Nanoscale mapping and functional analysis of individual adhesins on living bacteria

NRP1 is a receptor for mammalian orthoreovirus engaged by distinct capsid subunits

Mammalian orthoreovirus (reovirus) is a nonenveloped virus that establishes primary infection in the intestine and disseminates to sites of secondary infection, including the CNS. Reovirus entry involves multiple engagement factors, but how the virus disseminates systemically and targets neurons remains unclear. In this study, we identified murine neuropilin 1 (mNRP1) as a receptor for reovirus. mNRP1 binds reovirus with nanomolar affinity using a unique mechanism of virus-receptor interaction, which is coordinated by multiple interactions between distinct reovirus capsid subunits and multiple NRP1 extracellular domains. By exchanging essential capsid protein-encoding gene segments, we determined that the multivalent interaction is mediated by outer-capsid protein σ3 and capsid turret protein λ2. Using capsid mutants incapable of binding NRP1, we found that NRP1 contributes to reovirus dissemination and neurovirulence in mice. Collectively, our results demonstrate that NRP1 is an entry receptor for reovirus and uncover mechanisms by which NRPs promote viral entry and pathogenesis.

Deciphering molecular mechanisms stabilizing the reovirus-binding complex

Mammalian orthoreoviruses (reoviruses) serve as potential triggers of celiac disease and have oncolytic properties, making these viruses potential cancer therapeutics. Primary attachment of reovirus to host cells is mainly mediated by the trimeric viral protein, σ1, which engages cell-surface glycans, followed by high-affinity binding to junctional adhesion molecule-A (JAM-A). This multistep process is thought to be accompanied by major conformational changes in σ1, but direct evidence is lacking. By combining biophysical, molecular, and simulation approaches, we define how viral capsid protein mechanics influence virus-binding capacity and infectivity. Single-virus force spectroscopy experiments corroborated by in silico simulations show that GM2 increases the affinity of σ1 for JAM-A by providing a more stable contact interface. We demonstrate that conformational changes in σ1 that lead to an extended rigid conformation also significantly increase avidity for JAM-A. Although its associated lower flexibility impairs multivalent cell attachment, our findings suggest that diminished σ1 flexibility enhances infectivity, indicating that fine-tuning of σ1 conformational changes is required to successfully initiate infection. Understanding properties underlying the nanomechanics of viral attachment proteins offers perspectives in the development of antiviral drugs and improved oncolytic vectors.

Nanoscale clustering of mycobacterial ligands and DC-SIGN host receptors are key determinants for pathogen recognition

The bacterial pathogen Mycobacterium tuberculosis binds to the C-type lectin DC-SIGN (dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin) on dendritic cells to evade the immune system. While DC-SIGN glycoconjugate ligands are ubiquitous among mycobacterial species, the receptor selectively binds pathogenic species from the M. tuberculosis complex (MTBC). Here, we unravel the molecular mechanism behind this intriguing selective recognition by means of a multidisciplinary approach combining single-molecule atomic force microscopy with Förster resonance energy transfer and bioassays. Molecular recognition imaging of mycobacteria demonstrates that the distribution of DC-SIGN ligands markedly differs between Mycobacterium bovis Bacille Calmette-Guérin (BCG) (model MTBC species) and Mycobacterium smegmatis (non-MTBC species), the ligands being concentrated into dense nanodomains on M. bovis BCG. Upon bacteria-host cell adhesion, ligand nanodomains induce the recruitment and clustering of DC-SIGN. Our study highlights the key role of clustering of both ligands on MTBC species and DC-SIGN host receptors in pathogen recognition, a mechanism that might be widespread in host-pathogen interactions.

In Situ Probing of the Intrinsic Adhesion Strength of Single Anaerobic Microbial Cells

Probing the single-cell mechanobiology in situ is imperative for microbial processes in the medical, industrial, and agricultural realms, but it remains a challenge. Herein, we present a single-cell force microscopy method that can be used to measure microbial adhesion strength under anaerobic conditions in situ. This method integrates atomic force microscopy with an anaerobic liquid cell and inverted fluorescence microscopy. We obtained the nanomechanical measurements of the single anaerobic bacterium Ethanoligenens harbinense YUAN-3 and the methanogenic archaeon Methanosarcina acetivorans C2A and their nanoscale adhesion forces in the presence of sulfoxaflor, a successor of neonicotinoid pesticides. This study presents a new tool for in situ single-cell force measurements of various anoxic and anaerobic species and provides new perspectives for evaluating the potential environmental risk of neonicotinoid applications in ecosystems.

Contact Area and Deformation of Escherichia coli Cells Adhered on a Cationic Surface

When bacteria adhere to surfaces, the chemical and mechanical character of the cell-substrate interface guides cell function and the development of microcolonies and biofilms. Alternately on bactericidal surfaces, intimate contact is critical to biofilm prevention. The direct study of the buried cell-substrate interfaces at the heart of these behaviors is hindered by the small bacterial cell size and inaccessibility of the contact region. Here, we present a total internal reflectance fluorescence depletion approach to measure the size of the cell-substrate contact region and quantify the gap separation and curvature near the contact zone, providing an assessment of the shapes of the near-surface undersides of adhered bacterial cells. Resolution of the gap height is about 10%, down to a few nanometers at contact. Using 1 and 2 μm silica spheres as calibration standards we report that, for flagella-free Escherichia coli (E. coli) adhering on a cationic poly-l-lysine layer, the cell-surface contact and apparent cell deformation vary with adsorbed cell configuration. Most cells adhere by their ends, achieving small contact areas of 0.15 μm2, corresponding to about 1-2% of the cell's surface. The altered Gaussian curvatures of end-adhered cells suggest the flattening of the envelope within the small contact region. When cells adhere by their sides, the contact area is larger, in the range 0.3-1.1 μm2 and comprising up to ∼12% of the cell's total surface. A region of sharper curvature, greater than that of the cells' original spherocylindrical shape, borders the flat contact region in cases of side-on or end-on cell adhesion, suggesting envelope stress. From the measured curvatures, precise stress distributions over the cell surface could be calculated in future studies that incorporate knowledge of envelope moduli. Overall the small contact areas of end-adhered cells may be a limiting factor for antimicrobial surfaces that kill on contact rather than releasing bactericide.

Molecular Origins of Force-Dependent Protein Complex Stabilization during Bacterial Infections

The unbinding pathway of a protein complex can vary significantly depending on biochemical and mechanical factors. Under mechanical stress, a complex may dissociate through a mechanism different from that used in simple thermal dissociation, leading to different dissociation rates under shear force and thermal dissociation. This is a well-known phenomenon studied in biomechanics whose molecular and atomic details are still elusive. A particularly interesting case is the complex formed by bacterial adhesins with their human peptide target. These protein interactions have a force resilience equivalent to those of covalent bonds, an order of magnitude stronger than the widely used streptavidin:biotin complex, while having an ordinary affinity, much lower than that of streptavidin:biotin. Here, in an in silico single-molecule force spectroscopy approach, we use molecular dynamics simulations to investigate the dissociation mechanism of adhesin/peptide complexes. We show how the Staphylococcus epidermidis adhesin SdrG uses a catch-bond mechanism to increase complex stability with increasing mechanical stress. While allowing for thermal dissociation in a low-force regime, an entirely different mechanical dissociation path emerges in a high-force regime, revealing an intricate mechanism that does not depend on the peptide's amino acid sequence. Using a dynamic network analysis approach, we identified key amino acid contacts that describe the mechanics of this complex, revealing differences in dynamics that hinder thermal dissociation and establish the mechanical dissociation path. We then validate the information content of the selected amino acid contacts using their dynamics to successfully predict the rupture forces for this complex through a machine learning model.

High-Yield Characterization of Single Molecule Interactions with DeepTipTM Atomic Force Microscopy Probes

Single molecule interactions between biotin and streptavidin were characterized with functionalized DeepTip^TM probes and used as a model system to develop a comprehensive methodology for the high-yield identification and analysis of single molecular events. The procedure comprises the covalent binding of the target molecule to a surface and of the sensing molecule to the DeepTip^TM probe, so that the interaction between both chemical species can be characterized by obtaining force-displacement curves in an atomic force microscope. It is shown that molecular resolution is consistently attained with a percentage of successful events higher than 90% of the total number of recorded curves, and a very low level of unspecific interactions. The combination of both features is a clear indication of the robustness and versatility of the proposed methodology.

Interaction of the Staphylococcus aureus Surface Protein FnBPB with Corneodesmosin Involves Two Distinct, Extremely Strong Bonds

Attachment of Staphylococcus aureus to human skin corneocyte cells plays a critical role in exacerbating the severity of atopic dermatitis (AD). Pathogen-skin adhesion is mediated by bacterial cell-surface proteins called adhesins, including fibronectin-binding protein B (FnBPB). FnBPB binds to corneodesmosin (CDSN), a glycoprotein exposed on AD patient corneocytes. Using single-molecule experiments, we demonstrate that CDSN binding by FnBPB relies on a sophisticated two-site mechanism. Both sites form extremely strong bonds with binding forces of ∼1 and ∼2.5 nN albeit with faster dissociation rates than those reported for homologues of the adhesin. This previously unidentified two-binding site interaction in FnBPB illustrates its remarkable variety of adhesive functions and is of biological significance as the high strength and short bond lifetime will favor efficient skin colonization by the pathogen.

Crack patterns of drying dense bacterial suspensions

Drying of bacterial suspensions is frequently encountered in a plethora of natural and engineering processes. However, the evaporation-driven mechanical instabilities of dense consolidating bacterial suspensions have not been explored heretofore. Here, we report the formation of two different crack patterns of drying suspensions of Escherichia coli (E. coli) with distinct motile behaviors. Circular cracks are observed for wild-type E. coli with active swimming, whereas spiral-like cracks form for immotile bacteria. Using the elastic fracture mechanics and the poroelastic theory, we show that the formation of the circular cracks is determined by the tensile nature of the radial drying stress once the cracks are initiated by the local order structure of bacteria due to their collective swimming. Our study demonstrates the link between the microscopic swimming behaviors of individual bacteria and the mechanical instabilities and macroscopic pattern formation of drying bacterial films. The results shed light on the dynamics of active matter in a drying process and provide useful information for understanding various biological processes associated with drying bacterial suspensions.

Atomic force microscopy applied to interrogate nanoscale cellular chemistry and supramolecular bond dynamics for biomedical applications

Understanding biological interactions at a molecular level grants valuable information relevant to improving medical treatments and outcomes. Among the suite of technologies available, Atomic Force Microscopy (AFM) is unique in its ability to quantitatively probe forces and receptor-ligand interactions in real-time. The ability to assess the formation of supramolecular bonds and intermediates in real-time on surfaces and living cells generates important information relevant to understanding biological phenomena. Combining AFM with fluorescence-based techniques allows for an unprecedented level of insight not only concerning the formation and rupture of bonds, but understanding medically relevant interactions at a molecular level. As the ability of AFM to probe cells and more complex models improves, being able to assess binding kinetics, chemical topographies, and garner spectroscopic information will likely become key to developing further improvements in fields such as cancer, nanomaterials, and virology. The rapid response to the COVID-19 crisis, producing information regarding not just receptor affinities, but also strain-dependent efficacy of neutralizing nanobodies, demonstrates just how viable and integral to the pre-clinical development of information AFM techniques are in this era of medicine.

Advances in the Biological Application of Force-Induced Remnant Magnetization Spectroscopy

Biomolecules participate in various physiological and pathological processes through intermolecular interactions generally driven by non-covalent forces. In the present review, the force-induced remnant magnetization spectroscopy (FIRMS) is described and illustrated as a novel method to measure non-covalent forces. During the FIRMS measurement, the molecular magnetic probes are magnetized to produce an overall magnetization signal. The dissociation under the interference of external force yields a decrease in the magnetic signal, which is recorded and collected by atomic magnetometer in a spectrum to study the biological interactions. Furthermore, the recent FIRMS development with various external mechanical forces and magnetic probes is summarized.

Spatiotemporal Tracing of the Cellular Internalization Process of Rod-Shaped Nanostructures

Endocytosis, as one of the main ways for nanostructures enter cells, is affected by several aspects, and shape is an especially critical aspect during the endocytosis of nanostructures. However, it has remained challenging to capture the dynamic internalization behaviors of rod-shaped nanostructures while also probing the mechanical aspects of the internalization. Here, using the atomic force microscopy-based force tracing technique, transmission electron microscopy, and molecular dynamic simulation, we mapped the detailed internalization behaviors of rod-shaped nanostructures with different aspect ratios at the single-particle level. We found that the gold nanorod is endocytosed in a noncontinuous and force-rebound rotation manner, herein named "intermittent rotation". The force tracing test indicated that the internalization force (∼81 pN, ∼108 pN, and ∼157 pN) and time (∼0.56 s, ∼0.66 s, and ∼1.14 s for a 12.10 nm × 11.96 nm gold nanosphere and 26.15 nm × 13.05 nm and 48.71 nm × 12.45 nm gold nanorods, respectively) are positively correlated with the aspect ratios. However, internalization speed is negatively correlated with internalization time, irrespective of the aspect ratio. Further, the energy analysis suggested that intermittent rotation from the horizontal to vertical direction can reduce energy dissipation during the internalization process. Thus, to overcome the energy barrier of internalization, the number and angle of rotation increases with aspect ratios. Our findings provide critical missing evidence of rod-shaped nanostructure's internalization, which is essential for fundamentally understanding the internalization mechanism in living cells.

Mycobacterial Adhesion: From Hydrophobic to Receptor-Ligand Interactions

Adhesion is crucial for the infective lifestyles of bacterial pathogens. Adhesion to non-living surfaces, other microbial cells, and components of the biofilm extracellular matrix are crucial for biofilm formation and integrity, plus adherence to host factors constitutes a first step leading to an infection. Adhesion is, therefore, at the core of pathogens’ ability to contaminate, transmit, establish residency within a host, and cause an infection. Several mycobacterial species cause diseases in humans and animals with diverse clinical manifestations. Mycobacterium tuberculosis, which enters through the respiratory tract, first adheres to alveolar macrophages and epithelial cells leading up to transmigration across the alveolar epithelium and containment within granulomas. Later, when dissemination occurs, the bacilli need to adhere to extracellular matrix components to infect extrapulmonary sites. Mycobacteria causing zoonotic infections and emerging nontuberculous mycobacterial pathogens follow divergent routes of infection that probably require adapted adhesion mechanisms. New evidence also points to the occurrence of mycobacterial biofilms during infection, emphasizing a need to better understand the adhesive factors required for their formation. Herein, we review the literature on tuberculous and nontuberculous mycobacterial adhesion to living and non-living surfaces, to themselves, to host cells, and to components of the extracellular matrix.

The biophysics of bacterial infections: Adhesion events in the light of force spectroscopy

Bacterial infections are the most eminent public health challenge of the 21st century. The primary step leading to infection is bacterial adhesion to the surface of host cells or medical devices, which is mediated by a multitude of molecular interactions. At the interface of life sciences and physics, last years advances in atomic force microscopy (AFM)-based force spectroscopy techniques have made possible to measure the forces driving bacteria-cell and bacteria-materials interactions on a single molecule/cell basis (single molecule/cell force spectroscopy). Among the bacteria-(bio)materials surface interactions, the life-threatening infections associated to medical devices involving Staphylococcus aureus and Escherichia coli are the most eminent. On the other hand, Pseudomonas aeruginosa binding to the pulmonary and urinary tract or the Helicobacter pylori binding to the gastric mucosa, are classical examples of bacteria-host cell interactions that end in serious infections. As we approach the end of the antibiotic era, acquisition of a deeper knowledge of the fundamental forces involved in bacteria - host cells/(bio)materials surface adhesion is crucial for the identification of new ligand-binding events and its assessment as novel targets for alternative anti-infective therapies. This article aims to highlight the potential of AFM-based force spectroscopy for new targeted therapies development against bacterial infections in which adhesion plays a pivotal role and does not aim to be an extensive overview on the AFM technical capabilities and theory of single molecule force spectroscopy.

Dual-pulse photoactivated atomic force microscopy

Photoactivated atomic force microscopy (pAFM), which integrates light excitation and mechanical detection of the deflections of a cantilever tip, has become a widely used tool for probing nanoscale structures. Raising the illuminating laser power is an obvious way to boost the signal-to-noise ratio of pAFM, but strong laser power can damage both the sample and cantilever tip. Here, we demonstrate a dual-pulse pAFM (DP-pAFM) that avoids this problem by using two laser pulses with a time delay. The first laser heats the light absorber and alters the local Grüneisen parameter value, and the second laser boosts the mechanical vibration within the thermal relaxation time. Using this technique, we successfully mapped the optical structures of small-molecule semiconductor films. Of particular interest, DP-pAFM clearly visualized nanoscale cracks in organic semiconductor films, which create crucial problems for small-molecule semiconductors. DP-pAFM opens a promising new optical avenue for studying complex nanoscale phenomena in various research fields.

Seeing and Touching the Mycomembrane at the Nanoscale

Mycobacteria have unique cell envelopes, surface properties, and growth dynamics, which all play a part in the ability of these important pathogens to infect, evade host immunity, disseminate, and resist antibiotic challenges. Recent atomic force microscopy (AFM) studies have brought new insights into the nanometer-scale ultrastructural, adhesive, and mechanical properties of mycobacteria. The molecular forces with which mycobacterial adhesins bind to host factors, like heparin and fibronectin, and the hydrophobic properties of the mycomembrane have been unraveled by AFM force spectroscopy studies. Real-time correlative AFM and fluorescence imaging have delineated a complex interplay between surface ultrastructure, tensile stresses within the cell envelope, and cellular processes leading to division. The unique capabilities of AFM, which include subdiffraction-limit topographic imaging and piconewton force sensitivity, have great potential to resolve important questions that remain unanswered on the molecular interactions, surface properties, and growth dynamics of this important class of pathogens.

AFM in cellular and molecular microbiology

The unique capabilities of the atomic force microscope (AFM), including super-resolution imaging, piconewton force-sensitivity, nanomanipulation and ability to work under physiological conditions, have offered exciting avenues for cellular and molecular biology research. AFM imaging has helped unravel the fine architectures of microbial cell envelopes at the nanoscale, and how these are altered by antimicrobial treatment. Nanomechanical measurements have shed new light on the elasticity, tensile strength and turgor pressure of single cells. Single-molecule and single-cell force spectroscopy experiments have revealed the forces and dynamics of receptor-ligand interactions, the nanoscale distribution of receptors on the cell surface and the elasticity and adhesiveness of bacterial pili. Importantly, recent force spectroscopy studies have demonstrated that extremely stable bonds are formed between bacterial adhesins and their cognate ligands, originating from a catch bond behaviour allowing the pathogen to reinforce adhesion under shear or tensile stress. Here, we survey how the versatility of AFM has enabled addressing crucial questions in microbiology, with emphasis on bacterial pathogens. TAKE AWAYS: AFM topographic imaging unravels the ultrastructure of bacterial envelopes. Nanomechanical mapping shows what makes cell envelopes stiff and resistant to drugs. Force spectroscopy characterises the molecular forces in pathogen adhesion. Stretching pili reveals a wealth of mechanical and adhesive responses.

Nanomotion Detection-Based Rapid Antibiotic Susceptibility Testing

Rapid antibiotic susceptibility testing (AST) could play a major role in fighting multidrug-resistant bacteria. Recently, it was discovered that all living organisms oscillate in the range of nanometers and that these oscillations, referred to as nanomotion, stop as soon the organism dies. This finding led to the development of rapid AST techniques based on the monitoring of these oscillations upon exposure to antibiotics. In this review, we explain the working principle of this novel technique, compare the method with current ASTs, explore its application and give some advice about its implementation. As an illustrative example, we present the application of the technique to the slowly growing and pathogenic Bordetella pertussis bacteria.

The diagnostic tools for viable but nonculturable pathogens in the food industry: Current status and future prospects

Viable but nonculturable (VBNC) microorganisms have been recognized as pathogenic contaminants in foods and environments. The failure of VBNC cells to form the visible colonies hinders the ability to use conventional media for their detection. Efficient and rapid detection of pathogens in the VBNC state is a prerequisite to ensure the food safety and public health. Despite their nonculturability, VBNC cells have distinct characteristics, such as morphology, metabolism, chemical composition, and gene and protein expression, that have been used as the basis for the development of abundant diagnostic tools. This review covers the current status and advances in various approaches for examining microorganisms in the VBNC state, including but not limited to the methodological aspects, advantages, and drawbacks of each technique. Existing methods, such as direct viable count, SYTO/PI dual staining, and propidium monoazide quantitative polymerase chain reaction (PCR), as well as some techniques with potential to be applied in the future, such as digital PCR, enhanced-surface Raman spectroscopy, and impedance-based techniques, are summarized in depth. Finally, future prospects for the one-step detection of VBNC bacteria are proposed and discussed. We believe that this review can provide more optional methods for researchers and promote the development of rapid, accurate detecting methods, and for inspectors, the diagnostic tools can provide data to undertake risk analysis of VBNC cells.

The microbial adhesive arsenal deciphered by atomic force microscopy

Microbes employ a variety of strategies to adhere to abiotic and biotic surfaces, as well as host cells. In addition to their surface physicochemical properties (e.g. charge, hydrophobic balance), microbes produce appendages (e.g. pili, fimbriae, flagella) and express adhesion proteins embedded in the cell wall or cell membrane, with adhesive domains targeting specific ligands or chemical properties. Atomic force microscopy (AFM) is perfectly suited to deciphering the adhesive properties of microbial cells. Notably, AFM imaging has revealed the cell wall topographical organization of live cells at unprecedented resolution, and AFM has a dual capability to probe adhesion at the single-cell and single-molecule levels. AFM is thus a powerful tool for unravelling the molecular mechanisms of microbial adhesion at scales ranging from individual molecular interactions to the behaviours of entire cells. In this review, we cover some of the major breakthroughs facilitated by AFM in deciphering the microbial adhesive arsenal, including the exciting development of anti-adhesive strategies.

Atomic Force Microscopy-Based Force Spectroscopy and Multiparametric Imaging of Biomolecular and Cellular Systems

During the last three decades, a series of key technological improvements turned atomic force microscopy (AFM) into a nanoscopic laboratory to directly observe and chemically characterize molecular and cell biological systems under physiological conditions. Here, we review key technological improvements that have established AFM as an analytical tool to observe and quantify native biological systems from the micro- to the nanoscale. Native biological systems include living tissues, cells, and cellular components such as single or complexed proteins, nucleic acids, lipids, or sugars. We showcase the procedures to customize nanoscopic chemical laboratories by functionalizing AFM tips and outline the advantages and limitations in applying different AFM modes to chemically image, sense, and manipulate biosystems at (sub)nanometer spatial and millisecond temporal resolution. We further discuss theoretical approaches to extract the kinetic and thermodynamic parameters of specific biomolecular interactions detected by AFM for single bonds and extend the discussion to multiple bonds. Finally, we highlight the potential of combining AFM with optical microscopy and spectroscopy to address the full complexity of biological systems and to tackle fundamental challenges in life sciences.

Fast chemical force microscopy demonstrates that glycopeptidolipids define nanodomains of varying hydrophobicity on mycobacteria

Mycobacterium abscessus is an emerging multidrug-resistant bacterial pathogen causing severe lung infections in cystic fibrosis patients. A remarkable trait of this mycobacterial species is its ability to form morphologically smooth (S) and rough (R) colonies. The S-to-R transition is caused by the loss of glycopeptidolipids (GPLs) in the outer layer of the cell envelope and correlates with an increase in cording and virulence. Despite the physiological and medical importance of this morphological transition, whether it involves changes in cell surface properties remains unknown. Herein, we combine recently developed quantitative imaging (QI) atomic force microscopy (AFM) with hydrophobic tips to quantitatively map the surface structure and hydrophobicity of M. abscessus at high spatiotemporal resolution, and to assess how these properties are modulated by the S-to-R transition and by treatment with an inhibitor of the mycolic acid transporter MmpL3. We discover that loss of GPLs leads to major modifications in surface hydrophobicity, without any apparent change in cell surface ultrastructure. While R bacilli are homogeneously hydrophobic, S bacilli feature unusual variations of nanoscale hydrophobic properties. These previously undescribed cell surface nanodomains are likely to play critical roles in bacterial adhesion, aggregation, phenotypic heterogeneity and transmission, and in turn in virulence and pathogenicity. Our study also suggests that MmpL3 inhibitors show promise in nanomedicine as chemotherapeutic agents to interfere with the highly hydrophobic nature of the mycobacterial cell wall. The advantages of QI-AFM with hydrophobic tips are the ability to map chemical and structural properties simultaneously and at high resolution, applicable to a wide range of biosystems.

How Microbes Use Force To Control Adhesion

Microbial adhesion and biofilm formation are usually studied using molecular and cellular biology assays, optical and electron microscopy, or laminar flow chamber experiments. Today, atomic force microscopy (AFM) represents a valuable addition to these approaches, enabling the measurement of forces involved in microbial adhesion at the single-molecule level. In this minireview, we discuss recent discoveries made applying state-of-the-art AFM techniques to microbial specimens in order to understand the strength and dynamics of adhesive interactions. These studies shed new light on the molecular mechanisms of adhesion and demonstrate an intimate relationship between force and function in microbial adhesins.

Next Generation Methods for Single-Molecule Force Spectroscopy on Polyproteins and Receptor-Ligand Complexes

Single-molecule force spectroscopy with the atomic force microscope provides molecular level insights into protein function, allowing researchers to reconstruct energy landscapes and understand functional mechanisms in biology. With steadily advancing methods, this technique has greatly accelerated our understanding of force transduction, mechanical deformation, and mechanostability within single- and multi-domain polyproteins, and receptor-ligand complexes. In this focused review, we summarize the state of the art in terms of methodology and highlight recent methodological improvements for AFM-SMFS experiments, including developments in surface chemistry, considerations for protein engineering, as well as theory and algorithms for data analysis. We hope that by condensing and disseminating these methods, they can assist the community in improving data yield, reliability, and throughput and thereby enhance the information that researchers can extract from such experiments. These leading edge methods for AFM-SMFS will serve as a groundwork for researchers cognizant of its current limitations who seek to improve the technique in the future for in-depth studies of molecular biomechanics.

Together We Are Stronger: Protein Clustering at the Nanoscale

Cell surface proteins are known to assemble into nano- and microscale domains in order to govern biological processes, including cell adhesion, endocytosis, and immune responses. The small size and ephemerality of these structures have made their direct observation and functional analysis challenging. In this Perspective, I discuss recent progress made in applying nanotechniques to study protein clustering, emphasizing the use of state-of-the-art single-molecule atomic force microscopy, as reported by Strasser et al. in this issue of ACS Nano.

Mechanical Forces between Mycobacterial Antigen 85 Complex and Fibronectin

Adhesion to extracellular matrix proteins is an important first step in host invasion, employed by many bacterial pathogens. In mycobacteria, the secreted Ag85 complex proteins, involved in the synthesis of the cell envelope, are known to bind to fibronectin (Fn) through molecular forces that are currently unknown. In this study, single-molecule force spectroscopy is used to study the strength, kinetics and thermodynamics of the Ag85-Fn interaction, focusing on the multidrug-resistant Mycobacterium abscessus species. Single Ag85 proteins bind Fn with a strength of ~75 pN under moderate tensile loading, which compares well with the forces reported for other Fn-binding proteins. The binding specificity is demonstrated by using free Ag85 and Fn peptides with active binding sequences. The Ag85-Fn rupture force increases with mechanical stress (i.e., loading rate) according to the Friddle–Noy–de Yoreo theory. From this model, we extract thermodynamic parameters that are in good agreement with previous affinity determinations by surface plasmon resonance. Strong bonds (up to ~500 pN) are observed under high tensile loading, which may favor strong mycobacterial attachment in the lung where cells are exposed to high shear stress or during hematogenous spread which leads to a disseminated infection. Our results provide new insight into the pleiotropic functions of an important mycobacterial virulence factor that acts as a stress-sensitive adhesin.

Scratching the Surface: Bacterial Cell Envelopes at the Nanoscale

The bacterial cell envelope is essential for viability, the environmental gatekeeper and first line of defense against external stresses. For most bacteria, the envelope biosynthesis is also the site of action of some of the most important groups of antibiotics. It is a complex, often multicomponent structure, able to withstand the internally generated turgor pressure. Thus, elucidating the architecture and dynamics of the cell envelope is important, to unravel not only the complexities of cell morphology and maintenance of integrity but also how interventions such as antibiotics lead to death. To address these questions requires the capacity to visualize the cell envelope in situ via high-spatial resolution approaches. In recent years, atomic force microscopy (AFM) has brought novel molecular insights into the assembly, dynamics, and functions of bacterial cell envelopes. The ultrafine resolution and physical sensitivity of the technique have revealed a wealth of ultrastructural features that are invisible to traditional optical microscopy techniques or imperceptible in their true physiological state by electron microscopy. Here, we discuss recent progress in our use of AFM imaging for understanding the architecture and dynamics of the bacterial envelope. We survey recent studies that demonstrate the power of the technique to observe isolated membranes and live cells at (sub)nanometer resolution and under physiological conditions and to track in vitro structural dynamics in response to growth or to drugs.

Atomic Force Microscopy: The Characterisation of Amyloid Protein Structure in Pathology

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Proteins are versatile macromolecules that perform a variety of functions and participate in virtually all cellular processes. The functionality of a protein greatly depends on its structure and alterations may result in the development of diseases. Most well-known of these are protein misfolding disorders, which include Alzheimer’s and Parkinson’s diseases as well as type 2 diabetes mellitus, where soluble proteins transition into insoluble amyloid fibrils. Atomic Force Microscopy (AFM) is capable of providing a topographical map of the protein and/or its aggregates, as well as probing the nanomechanical properties of a sample. Moreover, AFM requires relatively simple sample preparation, which presents the possibility of combining this technique with other research modalities, such as confocal laser scanning microscopy, Raman spectroscopy and stimulated emission depletion microscopy. In this review, the basic principles of AFM are discussed, followed by a brief overview of how it has been applied in biological research. Finally, we focus specifically on its use as a characterisation method to study protein structure at the nanoscale in pathophysiological conditions, considering both molecules implicated in disease pathogenesis and the plasma protein fibrinogen. In conclusion, AFM is a userfriendly tool that supplies multi-parametric data, rendering it a most valuable technique.

Glycan–glycan interactions determine Leishmania attachment to the midgut of permissive sand fly vectors

Force spectroscopy was used to measure the adhesion of Leishmania to synthetic mimics of galectins on the sand fly midgut.

Imaging live bacteria at the nanoscale: comparison of immobilisation strategies

Atomic force microscopy (AFM) provides an effective, label-free technique enabling the imaging of live bacteria under physiological conditions with nanometre precision. However, AFM is a surface scanning technique, and the accuracy of its performance requires the effective and reliable immobilisation of bacterial cells onto substrates. Here, we compare the effectiveness of various chemical approaches to facilitate the immobilisation of Escherichia coli onto glass cover slips in terms of bacterial adsorption, viability and compatibility with correlative imaging by fluorescence microscopy. We assess surface functionalisation using gelatin, poly-l-lysine, Cell-Tak™, and Vectabond®. We describe how bacterial immobilisation, viability and suitability for AFM experiments depend on bacterial strain, buffer conditions and surface functionalisation. We demonstrate the use of such immobilisation by AFM images that resolve the porin lattice on the bacterial surface; local degradation of the bacterial cell envelope by an antimicrobial peptide (Cecropin B); and the formation of membrane attack complexes on the bacterial membrane.

Glycan-mediated enhancement of reovirus receptor binding

Viral infection is an intricate process that requires the concerted action of both viral and host cell components. Entry of viruses into cells is initiated by interactions between viral proteins and their cell surface receptors. Despite recent progress, the molecular mechanisms underlying the multistep reovirus entry process are poorly understood. Using atomic force microscopy, we investigated how the reovirus σ1 attachment protein binds to both α-linked sialic acid (α-SA) and JAM-A cell-surface receptors. We discovered that initial σ1 binding to α-SA favors a strong multivalent anchorage to JAM-A. The enhanced JAM-A binding by virions following α-SA engagement is comparable to JAM-A binding by infectious subvirion particles (ISVPs) in the absence of α-SA. Since ISVPs have an extended σ1 conformer, this finding suggests that α-SA binding triggers a conformational change in σ1. These results provide new insights into the function of viral attachment proteins in the initiation of infection and open new avenues for the use of reoviruses as oncolytic agents.

Dissecting the cytochrome c 2-reaction centre interaction in bacterial photosynthesis using single molecule force spectroscopy

The reversible docking of small, diffusible redox proteins onto a membrane protein complex is a common feature of bacterial, mitochondrial and photosynthetic electron transfer (ET) chains. Spectroscopic studies of ensembles of such redox partners have been used to determine ET rates and dissociation constants. Here, we report a single-molecule analysis of the forces that stabilise transient ET complexes. We examined the interaction of two components of bacterial photosynthesis, cytochrome c ₂ and the reaction centre (RC) complex, using dynamic force spectroscopy and PeakForce quantitative nanomechanical imaging. RC-LH1-PufX complexes, attached to silicon nitride AFM probes and maintained in a photo-oxidised state, were lowered onto a silicon oxide substrate bearing dispersed, immobilised and reduced cytochrome c ₂ molecules. Microscale patterns of cytochrome c ₂ and the cyan fluorescent protein were used to validate the specificity of recognition between tip-attached RCs and surface-tethered cytochrome c ₂ Following the transient association of photo-oxidised RC and reduced cytochrome c ₂ molecules, retraction of the RC-functionalised probe met with resistance, and forces between 112 and 887 pN were required to disrupt the post-ET RC-c ₂ complex, depending on the retraction velocities used. If tip-attached RCs were reduced instead, the probability of interaction with reduced cytochrome c ₂ molecules decreased 5-fold. Thus, the redox states of the cytochrome c ₂ haem cofactor and RC 'special pair' bacteriochlorophyll dimer are important for establishing a productive ET complex. The millisecond persistence of the post-ET cytochrome c ₂[oxidised]-RC[reduced] 'product' state is compatible with rates of cyclic photosynthetic ET, at physiologically relevant light intensities.

The Mycobacterial HBHA Protein: A Promising Biomarker for Tuberculosis

A major goal in tuberculosis (TB) research is the identification, among the subjects infected with Mycobacterium tuberculosis (Mtb), of those with active TB, or at higher risk of developing active disease, from the latently infected subjects. The classical heterogeneity of Mtb infection and TB disease is a major obstacle toward the identification of reliable biomarkers that can stratify Mtb infected subjects based on disease risk. The heparin-binding haemagglutinin (HBHA) is a mycobacterial surface antigen that is implicated in tuberculosis (TB) pathogenesis. The host immune response against HBHA varies depending on the TB status and several studies are supporting the role of HBHA as a useful biomarker of TB.

Development of a novel multiphysical approach for the characterization of mechanical properties of musculotendinous tissues

At present, there is a lack of well-validated protocols that allow for the analysis of the mechanical properties of muscle and tendon tissues. Further, there are no reports regarding characterization of mouse skeletal muscle and tendon mechanical properties in vivo using elastography thereby limiting the ability to monitor changes in these tissues during disease progression or response to therapy. Therefore, we sought to develop novel protocols for the characterization of mechanical properties in musculotendinous tissues using atomic force microscopy (AFM) and ultrasound elastography. Given that TIEG1 knockout (KO) mice exhibit well characterized defects in the mechanical properties of skeletal muscle and tendon tissue, we have chosen to use this model system in the present study. Using TIEG1 knockout and wild-type mice, we have devised an AFM protocol that does not rely on the use of glue or chemical agents for muscle and tendon fiber immobilization during acquisition of transversal cartographies of elasticity and topography. Additionally, since AFM cannot be employed on live animals, we have also developed an ultrasound elastography protocol using a new linear transducer, SLH20-6 (resolution: 38 µm, footprint: 2.38 cm), to characterize the musculotendinous system in vivo. This protocol allows for the identification of changes in muscle and tendon elasticities. Such innovative technological approaches have no equivalent to date, promise to accelerate our understanding of musculotendinous mechanical properties and have numerous research and clinical applications.

Bacterial-nanostructure interactions: The role of cell elasticity and adhesion forces

The attachment of single-celled organisms, namely bacteria and fungi, to abiotic surfaces is of great interest to both the scientific and medical communities. This is because the interaction of such cells has important implications in a range of areas, including biofilm formation, biofouling, antimicrobial surface technologies, and bio-nanotechnologies, as well as infection development, control, and mitigation. While central to many biological phenomena, the factors which govern microbial surface attachment are still not fully understood. This lack of understanding is a direct consequence of the complex nature of cell-surface interactions, which can involve both specific and non-specific interactions. For applications involving micro- and nano-structured surfaces, developing an understanding of such phenomenon is further complicated by the diverse nature of surface architectures, surface chemistry, variation in cellular physiology, and the intended technological output. These factors are extremely important to understand in the emerging field of antibacterial nanostructured surfaces. The aim of this perspective is to re-frame the discussion surrounding the mechanism of nanostructured-microbial surface interactions. Broadly, the article reviews our current understanding of these phenomena, while highlighting the knowledge gaps surrounding the adhesive forces which govern bacterial-nanostructure interactions and the role of cell membrane rigidity in modulating surface activity. The roles of surface charge, cell rigidity, and cell-surface adhesion force in bacterial-surface adsorption are discussed in detail. Presently, most studies have overlooked these areas, which has left many questions unanswered. Further, this perspective article highlights the numerous experimental issues and misinterpretations which surround current studies of antibacterial nanostructured surfaces.

Automated multi-sample acquisition and analysis using atomic force microscopy for biomedical applications

In the last 20 years, atomic force microscopy (AFM) has emerged as a ubiquitous technique in biological research, allowing the analysis of biological samples under near-physiological conditions from single molecules to living cells. Despite its growing use, the low process throughput remains a major drawback. Here, we propose a solution validated on a device allowing a fully automated, multi-sample analysis. Our approach is mainly designed to study samples in fluid and biological cells. As a proof of concept, we demonstrate its feasibility applied to detect and scan both fixed and living bacteria before completion of data processing. The effect of two distinct treatments (i.e. gentamicin and heating) is then evidenced on physical parameters of fixed Yersinia pseudotuberculosis bacteria. The multi-sample analysis presented allows an increase in the number of scanned samples while limiting the user's input. Importantly, cantilever cleaning and control steps are performed regularly-as part of the automated process-to ensure consistent scanning quality. We discuss how such an approach is paving the way to AFM developments in medical and clinical fields, in which statistical significance of results is a prerequisite.

Antimicrobial Peptides: Effect on Bacterial Cells

Antimicrobial peptides (AMPs) are one of the most promising alternatives to conventional antibiotics. Atomic force microscopy (AFM), as imaging and force spectroscopy tool, has been applied to study their mechanism of action and development. Here, we describe different methods to be applied in the study of AMP effects on bacteria, either by imaging or by force spectroscopy studies, essential to underlie their action and to identify possibly outcomes of the same.

Modified cantilever arrays improve sensitivity and reproducibility of nanomechanical sensing in living cells

Mechanical signaling involved in molecular interactions lies at the heart of materials science and biological systems, but the mechanisms involved are poorly understood. Here we use nanomechanical sensors and intact human cells to provide unique insights into the signaling pathways of connectivity networks, which deliver the ability to probe cells to produce biologically relevant, quantifiable and reproducible signals. We quantify the mechanical signals from malignant cancer cells, with 10 cells per ml in 1000-fold excess of non-neoplastic human epithelial cells. Moreover, we demonstrate that a direct link between cells and molecules creates a continuous connectivity which acts like a percolating network to propagate mechanical forces over both short and long length-scales. The findings provide mechanistic insights into how cancer cells interact with one another and with their microenvironments, enabling them to invade the surrounding tissues. Further, with this system it is possible to understand how cancer clusters are able to co-ordinate their migration through narrow blood capillaries.

Samadhan Patil et al. report a new method for improving the sensitivity and reproducibility of mechanobiological measurements in malignant cancer cells. Their findings provide insight into the interaction of cells with each other and the microenvironment and may impact our understanding of metastasis.

Heparin-Binding Hemagglutinin Adhesin (HBHA) Is Involved in Intracytosolic Lipid Inclusions Formation in Mycobacteria

The heparin-binding hemagglutinin adhesin (HBHA) is an important virulence factor of Mycobacterium tuberculosis. It is a surface-displayed protein that serves as an adhesin for non-phagocytic cells and is involved in extra-pulmonary dissemination of the tubercle bacillus. It is also an important latency antigen useful for the diagnosis of latently M. tuberculosis-infected individuals. Using fluorescence time-lapse microscopy on mycobacteria that produce HBHA-green fluorescent protein chimera, we show here that HBHA can be found at two different locations and dynamically alternates between the mycobacterial surface and the interior of the cell, where it participates in the formation of intracytosolic lipid inclusions (ILI). Compared to HBHA-producing mycobacteria, HBHA-deficient mutants contain significantly lower amounts of ILI when grown in vitro or within macrophages, and the sizes of their ILI are significantly smaller. Lipid-binding assays indicate that HBHA is able to specifically bind to phosphatidylinositol and in particular to 4,5 di-phosphorylated phosphatidylinositol, but not to neutral lipids, the main constituents of ILI. HBHA derivatives lacking the C-terminal methylated, lysine-rich repeat region fail to bind to these lipids and these derivatives also fail to complement the phenotype of HBHA-deficient mutants. These studies indicate that HBHA is a moonlighting protein that serves several functions depending on its location. When surface exposed, HBHA serves as an adhesin, and when intracellularly localized, it participates in the generation of ILI, possibly as a cargo to transport phospholipids from the plasma membrane to the ILI in the process of being formed.

Multivalent binding of herpesvirus to living cells is tightly regulated during infection

Viral infection, initiated by the landing of a virion on a cellular surface, is largely defined by the preliminary interactions established between viral particles and their receptors at the cell surface. While multiple parallel interactions would allow strong virus attachment, a low number of bonds could be preferred to allow lateral diffusion toward specific receptors and to promote efficient release of progeny virions from the cell surface. However, so far, the molecular mechanisms underlying the regulation of the multivalency in virus attachment to receptors are poorly understood. We introduce a new method to force-probe multivalent attachment directly on living cells, and we show, for the first time, direct evidence of a new mechanism by which a herpesvirus surface glycoprotein acts as a key negative regulator in the first step of herpesvirus binding. Using atomic force microscopy, we probe at the single-virion level the number and the strength of the bonds established with heparan sulfate both on model surfaces and on living cells. Our biophysical results, correlated with other techniques, show that the major envelope glycoprotein functions as a regulator of binding valency during both attachment and release steps, determining the binding, diffusion, and release potential of virions at the cellular surface.

Probing the influence of cell surface polysaccharides on nanodendrimer binding to Gram-negative and Gram-positive bacteria using single-nanoparticle force spectroscopy

The safe use and design of nanoparticles (NPs) ask for a comprehensive interpretation of their potentially adverse effects on (micro)organisms. In this respect, the prior assessment of the interactions experienced by NPs in the vicinity of - and in contact with - complex biological surfaces is mandatory. It requires the development of suitable techniques for deciphering the processes that govern nano-bio interactions when a single organism is exposed to an extremely low dose of NPs. Here, we used atomic force spectroscopy (AFM)-based force measurements to investigate at the nanoscale the interactions between carboxylate-terminated polyamidoamine (PAMAM) nanodendrimers (radius ca. 4.5 nm) and two bacteria with very distinct surface properties, Escherichia coli and Lactococcus lactis. The zwitterionic nanodendrimers exhibit a negative peripheral surface charge and/or a positive intraparticulate core depending on the solution pH and salt concentration. Following an original strategy according to which a single dendrimer NP is grafted at the very apex of the AFM tip, the density and localization of NP binding sites are probed at the surface of E. coli and L. lactis mutants expressing different cell surface structures (presence/absence of the O-antigen of the lipopolysaccharides (LPS) or of a polysaccharide pellicle). In line with electrokinetic analysis, AFM force measurements evidence that adhesion of NPs onto pellicle-decorated L. lactis is governed by their underlying electrostatic interactions as controlled by the pH-dependent charge of the peripheral and internal NP components, and the negatively-charged cell surface. In contrast, the presence of the O-antigen on E. coli systematically suppresses the adhesion of nanodendrimers onto cells, may the apparent NP surface charge be determined by the peripheral carboxylate groups or by the internal amine functions. Altogether, this work highlights the differentiated roles played by surface polysaccharides in mediating NP attachment to Gram-positive and Gram-negative bacteria. It further demonstrates that the assessment of NP bioadhesion features requires a critical analysis of the electrostatic contributions stemming from the various structures composing the stratified cell envelope, and those originating from the bulk and surface NP components. The joint use of electrokinetics and AFM provides a valuable option for rapidly addressing the binding propensity of NPs to microorganisms, as urgently needed in NP risk assessments.

Rv0613c/MSMEG_1285 Interacts with HBHA and Mediates Its Proper Cell-Surface Exposure in Mycobacteria

Heparin-binding haemagglutinin (HBHA) is a surface-exposed virulence factor of Mycobacterium tuberculosis and is involved in the binding of mycobacteria to non-phagocytic cells, allowing for extra-pulmonary dissemination of the bacilli. Despite its surface exposure, HBHA is not produced as a pre-protein containing a typical cleavable N-terminal signal peptide and is thus likely secreted by a Sec-independent, as of yet unknown mechanism. Here, we used the bacterial adenylate cyclase two-hybrid system to identify the proteins encoded by rv0613c and mmpL14 as being able to interact with HBHA. Our study was focused on Rv0613c, as it showed more consistent interactions with HBHA than MmpL14. Deletion of its orthologous gene MSMEG_1285 in recombinant Mycobacterium smegmatis producing HBHA from M. tuberculosis resulted in the loss of proper surface exposure of HBHA, as evidenced by atomic force microscopy. Furthermore, the lack of MSMEG_1285 also abolished the clumping phenotype and rough colony morphology of the recombinant M. smegmatis and reduced its adherence to A549 epithelial cells. These phenotypes have previously been associated with surface-exposed HBHA. Thus, MSMEG_1285 is directly involved in the proper cell-surface exposure of HBHA. These observations identify MSMEG_1285/Rv0613c as the first accessory protein involved in the cell surface exposure of HBHA.

Asymmetric adhesion of rod-shaped bacteria controls microcolony morphogenesis

Surface colonization underpins microbial ecology on terrestrial environments. Although factors that mediate bacteria–substrate adhesion have been extensively studied, their spatiotemporal dynamics during the establishment of microcolonies remains largely unexplored. Here, we use laser ablation and force microscopy to monitor single-cell adhesion during the course of microcolony formation. We find that adhesion forces of the rod-shaped bacteria Escherichia coli and Pseudomonas aeruginosa are polar. This asymmetry induces mechanical tension, and drives daughter cell rearrangements, which eventually determine the shape of the microcolonies. Informed by experimental data, we develop a quantitative model of microcolony morphogenesis that enables the prediction of bacterial adhesion strength from simple time-lapse measurements. Our results demonstrate how patterns of surface colonization derive from the spatial distribution of adhesive factors on the cell envelope.

It is unclear how cell adhesion and elongation coordinate during formation of bacterial microcolonies. Here, Duvernoy et al. monitor microcolony formation in rod-shaped bacteria, and show that patterns of surface colonization derive from the spatial distribution of adhesive factors on the cell envelope.

>1000-Fold Lifetime Extension of a Nickel Electromechanical Contact Device via Graphene

Micro-/nano-electromechanical (M/NEM) switches have received significant attention as promising switching devices for a wide range of applications such as computing, radio frequency communication, and power gating devices. However, M/NEM switches still suffer from unacceptably low reliability because of irreversible degradation at the contacting interfaces, hindering adoption in practical applications and further development. Here, we evaluate and verify graphene as a contact material for reliability-enhanced M/NEM switching devices. Atomic force microscopy experiments and quantum mechanics calculations reveal that energy-efficient mechanical contact-separation characteristics are achieved when a few layers of graphene are used as a contact material on a nickel surface, reducing the energy dissipation by 96.6% relative to that of a bare nickel surface. Importantly, graphene displays almost elastic contact-separation, indicating that little atomic-scale wear, including plastic deformation, fracture, and atomic attrition, is generated. We also develop a feasible fabrication method to demonstrate a MEM switch, which has high-quality graphene as the contact material, and verify that the devices with graphene show mechanically stable and elastic-like contact properties, consistent with our nanoscale contact experiment. The graphene coating extends the switch lifetime >10³ times under hot switching conditions.

Layered Structure and Complex Mechanochemistry Underlie Strength and Versatility in a Bacterial Adhesive

While designing synthetic adhesives that perform in aqueous environments has proven challenging, microorganisms commonly produce bioadhesives that efficiently attach to a variety of substrates, including wet surfaces. The aquatic bacterium Caulobacter crescentus uses a discrete polysaccharide complex, the holdfast, to strongly attach to surfaces and resist flow. The holdfast is extremely versatile and has impressive adhesive strength. Here, we used atomic force microscopy in conjunction with superresolution microscopy and enzymatic assays to unravel the complex structure of the holdfast and to characterize its chemical constituents and their role in adhesion. Our data support a model whereby the holdfast is a heterogeneous material organized as two layers: a stiffer nanoscopic core layer wrapped into a sparse, far-reaching, flexible brush layer. Moreover, we found that the elastic response of the holdfast evolves after surface contact from initially heterogeneous to more homogeneous. From a composition point of view, besides N-acetyl-d-glucosamine (NAG), the only component that had been identified to date, our data show that the holdfast contains peptides and DNA. We hypothesize that, while polypeptides are the most important components for adhesive force, the presence of DNA mainly impacts the brush layer and the strength of initial adhesion, with NAG playing a primarily structural role within the core. The unanticipated complexity of both the structure and composition of the holdfast likely underlies its versatility as a wet adhesive and its distinctive strength. Continued improvements in understanding of the mechanochemistry of this bioadhesive could provide new insights into how bacteria attach to surfaces and could inform the development of new adhesives.IMPORTANCE There is an urgent need for strong, biocompatible bioadhesives that perform underwater. To strongly adhere to surfaces and resist flow underwater, the bacterium Caulobacter crescentus produces an adhesive called the holdfast, the mechanochemistry of which remains undefined. We show that the holdfast is a layered structure with a stiff core layer and a polymeric brush layer and consists of polysaccharides, polypeptides, and DNA. The DNA appears to play a role in the structure of the brush layer and initial adhesion, the peptides in adhesive strength, and the polysaccharides in the structure of the core. The complex, multilayer organization and diverse chemistry described here underlie the distinctive adhesive properties of the holdfast and will provide important insights into the mechanisms of bacterial adhesion and bioadhesive applications.

Orthogonal Probing of Single-Molecule Heterogeneity by Correlative Fluorescence and Force Microscopy

Correlative imaging by fluorescence and force microscopy is an emerging technology to acquire orthogonal information at the nanoscale. Whereas atomic force microscopy excels at resolving the envelope structure of nanoscale specimens, fluorescence microscopy can detect specific molecular labels, which enables the unambiguous recognition of molecules in a complex assembly. Whereas correlative imaging at the micrometer scale has been established, it remains challenging to push the technology to the single-molecule level. Here, we used an integrated setup to systematically evaluate the factors that influence the quality of correlative fluorescence and force microscopy. Optimized data processing to ensure accurate drift correction and high localization precision results in image registration accuracies of ∼25 nm on organic fluorophores, which represents a 2-fold improvement over the state of the art in correlative fluorescence and force microscopy. Furthermore, we could extend the Atto532 fluorophore bleaching time ∼2-fold, by chemical modification of the supporting mica surface. In turn, this enables probing the composition of macromolecular complexes by stepwise photobleaching with high confidence. We demonstrate the performance of our method by resolving the stoichiometry of molecular subpopulations in a heterogeneous EcoRV-DNA nucleoprotein ensemble.

KCa2 channel localization and regulation in the axon initial segment

Small conductance calcium-activated potassium (K_Ca2) channels are expressed throughout the CNS and play a critical role in synaptic and neuronal excitability. K_Ca2 channels have a somatodendritic distribution with their highest expression in distal dendrites. It is unclear whether K_Ca2 channels are specifically present on the axon initial segment (AIS), the site at which action potentials are initiated in neurons. Through a powerful combination of toxin pharmacology, single-molecule atomic force microscopy, and dual-color fluorescence microscopy, we report here that K_Ca2 channels-predominantly the K_Ca2.3 subtype-are indeed present on the AIS. We also report that cAMP-PKA controls the axonal K_Ca2 channel surface expression. Surprisingly, and in contrast to K_Ca2 channels that were observed in the soma and dendrites, the inhibition of cAMP-PKA increased the surface expression of K_Ca2 channels without promoting nanoclustering. Lastly, we found that axonal K_Ca2 channels seem to undergo endocytosis in a dynamin-independent manner, unlike K_Ca2 channels in the soma and dendrites. Together, these novel results demonstrate that the distribution and membrane recycling of K_Ca2 channels differs among various neuronal subcompartments.-Abiraman, K., Tzingounis, A. V., Lykotrafitis, G. K_Ca2 channel localization and regulation in the axon initial segment.