Application of atomic force microscopy in bacterial research

Changes in cell surface properties of Pseudomonas fluorescens by adaptation to NaCl induced hypertonic stress

Determination of the effect of water stress on the surface properties of bacteria is crucial to study bacterial induced soil water repellency. Changes in the environmental conditions may affect several properties of bacteria such as the cell hydrophobicity and morphology. Here, we study the influence of adaptation to hypertonic stress on cell wettability, shape, adhesion, and surface chemical composition of Pseudomonas fluorescens. From this we aim to discover possible relations between the changes in wettability of bacterial films studied by contact angle and single cells studied by atomic and chemical force microscopy (AFM, CFM), which is still lacking. We show that by stress the adhesion forces of the cell surfaces towards hydrophobic functionalized probes increase while they decrease towards hydrophilic functionalized tips. This is consistent with the contact angle results. Further, cell size shrunk and protein content increased upon stress. The results suggest two possible mechanisms: Cell shrinkage is accompanied by the release of outer membrane vesicles by which the protein to lipid ratio increases. The higher protein content increases the rigidity and the number of hydrophobic nano-domains per surface area.

Bacterial surface interactions with organic colloidal particles: Nanoscale hotspots of organic matter in the ocean

Colloidal particles constitute a substantial fraction of organic matter in the global ocean and an abundant component of the organic matter interacting with bacterial surfaces. Using E. coli ribosomes as model colloidal particles, we applied high-resolution atomic force microscopy to probe bacterial surface interactions with organic colloids to investigate particle attachment and relevant surface features. We observed the formation of ribosome films associating with marine bacteria isolates and natural seawater assemblages, and that bacteria readily utilized the added ribosomes as growth substrate. In exposure experiments ribosomes directly attached onto bacterial surfaces as 40–200 nm clusters and patches of individual particles. We found that certain bacterial cells expressed surface corrugations that range from 50–100 nm in size, and 20 nm deep. Furthermore, our AFM studies revealed surface pits in select bacteria that range between 50–300 nm in width, and 10–50 nm in depth. Our findings suggest novel adaptive strategies of pelagic marine bacteria for colloid capture and utilization as nutrients, as well as storage as nanoscale hotspots of DOM.

Spectroscopic and microscopic comparisons of cell topology and chemistry analysis of mouse embryonic stem cell, somatic cell and cancer cell

While embryonic stem cells and cancer cells are known to have many similarities in signalling pathways, healthy somatic cells are known to be different in many ways. Characterization of embryonic stem cell is crucial for cancer development and cancer recurrence due to the shared signalling pathways and life course with cancer initiator and cancer stem cells. Since embryonic stem cells are the sources of the somatic and cancer cells, it is necessary to reveal the relevance between them. The past decade has seen the importance of interdisciplinary studies and it is obvious that the reflection of the physical/chemical phenomena occurring on the cell biology has attracted much more attention. For this reason, the aim of this study is to elementally and topologically characterize the mouse embryonic stem cells, mouse lung squamous cancer cells, and mouse skin fibroblast cells by using Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) supported with Electron Dispersive Spectroscopy (EDS) techniques in a complementary way. Our AFM findings revealed that roughness data of the mouse embryonic stem cells and cancer cells were similar and somatic cells were found to be statistically different from these two cell types. However, based on both XPS and SEM-EDS results, surface elemental ratios vary in mouse embryonic stem cells, cancer cells and somatic cells. Our results showed that these complementary spectroscopic and microscopic techniques used in this work are very effective in cancer and stem cell characterization and have the potential to gather more detailed information on relevant biological samples.

Investigation of the bacterial cell envelope nanomechanical properties after long-term exposure to nitrofurans

Antibiotic residues in the environment may negatively affect biological communities in the natural ecosystems. However, their influence on environmental bacterial strains has not been thoroughly investigated. In this study, two representatives of 5-nitrofuran antibiotics (nitrofurantoin and furaltadone) were investigated in terms of their long-term influence on the cell envelopes of newly isolated environmental bacterial strains (Sphingobacterium caeni FTD2, Achromobacter xylosoxidans NFZ2 and Pseudomonas hibiscicola FZD2). A 12-month exposure of bacterial cells to nitrofurans at a concentration of 20 mg L^-1 induced changes in the cell structure and texture (bacteria under stress conditions showed a loss of their original shape and seemed to be vastly inflated, the cells increased average surface roughness after exposure to NFT and FTD, respectively). AFM observations allowed the calculation of the bacterial cell nanomechanical properties. Significant increase in adhesion energy of bacteria after prolonged contact with nitrofurantoin was demonstrated. Changes in the permeability of bacterial membrane, fatty acids' composition and bacterial cell surface hydrophobicity were determined. Despite visible bacterial adaptation to nitrofurans, prolonged presence of pharmaceuticals in the environment has led to significant alterations in the cells' structures which was particularly visible in P. hibiscicola.

Bacterial-Derived Plant Protection Metabolite 2,4-Diacetylphloroglucinol: Effects on Bacterial Cells at Inhibitory and Subinhibitory Concentrations

2,4-Diacetylphloroglucinol (2,4-DAPG) is a well-known bacterial secondary metabolite, however, its mechanism of inhibitory and subinhibitory action on bacterial cells is still poorly understood. The mechanism of 2,4-DAPG action on model bacterial strains was investigated using fluorescent spectroscopy and the action of the antibiotic was found to involve a rapid increase in membrane permeability that was accompanied by a reduction in its viability in nutrient-poor medium. At the same time, antibacterial action in nutrient-rich medium developed for several hours. Atomic force microscopy demonstrated time-dependent disturbances in the outer membrane of Escherichia coli when exposed to 2,4-DAPG, while Staphylococcusaureus cells have been visualized with signs of intracellular leakage. In addition, 2,4-DAPG inhibited the metabolic activity of S. aureus and E. coli bacterial cells in mature biofilms. Observed differences in the antibiofilm activity were dependent upon antibiotic concentration. The intracellular targets of the action of 2,4-DAPG were assessed using bacterial biosensors with inducible bioluminescence corresponding to DNA and protein damage. It was unable to register any positive response from either sensor. As a result, the bactericidal action of 2,4-DAPG is believed to be associated with the destruction of the bacterial barrier structures. The subinhibitory effect of 2,4-diacetylphloroglucinol was tested on quorum-sensing mediated processes in Pectobacterium carotovorum. Subinhibitory concentrations of 2,4-DAPG were found to lower the biosynthesis of acyl-homoserine lactones in P. carotovorum in a dose-dependent manner. Further investigation elucidated that 2,4-DAPG inhibits the metabolic activity of bacteria without affecting their viability.

Adhesin Protein Interaction of Staphylococcus Aureus Bacteria with Various Biomaterial Surfaces

The primary stage of adhesion during implant infection is dominated by interactions of the surface proteins of the bacteria with the substrate atoms. In the current work, molecular dynamics (MD) simulations have been utilized to investigate the mechanics of the associated adhesion forces of bacteria on different surfaces. The unfolding of these adhesion proteins is investigated in order to map these events to earlier experiments on bacterial de-adhesion (using single cell force spectroscopy) with real-life substrates (i.e., ultrahigh molecular weight polyethylene, hydroxyapatite, Ti alloy, and stainless steel). The adhesion of Staphylococcus aureus adhesin (i.e., SpA) is observed by altering their orientation on the silica substrate through MD simulations, followed by capturing unfolding events of three adhesins (SpA, ClfA, and SraP) of variable lengths possessing different secondary structures. The output long-range and short-range interaction forces and consequent visualization of changes in the secondary structure of protein segments are presented during the de-adhesion process. Simulation results are correlated with extracted short-range forces (using Poisson regression) from real-life bacterial de-adhesion experiments. Insights into such protein-substrate interactions may allow for engineering of biomaterials and designing of nonbiofouling surfaces.

A How-To Guide for Mode of Action Analysis of Antimicrobial Peptides

Antimicrobial peptides (AMPs) are a promising alternative to classical antibiotics in the fight against multi-resistant bacteria. They are produced by organisms from all domains of life and constitute a nearly universal defense mechanism against infectious agents. No drug can be approved without information about its mechanism of action. In order to use them in a clinical setting, it is pivotal to understand how AMPs work. While many pore-forming AMPs are well-characterized in model membrane systems, non-pore-forming peptides are often poorly understood. Moreover, there is evidence that pore formation may not happen or not play a role in vivo. It is therefore imperative to study how AMPs interact with their targets in vivo and consequently kill microorganisms. This has been difficult in the past, since established methods did not provide much mechanistic detail. Especially, methods to study membrane-active compounds have been scarce. Recent advances, in particular in microscopy technology and cell biological labeling techniques, now allow studying mechanisms of AMPs in unprecedented detail. This review gives an overview of available in vivo methods to investigate the antibacterial mechanisms of AMPs. In addition to classical mode of action classification assays, we discuss global profiling techniques, such as genomic and proteomic approaches, as well as bacterial cytological profiling and other cell biological assays. We cover approaches to determine the effects of AMPs on cell morphology, outer membrane, cell wall, and inner membrane properties, cellular macromolecules, and protein targets. We particularly expand on methods to examine cytoplasmic membrane parameters, such as composition, thickness, organization, fluidity, potential, and the functionality of membrane-associated processes. This review aims to provide a guide for researchers, who seek a broad overview of the available methodology to study the mechanisms of AMPs in living bacteria.

Atomic Force Microscopy (AFM) As a Surface Mapping Tool in Microorganisms Resistant Toward Antimicrobials: A Mini-Review

The worldwide emergence of antimicrobial resistance (AMR) in pathogenic microorganisms, including bacteria and viruses due to a plethora of reasons, such as genetic mutation and indiscriminate use of antimicrobials, is a major challenge faced by the healthcare sector today. One of the issues at hand is to effectively screen and isolate resistant strains from sensitive ones. Utilizing the distinct nanomechanical properties (e.g., elasticity, intracellular turgor pressure, and Young’s modulus) of microbes can be an intriguing way to achieve this; while atomic force microscopy (AFM), with or without modification of the tips, presents an effective way to investigate such biophysical properties of microbial surfaces or an entire microbial cell. Additionally, advanced AFM instruments, apart from being compatible with aqueous environments—as often is the case for biological samples—can measure the adhesive forces acting between AFM tips/cantilevers (conjugated to bacterium/virion, substrates, and molecules) and target cells/surfaces to develop informative force-distance curves. Moreover, such force spectroscopies provide an idea of the nature of intercellular interactions (e.g., receptor-ligand) or propensity of microbes to aggregate into densely packed layers, that is, the formation of biofilms—a property of resistant strains (e.g., Staphylococcus aureus, Pseudomonas aeruginosa). This mini-review will revisit the use of single-cell force spectroscopy (SCFS) and single-molecule force spectroscopy (SMFS) that are emerging as powerful additions to the arsenal of researchers in the struggle against resistant microbes, identify their strengths and weakness and, finally, prioritize some future directions for research.

Lethality of Brucella microti in a murine model of infection depends on the wbkE gene involved in O-polysaccharide synthesis

Brucella microti was isolated a decade ago from wildlife and soil in Europe. Compared to the classical Brucella species, it exhibits atypical virulence properties such as increased growth in human and murine macrophages and lethality in experimentally infected mice. A spontaneous rough (R) mutant strain, derived from the smooth reference strain CCM4915^T, showed increased macrophage colonization and was non-lethal in murine infections. Whole-genome sequencing and construction of an isogenic mutant of B. microti and Brucella suis 1330 revealed that the R-phenotype was due to a deletion in a single gene, namely wbkE (BMI_I539), encoding a putative glycosyltransferase involved in lipopolysaccharide (LPS) O-polysaccharide biosynthesis. Complementation of the R-strains with the wbkE gene restored the smooth phenotype and the ability of B. microti to kill infected mice. LPS with an intact O-polysaccharide is therefore essential for lethal B. microti infections in the murine model, demonstrating its importance in pathogenesis.

Synthesis and characterization of pyridine-based organic salts: Their antibacterial, antibiofilm and wound healing activities

In treating wounds, long lasting infection is considered the major impediment. Drugs are rendered ineffective by pathogenic microorganisms via antibiotic resistance and calls for designing and development of new drugs. Herein, we report synthesis of eight different N-alkylated pyridine-based organic salts QAS 1-8 and their antibacterial, antibiofilm and wound healing activities. 3-(2-R-hydrazinecarbonyl)-1-propylpyridinium Bromide was the parent compound while R group was varying in each salt composed of different aromatic aldehyde moieties. In the antibacterial activity against S. aureus and E. coli, amoxicillin shows IC₅₀ near to 25 µg/mL inhibiting 58 ± 0.4% S. aureus while ceftriaxone inhibited 55 ± 0.5% E. coli at a concentration of 10 µg/mL. The highest IC₅₀ (56 ± 0.5% against S. aureus; 55 ± 0.5% against E. coli) was shown by compound QAS 7 at the concentration of 100 µg/mL; followed by the QAS 6 (55 ± 0.5% against E. coli) and QAS 2 (55 ± 0.5% against E. coli). In the antibiofilm activity, QAS 6, QAS 1 and QAS 8 inhibited 58 ± 0.4% S. aureus at a concentration of 75 µg/mL, while QAS 2 inhibited E. coli at the same concentration and amount. QAS 7, 3 and 1 inhibited almost 90% while QAS 6 inhibited 95 ± 1.1%of E. coli at a concentration of 250 µg/mL. Highest MBIC was provided by QAS 7 (52 ± 0.4%) against S. aureus at a concentration of 50 µg/mL that is very near to the standard amoxicillin. Antibacterial and antibiofilm activity results were also supported by the atomic force microscopy (AFM). In the wound healing activity, QAS 8 healed 90.8 ± 4.3% of the wound in 21 days with an average period of epithelialization (POE) of 19 ± 1.4 days; that is far better than povidone iodine ointment (81.5 ± 3.3% of the wound in the 21 days with 22.4 ± 2.9 days of POE). It is concluded from this study that the synthesized compounds QAS 2, 7 and 8 can be used for further mechanistic studies to be employed as antibacterial, antibiofilm and wound healing agents.

Halogenated boroxine dipotassium trioxohydroxytetrafluorotriborate K2[B3O3F4OH] inhibits emerging multidrug-resistant and β-lactamase-producing opportunistic pathogens

Halogenated boroxine dipotassium trioxohydroxytetrafluorotriborate, K₂[B₃O₃F₄OH] (boroxine) was previously shown to be very effective in inhibition of several carcinoma cell lines, including the skin cancer. Here, we investigated its antimicrobial potential by targeting the multidrug-resistant opportunistic pathogens associated with skin and wound infections. The antimicrobial testing against eleven bacterial and four fungal species revealed good activity of boroxine against pathogenic filamentous fungi Penicillium funiculosum and Aspergillus niger (MIC₅₀ 64 and 128 µg/ml), and a moderate bioactivity against the yeast Candida albicans (MIC₅₀ 512 µg/ml). Among the tested multidrug-resistant bacteria, the best antibacterial effect, stable over a 24-h period, was observed against the methicillin-resistant Staphylococcus aureus strain (MRSA) at MIC of 1024 µg/ml. The atomic force microscopy (AFM) used to investigate the morphology of S. aureus cells revealed indentations on its cell envelope after the boroxine exposure. These results show that in addition to the antitumor effect, boroxine exerts wide spectrum antimicrobial activity, thus may help preventing the development of skin and wound-related opportunistic infections.

Behavior of a Metal Organic Framework Thin-Film at Elevated Temperature and Pressure as Studied with an Autoclave-Inserted Atomic Force Microscope

Bridging the gap in studying surface reactions, processes, and morphology and measuring at (catalytic) relevant conditions is crucial for our understanding of the working principles of porous crystalline materials. Scanning tunneling microscopy is limited because of the required conductivity of the sample, whereas atomic force microscopy (AFM) is often challenging in use owing to the physical mechanism underlying the technique. Herein, we report a tailor-made autoclave-inserted AFM, able to measure at ∼20 bar and ∼110 °C. First, we show the ability to obtain nanometer resolution on well-defined test samples at before-mentioned conditions. Second, to demonstrate the possibilities of analyzing morphological evolutions at elevated temperatures and pressures, we use this setup to measure the stability of a surface-anchored metal-organic framework (SURMOF) in-situ at pressures of 1-20 bar in the temperature range between 20 and 60 °C. It was found that the showcase HKUST-1 material has a good physical stability, as it is hardly damaged from exposure to pressures up to 20 bar. However, its thermal stability is weaker, as exposure to elevated T damaged the material by influencing the interaction between organic linker and metal cluster. In-situ measurements at elevated T also showed an increased mobility of the material when working at such conditions. Combining the strength of AFM at elevated T and p with ex-situ AFM and spectroscopic measurements on this MOF showcases an example of how porous materials can be studied at (industrially) relevant conditions using the autoclave-inserted AFM.

Fast Specimen Boundary Tracking and Local Imaging with Scanning Probe Microscopy

An efficient and adaptive boundary tracking method is developed to confine area of interest for high-efficiency local scanning. By using a boundary point determination criterion, the scanning tip is steered with a sinusoidal waveform while estimating azimuth angle and radius ratio of each boundary point to accurately track the boundary of targets. A local scan region and path are subsequently planned based on the prior knowledge of boundary tracking to reduce the scan time. Boundary tracking and local scanning methods have great potential not only for fast dimension measurement but also for sample surface topography and physical characterization, with only scanning region of interest. The performance of the proposed methods was verified by using the alternate current mode scanning ion-conductance microscopy, tapping, and PeakForce modulation atomic force microscopy. Experimental results of single/multitarget boundary tracking and local scanning of target structures with complex boundaries demonstrate the flexibility and validity of the proposed method.

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.

Substrate effects on cell-envelope deformation during early-stage Staphylococcus aureus biofilm formation

Bacterial adhesion to a surface is the first step in biofilm formation, and adhesive forces between the surface and a bacterium are believed to give rise to planktonic-to-biofilm phenotypic changes. Here we use Focused-Ion-Beam (FIB) tomography with backscattered scanning electron microscopy (SEM) to image Staphyolococcus aureus (S. aureus) biofilms grown on Au-coated polystyrene (PS) and Au-coated PS modified by mixed thiols of triethylene glycol mono-11-mercaptoundecyl ether (EG₃) and 1-dodecanethiol (CH₃). The FIB-SEM technique enables a direct measurement of the contact area between individual bacteria and the substrate. The area of adhesion is effectively zero on the EG₃ substrate. It is nonzero on all of the other substrates and increases with increasing hydrophobicity. The fact that the contact area is highest on the unmodified gold, however, indicates that other forces beyond hydrophobicity are significant. The magnitude of bacterial deformation suggests that the adhesive forces are on the order of a few nN, consistent with AFM force measurements reported in the literature. The resolution afforded by electron microscopy furthermore enables us to probe changes in the cell-envelope thickness, which decreases within and near the contact area relative to other parts of the same bacterium. This finding supports the idea that mechanosensing due to stress-induced membrane thinning plays a role in the planktonic-to-biofilm transition associated with bacterial adhesion.

Effects of Piper cubeba L. essential oil on methicillin-resistant Staphylococcus aureus: an AFM and TEM study

The increasing prevalence of antibiotic-resistant bacteria is creating a real challenge for health care systems worldwide, making the development of novel antibiotics a necessity. In addition to the development of new antibiotics, there is an urgent need for in-depth characterization of the mechanisms of bacterial resistance toward new drugs. Here, we used essential oils extracted in our laboratory from Piper cubeba against methicillin-resistant Staphylococcus aureus ATCC 43300, one of the most prominent antibiotic-resistant bacteria. Effects of the essential oils extracted from P cubeba on bacteria were mainly evaluated using 2 powerful microscopy techniques: atomic force microscopy and transmission electron microscopy. High-resolution atomic force microscopy images of the cells were obtained close to their native environment by immobilizing the cells on porous Polyether sulfone membranes, which were prepared in our laboratory with a wide range and distribution of pore sizes and depth. Inhibition zones (mm) and minimum inhibitory concentrations were determined. Two different concentrations of the oil were used to treat the cells: 50 μg/mL minimum inhibitory concentration and 25 μg/mL. The 50 μg/mL oil solution caused severe damage to the bacterial cells at microscopic levels while the 25 μg/mL solution showed no effects compared to the control. However, at nanoscopic levels, the 25 μg/mL oil solution caused significant changes in the cell wall, which could potentially impair bacterial activities. These results were also confirmed by transmission electron microscopy micrographs. Our results indicate that the extract has a good biological activity against methicillin- and oxacillin-resistant S aureus and that it acts on the cell wall and plasma (cytoplasmic) membrane.

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.

Implications for directionality of nanoscale forces in bacterial attachment

Adhesion and friction are closely related and play a predominant role in many natural processes. From the wall-clinging feet of the gecko to bacteria forming a biofilm, in many cases adhesion is a necessity to survive. The direction in which forces are applied has shown to influence the bond strength of certain systems tremendously and can mean the difference between adhesion and detachment. The spatula present on the extension of the feet of the gecko can either attach or detach, based on the angle at which they are loaded. Certain proteins are known to unfold at different loads, depending on the direction at which the load is applied and some bacteria have specific receptors which increase their bond strength in the presence of shear. Bacteria adhere to any man-made surface despite the presence of shear forces due to running fluids, air flow, and other causes. In bacterial adhesion research, however, adhesion forces are predominantly measured perpendicularly to surfaces, whereas other directions are often neglected. The angle of shear forces acting on bacteria or biofilms will not be at a 90° angle, as shear induced by flow is often along the surface. Measuring at different angles or even lateral to the surface will give a more complete overview of the adhesion forces and mechanism, perhaps even resulting in alternative means to discourage bacterial adhesion or promote removal.

In vitro biophysical, microspectroscopic and cytotoxic evaluation of metastatic and non-metastatic cancer cells in responses to anti-cancer drug

The Breast Cancer Metastasis Suppressor 1 (BRMS1) is a nucleo-cytoplasmic protein that suppresses cancer metastasis without affecting the growth of the primary tumor. Previous work has shown that it decreases the expression of protein mediators involved in chemoresistance. This study measured the biomechanical and biochemical changes in BRMS1 expression and the responses of BRMS1 to drug treatments on cancer cells in vitro. The results show that BRMS1 expression affects biomechanical properties by decreasing the Young's modulus and adhesion force of breast cancer cells after doxorubicin (DOX) exposure. Raman spectral bands corresponding to DNA/RNA, lipids and proteins were similar for all cells after DOX treatment. The expression of cytokines were similar for cancer cells after DOX exposure, although BRMS1 expression had different effects on the secretion of cytokines for breast cancer cells. The absence of significant changes on apoptosis, reactive oxygen species (ROS) expression and cell viability after BRMS1 expression shows that BRMS1 has little effect on cellular chemoresistance. Analyzing cancer protein expression is critical in evaluating therapeutics. Our study may provide evidence of the benefit of metastatic suppressor expression before chemotherapy.

Useful surface parameters for biomaterial discrimination

Topographical features of biomaterials' surfaces are determinant when addressing their application site. Unfortunately up to date there has not been an agreement regarding which surface parameters are more representative in discriminating between materials. Discs (n = 16) of different currently used materials for implant prostheses fabrication, such as cast cobalt-chrome, direct laser metal soldered (DLMS) cobalt-chrome, titanium grade V, zirconia (Y-TZP), E-glass fiber-reinforced composite and polyetheretherketone (PEEK) were manufactured. Nanoscale topographical surface roughness parameters generated by atomic force microscopy (AFM), microscale surface roughness parameters obtained by white light interferometry (WLI) and water angle values obtained by the sessile-water-drop method were analyzed in order to assess which parameter provides the best optimum surface characterization method. Correlations between nanoroughness, microroughness, and hydrophobicity data were performed to achieve the best parameters giving the highest discriminatory power. A subset of six parameters for surface characterization were proposed. AFM and WLI techniques gave complementary information. Wettability did not correlate with any of the nanoroughness parameters while it however showed a weak correlation with microroughness parameters.

Label-free and non-invasive monitoring of porcine trophoblast derived cells: differentiation in serum and serum-free media

Traditional approaches to characterize stem cell differentiation are time-consuming, lengthy and invasive. Here, Raman microspectroscopy (RM) and atomic force microscopy (AFM) - both considered as non-invasive techniques - are applied to detect the biochemical and biophysical properties of trophoblast derived stem-like cells incubated up to 10 days under conditions designed to induce differentiation. Significant biochemical and biophysical differences between control cells and differentiated cells were observed. Quantitative real time PCR was also applied to analyze gene expression. The relationship between cell differentiation and associated cellular biochemical and biomechanical changes were discussed. Monitoring trophoblast cells differentiation.

Single-bacterium nanomechanics in biomedicine: unravelling the dynamics of bacterial cells

The use of the atomic force microscope (AFM) in microbiology has progressed significantly throughout the years since its first application as a high-resolution imaging instrument. Modern AFM setups are capable of characterizing the nanomechanical behaviour of bacterial cells at both the cellular and molecular levels, where elastic properties and adhesion forces of single bacterium cells can be examined under different experimental conditions. Considering that bacterial and biofilm-mediated infections continue to challenge the biomedical field, it is important to understand the biophysical events leading towards bacterial adhesion and colonization on both biological and non-biological substrates. The purpose of this review is to present the latest findings concerning the field of single-bacterium nanomechanics, and discuss future trends and applications of nanoindentation and single-cell force spectroscopy techniques in biomedicine.

Atomic force microscopy in biomaterials surface science

Recent progress in surface science, nanotechnology and biophysics has cast new light on the correlation between the physicochemical properties of biomaterials and the resulting biological response. One experimental tool that promises to generate an increasingly more sophisticated knowledge of how proteins, cells and bacteria interact with nanostructured surfaces is the atomic force microscope (AFM). This unique instrument permits to close in on interfacial events at the scale at which they occur, the nanoscale. This perspective covers recent developments in the exploitation of the AFM, and suggests insights on future opportunities that can arise from the exploitation of this powerful technique.

A concise review of nanoscopic aspects of bioleaching bacteria-mineral interactions

Bioleaching is a technology for the recovery of metals from minerals by means of microorganisms, which accelerate the oxidative dissolution of the mineral by regenerating ferric ions. Bioleaching processes take place at the interface of bacteria, sulfide mineral and leaching solution. The fundamental forces between a bioleaching bacterium and mineral surface are central to understanding the intricacies of interfacial phenomena, such as bacterial adhesion or detachment from minerals and the mineral dissolution. This review focuses on the current state of knowledge in the colloidal aspect of bacteria-mineral interactions, particularly for bioleaching bacteria. Special consideration is given to the microscopic structure of bacterial cells and the atomic force microscopy technique used in the quantification of fundamental interaction forces at nanoscale.

Thermodynamic prediction of growth temperature dependence in the adhesion of Pseudomonas aeruginosa and Staphylococcus aureus to stainless steel and polycarbonate

This study investigated the effect of growth temperature changes (20, 30, and 37°C) on the adhesion behavior of Pseudomonas aeruginosa and Staphylococcus aureus to stainless steel and polycarbonate. Adhesion assays were performed under static conditions at 20°C. In addition, the validity of the thermodynamic and extended Derjaguin, Landau, Verwey, and Overbeek theories as predictive tools of bacterial adhesion were studied. The surface properties of the bacterial cells and the substrates of attachment were characterized, and atomic force microscopy was used to analyze the surface topography. The results indicated that the highest adhesion rate of P. aeruginosa and S. aureus on both surfaces was observed when the cells were grown at 37°C. The bacterial adhesion to stainless steel was found to be two times higher than to polycarbonate for both bacteria, whatever the condition used. The present study underlined that the thermodynamic and the extended Derjaguin, Landau, Verwey, and Overbeek theories were able to partially predict the empirical results of P. aeruginosa adhesion. However, these theories failed to predict the adhesion behavior of S. aureus to both surfaces when the growth temperature was changed. The results of the microbial adhesion to solvent indicated that the adhesion rate to abiotic surfaces may correlate with the hydrophobicity of bacterial surfaces. The effect of surface topography on bacterial adhesion showed that surface roughness, even on the very low nanometer scale, has a significant effect on bacterial adhesion behavior.

Modification of the surfaces of medical devices to prevent microbial adhesion and biofilm formation

BACKGROUND

The development of devices with surfaces that have an effect against microbial adhesion or viability is a promising approach to the prevention of device-related infections.

AIM

To review the strategies used to design devices with surfaces able to limit microbial adhesion and/or growth.

METHODS

A PubMed search of the published literature.

FINDINGS

One strategy is to design medical devices with a biocidal agent. Biocides can be incorporated into the materials or coated or covalently bonded, resulting either in release of the biocide or in contact killing without release of the biocide. The use of biocides in medical devices is debated because of the risk of bacterial resistance and potential toxicity. Another strategy is to modify the chemical or physical surface properties of the materials to prevent microbial adhesion, a complex phenomenon that also depends directly on microbial biological structure and the environment. Anti-adhesive chemical surface modifications mostly target the hydrophobicity features of the materials. Topographical modifications are focused on roughness and nanostructures, whose size and spatial organization are controlled. The most effective physical parameters to reduce bacterial adhesion remain to be determined and could depend on shape and other bacterial characteristics.

CONCLUSIONS

A prevention strategy based on reducing microbial attachment rather than on releasing a biocide is promising. Evidence of the clinical efficacy of these surface-modified devices is lacking. Additional studies are needed to determine which physical features have the greatest potential for reducing adhesion and to assess the usefulness of antimicrobial coatings other than antibiotics.

Reduced adhesion of oral bacteria on hydroxyapatite by fluoride treatment

The mechanisms of action of fluoride have been discussed controversially for decades. The cavity-preventive effect for teeth is often traced back to effects on demineralization. However, an effect on bacterial adhesion was indicated by indirect macroscopic studies. To characterize adhesion on fluoridated samples on a single bacterial level, we used force spectroscopy with bacterial probes to measure adhesion forces directly. We tested the adhesion of Streptococcus mutans , Streptococcus oralis , and Staphylococcus carnosus on smooth, high-density hydroxyapatite surfaces, pristine and after treatment with fluoride solution. All bacteria species exhibit lower adhesion forces after fluoride treatment of the surfaces. These findings suggest that the decrease of adhesion properties is a further key factor for the cariostatic effect of fluoride besides the decrease of demineralization.

In-situ determination of the mechanical properties of gliding or non-motile bacteria by atomic force microscopy under physiological conditions without immobilization

We present a study about AFM imaging of living, moving or self-immobilized bacteria in their genuine physiological liquid medium. No external immobilization protocol, neither chemical nor mechanical, was needed. For the first time, the native gliding movements of Gram-negative Nostoc cyanobacteria upon the surface, at speeds up to 900 µm/h, were studied by AFM. This was possible thanks to an improved combination of a gentle sample preparation process and an AFM procedure based on fast and complete force-distance curves made at every pixel, drastically reducing lateral forces. No limitation in spatial resolution or imaging rate was detected. Gram-positive and non-motile Rhodococcus wratislaviensis bacteria were studied as well. From the approach curves, Young modulus and turgor pressure were measured for both strains at different gliding speeds and are ranging from 20±3 to 105±5 MPa and 40±5 to 310±30 kPa depending on the bacterium and the gliding speed. For Nostoc, spatially limited zones with higher values of stiffness were observed. The related spatial period is much higher than the mean length of Nostoc nodules. This was explained by an inhomogeneous mechanical activation of nodules in the cyanobacterium. We also observed the presence of a soft extra cellular matrix (ECM) around the Nostoc bacterium. Both strains left a track of polymeric slime with variable thicknesses. For Rhodococcus, it is equal to few hundreds of nanometers, likely to promote its adhesion to the sample. While gliding, the Nostoc secretes a slime layer the thickness of which is in the nanometer range and increases with the gliding speed. This result reinforces the hypothesis of a propulsion mechanism based, for Nostoc cyanobacteria, on ejection of slime. These results open a large window on new studies of both dynamical phenomena of practical and fundamental interests such as the formation of biofilms and dynamic properties of bacteria in real physiological conditions.

Bacterial cell surface deformation under external loading

Viscoelastic deformation of the contact volume between adhering bacteria and substratum surfaces plays a role in their adhesion and detachment. Currently, there are no deformation models that account for the heterogeneous structure and composition of bacteria, consisting of a relatively soft outer layer and a more rigid, hard core enveloped by a cross-linked peptidoglycan layer. The aim of this paper is to present a new, simple model to derive the reduced Young’s modulus of the contact volume between adhering bacteria and substratum surfaces based on the relationship between deformation and applied external loading force, measured using atomic force microscopy. The model assumes that contact is established through a cylinder with constant volume and does not require assumptions on the properties and dimensions of the contact cylinder. The reduced Young’s moduli obtained (8 to 47 kPa) and dimensions of the contact cylinders could be interpreted on the basis of the cell surface features and cell wall characteristics, i.e., surfaces that are more rigid (because of either less fibrillation, less extracellular polymeric substance production, or a higher degree of cross-linking of the peptidoglycan layer) had shorter contact cylinders and higher reduced Young’s moduli. Application of an existing Hertz model to our experimental data yielded reduced Young’s moduli that were up to 100 times higher for all strains investigated, likely because the Hertz model pertains to a major extent to the more rigid peptidoglycan layer and not only to the soft outer bacterial cell surface, involved in the bond between a bacterium and a substratum surface.

The viscoelastic properties of the bond between an adhering bacterium and a substratum surface play a role in determining bacterial detachment. For instance, removal of an oral biofilm proceeds according to a viscoelastic failure model, and biofilm left behind after toothbrushing has been found to possess expanded bond lengths between adhering bacteria due to viscoelastic deformation. Current elastic deformation models are unable to distinguish between the soft outer bacterial cell surface and the hard core of a bacterium, enveloped by a peptidoglycan layer. Therefore, here we present a simple model to calculate the Young’s modulus and deformation of the contact volume between an adhering bacterium and a substratum surface that accounts for the heterogeneous structure of a bacterium.

Surface topographical factors influencing bacterial attachment

Substratum surface roughness is known to be one of the key factors in determining the extent of bacterial colonization. Understanding the way by which the substratum topography, especially at the nanoscale, mediates bacterial attachment remains ambiguous at best, despite the volume of work available on the topic. This is because the vast majority of bacterial attachment studies do not perform comprehensive topographical characterization analyses, and typically consider roughness parameters that describe only one aspect of the surface topography. The most commonly reported surface roughness parameters are average and root mean square (RMS) roughness (R(a) and R(q) respectively), which are both measures of the typical height variation of the surface. They offer no insights into the spatial distribution or shape of the surface features. Here, a brief overview of the current state of research on topography-mediated bacterial adhesion is presented, as well as an outline of the suite of roughness characterization parameters that are available for the comprehensive description of the surface architecture of a substratum. Finally, a set of topographical parameters is proposed as a new standard for surface roughness characterization in bacterial adhesion studies to improve the likelihood of identifying direct relationships between substratum topography and the extent of bacterial adhesion.

Nanoscale analysis of the effects of antibiotics and CX1 on a Pseudomonas aeruginosa multidrug-resistant strain

Drug resistance is a challenge that can be addressed using nanotechnology. We focused on the resistance of the bacteria Pseudomonas aeruginosa and investigated, using Atomic Force Microscopy (AFM), the behavior of a reference strain and of a multidrug resistant clinical strain, submitted to two antibiotics and to an innovative antibacterial drug (CX1). We measured the morphology, surface roughness and elasticity of the bacteria under physiological conditions and exposed to the antibacterial molecules. To go further in the molecules action mechanism, we explored the bacterial cell wall nanoscale organization using functionalized AFM tips. We have demonstrated that affected cells have a molecularly disorganized cell wall; surprisingly long molecules being pulled off from the cell wall by a lectin probe. Finally, we have elucidated the mechanism of action of CX1: it destroys the outer membrane of the bacteria as demonstrated by the results on artificial phospholipidic membranes and on the resistant strain.

Roughness parameters for standard description of surface nanoarchitecture

The nanoarchitecture and surface roughness of metallic thin films prepared by magnetron sputtering were analyzed to determine the topographical statistics that give the optimum description of their nanoarchitechture. Nanoscale topographical profiles were generated by performing atomic force microscopy (AFM) scans of 1 μm × 1 μm areas of titanium and silver films of three different thicknesses (3 nm, 12 nm, and 150 nm). Of the titanium films, the 150-nm film had the highest average roughness (R(a) = 2.63 nm), more than four times that of the 3-nm and 12-nm titanium films. When silver films were coated on top of 150-nm titanium films, the average roughness increased further; the 3-nm (R(a) = 4.96 nm) and 150-nm (R(a) = 4.65 nm) silver films average roughnesses were approximately twice that of the 150-nm titanium film. For topographical analysis, seven statistical parameters were calculated. These parameters included commonly used roughness measurements, as well as some less commonly used measurements, in order to determine which combination of parameters gave the best overall description of the nanoarchitecture of the films presented. Skewness (R(skw)), surface area increase (R(sa)), and peak counts (R(pc)) provided the best description of horizontal surface dimensions, and in conjunction with vertical descriptors R(a) and R(q) gave the best characterization of surface architecture. The five roughness parameters R(a), R(q), R(skw), R(sa), and R(pc) are proposed as a new standard for describing surface nanoarchitecture.

Glyphosate-induced stiffening of HaCaT keratinocytes, a Peak Force Tapping study on living cells

The skin is the first physiological barrier, with a complex constitution, that provides defensive functions against multiple physical and chemical aggressions. Glyphosate is an extensively used herbicide that has been shown to increase the risk of cancer. Moreover there is increasing evidence suggesting that the mechanical phenotype plays an important role in malignant transformation. Atomic force microscopy (AFM) has emerged within the last decade as a powerful tool for providing a nanometer-scale resolution imaging of biological samples. Peak Force Tapping (PFT) is a newly released AFM-based investigation technique allowing extraction of chemical and mechanical properties from a wide range of samples at a relatively high speed and a high resolution. The present work uses the PFT technology to investigate HaCaT keratinocytes, a human epidermal cell line, and offers an original approach to study chemically-induced changes in the cellular mechanical properties under near-physiological conditions. These experiments indicate glyphosate induces cell membrane stiffening, and the appearance of cytoskeleton structures at a subcellular level, for low cytotoxic concentrations whereas cells exposed to IC50 (inhibitory concentration 50%) treatment exhibit control-like mechanical behavior despite obvious membrane damages. Quercetin, a well-known antioxidant, reverses the glyphosate-induced mechanical phenotype.

Assembly of live micro-organisms on microstructured PDMS stamps by convective/capillary deposition for AFM bio-experiments

Immobilization of live micro-organisms on solid substrates is an important prerequisite for atomic force microscopy (AFM) bio-experiments. The method employed must immobilize the cells firmly enough to enable them to withstand the lateral friction forces exerted by the tip during scanning but without denaturing the cell interface. In this work, a generic method for the assembly of living cells on specific areas of substrates is proposed. It consists in assembling the living cells within the patterns of microstructured, functionalized poly-dimethylsiloxane (PDMS) stamps using convective/capillary deposition. This versatile approach is validated by applying it to two systems of foremost importance in biotechnology and medicine: Saccharomyces cerevisiae yeasts and Aspergillus fumigatus fungal spores. We show that this method allows multiplexing AFM nanomechanical measurements by force spectroscopy on S. cerevisiae yeasts and high-resolution AFM imaging of germinated Aspergillus conidia in buffer medium. These two examples clearly demonstrate the immense potential of micro-organism assembly on functionalized, microstructured PDMS stamps by convective/capillary deposition for performing rigorous AFM bio-experiments on living cells.

Atomic force microscopy of a ctpA mutant in Rhizobium leguminosarum reveals surface defects linking CtpA function to biofilm formation

Atomic force microscopy was used to investigate the surface ultrastructure, adhesive properties and biofilm formation of Rhizobium leguminosarum and a ctpA mutant strain. The surface ultrastructure of wild-type R. leguminosarum consists of tightly packed surface subunits, whereas the ctpA mutant has much larger subunits with loose lateral packing. The ctpA mutant strain is not capable of developing fully mature biofilms, consistent with its altered surface ultrastructure, greater roughness and stronger adhesion to hydrophilic surfaces. For both strains, surface roughness and adhesive forces increased as a function of calcium ion concentration, and for each, biofilms were thicker at higher calcium concentrations.

Statistical analysis of long- and short-range forces involved in bacterial adhesion to substratum surfaces as measured using atomic force microscopy

Surface thermodynamic analyses of microbial adhesion using measured contact angles on solid substrata and microbial cell surfaces are widely employed to determine the nature of the adhesion forces, i.e., the interplay between Lifshitz-van der Waals and acid-base forces. While surface thermodynamic analyses are often viewed critically, atomic force microscopy (AFM) can also provide information on the nature of the adhesion forces by means of Poisson analysis of the measured forces. This review first presents a description of Poisson analysis and its underlying assumptions. The data available from the literature for different combinations of bacterial strains and substrata are then summarized, leading to the conclusion that bacterial adhesion to surfaces is generally dominated by short-range, attractive acid-base interactions, in combination with long-range, weaker Lifshitz-van der Waals forces. This is in line with the findings of surface thermodynamic analyses of bacterial adhesion. Comparison with single-molecule ligand-receptor forces from the literature suggests that the short-range-force contribution from Poisson analysis involves a discrete adhesive bacterial cell surface site rather than a single molecular force. The adhesion force arising from these cell surface sites and the number of sites available may differ from strain to strain. Force spectroscopy, however, involves the tedious task of identifying the minor peaks in the AFM retraction force-distance curve. This step can be avoided by carrying out Poisson analysis on the work of adhesion, which can also be derived from retraction force-distance curves. This newly proposed way of performing Poisson analysis confirms that multiple molecular bonds, rather than a single molecular bond, contribute to a discrete adhesive bacterial cell surface site.