy

Bacterial adhesion is the first and a significant step in establishing infection. This adhesion normally occurs in the presence of flow of fluids. Therefore, bacterial adhesins must be able to provide high strength interactions with their target surface in order to maintain the adhered bacteria under hydromechanical stressing conditions. In the case of B. pertussis, a Gram-negative bacterium responsible for pertussis, a highly contagious human respiratory tract infection, an important protein participating in the adhesion process is a 220 kDa adhesin named filamentous haemagglutinin (FHA), an outer membrane and also secreted protein that contains recognition domains to adhere to ciliated respiratory epithelial cells and macrophages. In this work, we obtained information on the cell-surface localization and distribution of the B. pertussis adhesin FHA using an antibody-functionalized AFM tip. Through the analysis of specific molecular recognition events we built a map of the spatial distribution of the adhesin which revealed a non-homogeneous pattern. Moreover, our experiments showed a force induced reorganization of the adhesin on the surface of the cells, which could explain a reinforced adhesive response under external forces. This single-molecule information contributes to the understanding of basic molecular mechanisms used by bacterial pathogens to cause infectious disease and to gain insights into the structural features by which adhesins can act as force sensors under mechanical shear conditions.

In situ and real time investigation of the evolution of a Pseudomonas fluorescens nascent biofilm in the presence of an antimicrobial peptide

Against the increase of bacterial resistance to traditional antibiotics, antimicrobial peptides (AMP) are considered as promising alternatives. Bacterial biofilms are more resistant to antibiotics that their planktonic counterpart. The purpose of this study was to investigate the action of an AMP against a nascent bacterial biofilm. The activity of dermaseptin S4 derivative S4(1-16)M4Ka against 6 h-old Pseudomonas fluorescens biofilms was assessed by using a combination of Attenuated Total Reflectance-Fourier Transform InfraRed (ATR-FTIR) spectroscopy in situ and in real time, fluorescence microscopy using the Baclight™ kit, and Atomic Force Microscopy (AFM, imaging and force spectroscopy). After exposure to the peptide at three concentrations, different dramatic and fast changes over time were observed in the ATR-FTIR fingerprints reflecting a concentration-dependent action of the AMP. The ATR-FTIR spectra revealed major biochemical and physiological changes, adsorption/accumulation of the AMP on the bacteria, loss of membrane lipids, bacterial detachment, bacterial regrowth, or inhibition of biofilm growth. AFM allowed estimating at the nanoscale the effect of the AMP on the nanomechanical properties of the sessile bacteria. The bacterial membrane elasticity data measured by force spectroscopy were consistent with ATR-FTIR spectra, and they allowed suggesting a mechanism of action of this AMP on sessile P. fluorescens. The combination of these three techniques is a powerful tool for in situ and in real time monitoring the activity of AMPs against bacteria in a biofilm.

Reduction of the peptidoglycan crosslinking causes a decrease in stiffness of the Staphylococcus aureus cell envelope

We have used atomic-force microscopy (AFM) to probe the effect of peptidoglycan crosslinking reduction on the elasticity of the Staphylococcus aureus cell wall, which is of particular interest as a target for antimicrobial chemotherapy. Penicillin-binding protein 4 (PBP4) is a nonessential transpeptidase, required for the high levels of peptidoglycan crosslinking characteristic of S. aureus. Importantly, this protein is essential for β-lactam resistance in community-acquired, methicillin-resistant S. aureus (MRSA) strains but not in hospital-acquired MRSA strains. Using AFM in a new mode for recording force/distance curves, we observed that the absence of PBP4, and the concomitant reduction of the peptidoglycan crosslinking, resulted in a reduction in stiffness of the S. aureus cell wall. Importantly, the reduction in cell wall stiffness in the absence of PBP4 was observed both in community-acquired and hospital-acquired MRSA strains, indicating that high levels of peptidoglycan crosslinking modulate the overall structure and mechanical properties of the S. aureus cell envelope in both types of clinically relevant strains. Additionally, we were able to show that the applied method enables the separation of cell wall properties and turgor pressure.

Quantitative characterization of adhesion and stiffness of corneal lens of Drosophila melanogaster using atomic force microscopy

Atomic force Microscopy (AFM) has become a versatile tool in biology due to its advantage of high-resolution imaging of biological samples close to their native condition. Apart from imaging, AFM can also measure the local mechanical properties of the surfaces. In this study, we explore the possibility of using AFM to quantify the rough eye phenotype of Drosophila melanogaster through mechanical properties. We have measured adhesion force, stiffness and elastic modulus of the corneal lens using AFM. Various parameters affecting these measurements like cantilever stiffness and tip geometry are systematically studied and the measurement procedures are standardized. Results show that the mean adhesion force of the ommatidial surface varies from 36 nN to 16 nN based on the location. The mean stiffness is 483 ± 5 N/m, and the elastic modulus is 3.4 ± 0.05 GPa (95% confidence level) at the center of ommatidia. These properties are found to be different in corneal lens of eye expressing human mutant tau gene (mutant). The adhesion force, stiffness and elastic modulus are decreased in the mutant. We conclude that the measurement of surface and mechanical properties of D. melanogaster using AFM can be used for quantitative evaluation of 'rough eye' surface.

Role of Capsular Polysaccharides in Biofilm Formation: An AFM Nanomechanics Study

Bacteria form biofilms to facilitate colonization of biotic and abiotic surfaces, and biofilm formation on indwelling medical devices is a common cause of hospital-acquired infection. Although it is well-recognized that the exopolysaccharide capsule is one of the key bacterial components for biofilm formation, the underlying biophysical mechanism is poorly understood. In the present study, nanomechanical measurements of wild type and specific mutants of the pathogen, Klebsiella pneumoniae, were performed in situ using atomic force microscopy (AFM). Theoretical modeling of the mechanical data and static microtiter plate biofilm assays show that the organization of the capsule can influence bacterial adhesion, and thereby biofilm formation. The capsular organization is affected by the presence of type 3 fimbriae. Understanding the biophysical mechanisms for the impact of the structural organization of the bacterial polysaccharide capsule on biofilm formation will aid the development of strategies to prevent biofilm formation.

Lactic acid bacteria in dairy food: surface characterization and interactions with food matrix components

This review gives an overview of the importance of interactions occurring in dairy matrices between Lactic Acid Bacteria and milk components. Dairy products are important sources of biological active compounds of particular relevance to human health. These compounds include immunoglobulins, whey proteins and peptides, polar lipids, and lactic acid bacteria including probiotics. A better understanding of interactions between bioactive components and their delivery matrix may successfully improve their transport to their target site of action. Pioneering research on probiotic lactic acid bacteria has mainly focused on their host effects. However, very little is known about their interaction with dairy ingredients. Such knowledge could contribute to designing new and more efficient dairy food, and to better understand relationships between milk constituents. The purpose of this review is first to provide an overview of the current knowledge about the biomolecules produced on bacterial surface and the composition of the dairy matter. In order to understand how bacteria interact with dairy molecules, adhesion mechanisms are subsequently reviewed with a special focus on the environmental conditions affecting bacterial adhesion. Methods dedicated to investigate the bacterial surface and to decipher interactions between bacteria and abiotic dairy components are also detailed. Finally, relevant industrial implications of these interactions are presented and discussed.

Conidia of the insect pathogenic fungus, Metarhizium anisopliae, fail to adhere to mosquito larval cuticle

Adhesion of conidia of the insect pathogenic fungus, Metarhizium anisopliae, to the arthropod host cuticle initially involves hydrophobic forces followed by consolidation facilitated by the action of extracellular enzymes and secretion of mucilage. Gene expression analysis and atomic force microscopy were used to directly quantify recognition and adhesion between single conidia of M. anisopliae and the cuticle of the aquatic larval stage of Aedes aegypti and a representative terrestrial host, Tenebrio molitor. Gene expression data indicated recognition by the pathogen of both hosts; however, the forces for adhesion to the mosquito were approximately five times lower than those observed for Tenebrio. Although weak forces were recorded in response to Aedes, Metarhizium was unable to consolidate firm attachment. An analysis of the cuticular composition revealed an absence of long-chain hydrocarbons in Aedes larvae which are thought to be required for fungal development on host cuticle. This study provides, to our knowledge, the first evidence that Metarhizium does not form firm attachment to Ae. aegypti larvae in situ, therefore preventing the normal route of invasion and pathogenesis from occuring.

Spectroscopic investigation of local mechanical impedance of living cells

We studied nanoscale mechanical properties of PC12 living cells with a Force Feedback Microscope using two experimental approaches. The first one consists in measuring the local mechanical impedance of the cell membrane while simultaneously mapping the cell morphology at constant force. As the interaction force is increased, we observe the appearance of the sub-membrane cytoskeleton. We compare our findings with the outcome of other techniques. The second experimental approach consists in a spectroscopic investigation of the cell while varying the tip indentation into the membrane and consequently the applied force. At variance with conventional dynamic Atomic Force Microscopy techniques, here it is not mandatory to work at the first oscillation eigenmode of the cantilever: the excitation frequency of the tip can be chosen arbitrary leading then to new spectroscopic AFM techniques. We found in this way that the mechanical response of the PC12 cell membrane is found to be frequency dependent in the 1 kHz - 10 kHz range. In particular, we observe that the damping coefficient consistently decreases when the excitation frequency is increased.