N-Formylation modifies membrane damage associated with PSMα3 interfacial fibrillation

The virulence of Staphylococcus aureus, a multi-drug resistant pathogen, notably depends on the expression of the phenol soluble modulins α3 (PSMα3) peptides, able to self-assemble into amyloid-like cross-α fibrils. Despite remarkable advances evidencing the crucial, yet insufficient, role of fibrils in PSMα3 cytotoxic activities towards host cells, the relationship between its molecular structures, assembly propensities, and modes of action remains an open intriguing problem. In this study, combining atomic force microscopy (AFM) imaging and infrared spectroscopy, we first demonstrated in vitro that the charge provided by the N-terminal capping of PSMα3 alters its interactions with model membranes of controlled lipid composition without compromising its fibrillation kinetics or morphology. N-formylation eventually dictates PSMα3-membrane binding via electrostatic interactions with the lipid head groups. Furthermore, PSMα3 insertion within the lipid bilayer is favoured by hydrophobic interactions with the lipid acyl chains only in the fluid phase of membranes and not in the gel-like ordered domains. Strikingly, our real-time AFM imaging emphasizes how intermediate protofibrillar entities, formed along PSMα3 self-assembly and promoted at the membrane interface, likely disrupt membrane integrity via peptide accumulation and subsequent membrane thinning in a peptide concentration and lipid-dependent manner. Overall, our multiscale and multimodal approach sheds new light on the key roles of N-formylation and intermediate self-assembling entities, rather than mature fibrils, in dictating deleterious interactions of PSMα3 with membrane lipids, likely underscoring its ultimate cellular toxicity in vivo, and in turn S. aureus pathogenesis.

The impact of lipid polyunsaturation on the physical and mechanical properties of lipid membranes

The lipid composition of cellular membranes and the balance between the different lipid components can be impacted by aging, certain pathologies, specific diets and other factors. This is the case in a subgroup of individuals with psychiatric disorders, such as schizophrenia, where cell membranes of patients have been shown to be deprived in polyunsaturated fatty acids (PUFAs), not only in brain areas where the target receptors are expressed but also in peripheral tissues. This PUFA deprivation thus represents a biomarker of such disorders that might impact not only the interaction of antipsychotic medications with these membranes but also the activation and signaling of the targeted receptors embedded in the lipid membrane. Therefore, it is crucial to understand how PUFAs levels alterations modulate the different physical properties of membranes. In this paper, several biophysical approaches were combined (Laurdan fluorescence spectroscopy, atomic force microscopy, differential scanning calorimetry, molecular modeling) to characterize membrane properties such as fluidity, elasticity and thickness in PUFA-enriched cell membranes and lipid model systems reflecting the PUFA imbalance observed in some diseases. The impact of both the number of unsaturations and their position along the chain on the above properties was investigated. Briefly, data revealed that PUFA presence in membranes increases membrane fluidity, elasticity and flexibility and decreases its thickness and order parameter. Both the level of unsaturation and their position affect these membrane properties.

Mechanistic basis of staphylococcal interspecies competition for skin colonization

Staphylococci, whether beneficial commensals or pathogens, often colonize human skin, potentially leading to competition for the same niche. In this multidisciplinary study we investigate the structure, binding specificity, and mechanism of adhesion of the Aap lectin domain required for Staphylococcus epidermidis skin colonization and compare its characteristics to the lectin domain from the orthologous Staphylococcus aureus adhesin SasG. The Aap structure reveals a legume lectin-like fold with atypical architecture, showing specificity for N-acetyllactosamine and sialyllactosamine. Bacterial adhesion assays using human corneocytes confirmed the biological relevance of these Aap-glycan interactions. Single-cell force spectroscopy experiments measured individual binding events between Aap and corneocytes, revealing an extraordinarily tight adhesion force of nearly 900 nN and a high density of receptors at the corneocyte surface. The SasG lectin domain shares similar structural features, glycan specificity, and corneocyte adhesion behavior. We observe cross-inhibition of Aap-and SasG-mediated staphylococcal adhesion to corneocytes. Together, these data provide insights into staphylococcal interspecies competition for skin colonization and suggest potential avenues for inhibition of S. aureus colonization.

The staphylococcal biofilm protein Aap mediates cell-cell adhesion through mechanically distinct homophilic and lectin interactions

The accumulation phase of staphylococcal biofilms relies on both the production of an extracellular polysaccharide matrix and the expression of bacterial surface proteins. A prototypical example of such adhesive proteins is the long multidomain protein Aap (accumulation-associated protein) from Staphylococcus epidermidis, which mediates zinc-dependent homophilic interactions between Aap B-repeat regions through molecular forces that have not been investigated yet. Here, we unravel the remarkable mechanical strength of single Aap-Aap homophilic bonds between living bacteria and we demonstrate that intercellular adhesion also involves sugar binding through the lectin domain of the Aap A region. We find that the mechanical force needed to unfold individual β-sheet-rich G5-E domains from the Aap B-repeat regions is very high, ranging from 300 up to 1,000 pN at high loading rates, indicating these are extremely stable. This high mechanostability provides a means to the cells to form highly adhesive and cohesive biofilms capable of sustaining high physiological shear stress. Importantly, we identify a previously undescribed role of Aap in bacterial-bacterial adhesion, that is, heterophilic sugar binding by a specific lectin domain located in the N-terminal A region, which might be important to establish initial contacts between cells before strong homophilic bonds come into play. This study emphasizes the remarkable mechanical and binding properties of Aap as well as its wide diversity of adhesive functions.

AFM Identifies a Protein Complex Involved in Pathogen Adhesion Which Ruptures at Three Nanonewtons

Staphylococci bind to the blood protein von Willebrand Factor (vWF), thereby causing endovascular infections. Whether and how this interaction occurs with the medically important pathogen Staphylococcus epidermidis is unknown. Using single-molecule experiments, we demonstrate that the S. epidermidis protein Aap binds vWF via an ultrastrong force, ∼3 nN, the strongest noncovalent biological bond ever reported, and we show that this interaction is activated by tensile loading, suggesting a catch-bond behavior. Aap-vWF binding involves exclusively the A1 domain of vWF but requires both the A and B domains of Aap, as revealed by inhibition assays using specific monoclonal antibodies. Collectively, our results point to a mechanism where force-induced unfolding of the B repeats activates the A domain of Aap, shifting it from a weak- to a strong-binding state, which then engages into an ultrastrong interaction with vWF A1. This shear-dependent function of Aap offers promise for innovative antistaphylococcal therapies.

A New Function for Amyloid-Like Interactions: Cross-Beta Aggregates of Adhesins form Cell-to-Cell Bonds

Amyloid structures assemble through a repeating type of bonding called "cross-β", in which identical sequences in many protein molecules form β-sheets that interdigitate through side chain interactions. We review the structural characteristics of such bonds. Single cell force microscopy (SCFM) shows that yeast expressing Als5 adhesin from Candida albicans demonstrate the empirical characteristics of cross-β interactions. These properties include affinity for amyloid-binding dyes, birefringence, critical concentration dependence, repeating structure, and inhibition by anti-amyloid agents. We present a model for how cross-β bonds form in trans between two adhering cells. These characteristics also apply to other fungal adhesins, so the mechanism appears to be an example of a new type of cell-cell adhesion.

AFM force-clamp spectroscopy captures the nanomechanics of the Tad pilus retraction

Motorization of bacterial pili is key to generate traction forces to achieve cellular function. The Tad (or Type IVc) pilus from Caulobacter crescentus is a widespread motorized nanomachine crucial for bacterial survival, evolution and virulence. An unusual bifunctional ATPase motor drives Tad pilus retraction, which helps the bacteria to land on target surfaces. Here, we use a novel platform combining a fluorescence-based screening of piliated bacteria and atomic force microscopy (AFM) force-clamp spectroscopy, to monitor over time (30 s) the nanomechanics and dynamics of the Tad nanofilament retraction under a high constant tension (300 pN). We observe striking transient variations of force and height originating from two phenomena: active pilus retraction and passive hydrophobic interactions between the pilus and the hydrophobic substrate. That the Tad pilus is able to retract under high tensile loading - at a velocity of ∼150 nm s-1 - indicates that this nanomachine is stronger than previously anticipated. Our findings show that pilus retraction and hydrophobic interactions work together to mediate bacterial cell landing and surface adhesion. The motorized pilus retraction actively triggers the cell to approach the substrate. At short distances, passive hydrophobic interactions accelerate the approach phenomenon and promote strong cell-substrate adhesion. This mechanism could provide a strategy to save ATP-based energy by the retraction ATPase. Our force-clamp AFM methodology offers promise to decipher the physics of bacterial nanomotors with high sensitivity and temporal resolution.

AFM Unravels the Unique Adhesion Properties of the Caulobacter Type IVc Pilus Nanomachine

Bacterial pili are proteinaceous motorized nanomachines that play various functional roles including surface adherence, bacterial motion, and virulence. The surface-contact sensor type IVc (or Tad) pilus is widely distributed in both Gram-positive and Gram-negative bacteria. In Caulobacter crescentus, this nanofilament, though crucial for surface colonization, has never been thoroughly investigated at the molecular level. As Caulobacter assembles several surface appendages at specific stages of the cell cycle, we designed a fluorescence-based screen to selectively study single piliated cells and combined it with atomic force microscopy and genetic manipulation to quantify the nanoscale adhesion of the type IVc pilus to hydrophobic substrates. We demonstrate that this nanofilament exhibits high stickiness compared to the canonical type IVa/b pili, resulting mostly from multiple hydrophobic interactions along the fiber length, and that it features nanospring mechanical properties. Our findings may be helpful to better understand the structure-function relationship of bacterial pilus nanomachines.

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.

Entropic Bristles Tune the Seeding Efficiency of Prion-Nucleating Fragments

Prions of lower eukaryotes are self-templating protein aggregates with cores formed by parallel in-register beta strands. Short aggregation-prone glutamine (Q)- and asparagine (N)-rich regions embedded in longer disordered domains have been proposed to act as nucleation sites that initiate refolding of soluble prion proteins into highly ordered fibrils, termed amyloid. We demonstrate that a short Q/N-rich peptide corresponding to a proposed nucleation site in the prototype Saccharomyces cerevisiae prion protein Sup35 is sufficient to induce infectious cytosolic prions in mouse neuroblastoma cells ectopically expressing the soluble Sup35 NM prion domain. Embedding this nucleating core in a non-native N-rich sequence that does not form amyloid but acts as an entropic bristle quadruples seeding efficiency. Our data suggest that large disordered sequences flanking an aggregation core in prion proteins act as not only solubilizers of the monomeric protein but also breakers of the formed amyloid fibrils, enhancing infectivity of the prion seeds.

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A short peptide derived from Sup35 (p103–113) forms rigid amyloid fibrils

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p103–113 fibrils can induce infectious Sup35 NM prions in mammalian cells

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Embedding p103–113 in an N-rich sequence increases fibril brittleness

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Increased fibril brittleness enhances prion-inducing capacity

Adhesion of Staphylococcus aureus to Candida albicans During Co-Infection Promotes Bacterial Dissemination Through the Host Immune Response

Interspecies interactions greatly influence the virulence, drug tolerance and ultimately the outcome of polymicrobial biofilm infections. A synergistic interaction is observed between the fungus Candida albicans and the bacterium Staphylococcus aureus. These species are both normal commensals of most healthy humans and co-exist in several niches of the host. However, under certain circumstances, they can cause hospital-acquired infections with high morbidity and mortality rates. Using a mouse model of oral co-infection, we previously showed that an oral infection with C. albicans predisposes to a secondary systemic infection with S. aureus. Here, we unraveled this intriguing mechanism of bacterial dissemination. Using static and dynamic adhesion assays in combination with single-cell force spectroscopy, we identified C. albicans Als1 and Als3 adhesins as the molecular players involved in the interaction with S. aureus and in subsequent bacterial dissemination. Remarkably, we identified the host immune response as a key element required for bacterial dissemination. We found that the level of immunosuppression of the host plays a critical yet paradoxical role in this process. In addition, secretion of candidalysin, the C. albicans peptide responsible for immune activation and cell damage, is required for C. albicans colonization and subsequent bacterial dissemination. The physical interaction with C. albicans enhances bacterial uptake by phagocytic immune cells, thereby enabling an opportunity to disseminate.

Single-cell fluidic force microscopy reveals stress-dependent molecular interactions in yeast mating

Sexual agglutinins of the budding yeast Saccharomyces cerevisiae are proteins mediating cell aggregation during mating. Complementary agglutinins expressed by cells of opposite mating types "a" and "α" bind together to promote agglutination and facilitate fusion of haploid cells. By means of an innovative single-cell manipulation assay combining fluidic force microscopy with force spectroscopy, we unravel the strength of single specific bonds between a- and α-agglutinins (~100 pN) which require pheromone induction. Prolonged cell-cell contact strongly increases adhesion between mating cells, likely resulting from an increased expression of agglutinins. In addition, we highlight the critical role of disulfide bonds of the a-agglutinin and of histidine residue H₂₇₃ of α-agglutinin. Most interestingly, we find that mechanical tension enhances the interaction strength, pointing to a model where physical stress induces conformational changes in the agglutinins, from a weak-binding folded state, to a strong-binding extended state. Our single-cell technology shows promises for understanding and controlling the complex mechanism of yeast sexuality.

Stress-Induced Catch-Bonds to Enhance Bacterial Adhesion

Physical forces have a profound influence on bacterial cell physiology and disease. A striking example is the formation of catch-bonds that reinforce under mechanical stress. While mannose-binding by the Escherichia coli FimH adhesin has long been the only thoroughly studied microbial catch-bond, it has recently become clear that proteins from other species, such as staphylococci, are also engaged in such stress-dependent interactions.

Nanonewton forces between Staphylococcus aureus surface protein IsdB and vitronectin

Single-molecule experiments have recently revealed that the interaction between staphylococcal surface proteins and their ligands can be extremely strong, equivalent to the strength of covalent bonds. Here, we report on the unusually high binding strength between Staphylococcus aureus iron-regulated surface determinant B (IsdB) and vitronectin (Vn), an essential human blood protein known to interact with bacterial pathogens. The IsdB-Vn interaction is dramatically strengthened by mechanical tension, with forces up to 2000 pN at a loading rate of 105 pN s-1. In line with this, flow experiments show that IsdB-mediated bacterial adhesion to Vn is enhanced by fluid shear stress. The stress-dependent binding of IsdB to Vn is likely to play a role in promoting bacterial adhesion to human cells under fluid shear stress conditions.

Single-Molecule Analysis Demonstrates Stress-Enhanced Binding between Staphylococcus aureus Surface Protein IsdB and Host Cell Integrins

Binding of Staphylococcus aureus surface proteins to endothelial cell integrins plays essential roles in host cell adhesion and invasion, eventually leading to life-threatening diseases. The staphylococcal protein IsdB binds to β3-containing integrins through a mechanism that has never been thoroughly investigated. Here, we identify and characterize at the nanoscale a previously undescribed stress-dependent adhesion between IsdB and integrin α_Vβ₃. The strength of single IsdB-α_Vβ₃ interactions is moderate (∼100 pN) under low stress, but it increases dramatically under high stress (∼1000-2000 pN) to exceed the forces traditionally reported for the binding between integrins and Arg-Gly-Asp (RGD) sequences. We suggest a mechanism where high mechanical stress induces conformational changes in the integrin from a low-affinity, weak binding state to a high-affinity, strong binding state. This single-molecule study highlights that direct adhesin-integrin interactions represent potential targets to fight staphylococcal infections.

Nanomechanics of the molecular complex between staphylococcal adhesin SpsD and elastin

Staphylococcus pseudintermedius surface protein SpsD binds to extracellular matrix proteins to invade canine epithelial cells. Using single-molecule experiments, we show that SpsD engages in two modes of interaction with elastin that are tightly controlled by physical stress. Binding is weak (∼100 pN) at low tensile force (i.e. loading rate), but is dramatically enhanced (up to ∼1500 pN) by mechanical tension. Consistent with a "dock, lock, and latch" (DLL) mechanism, this force represents among the highest mechanical strengths known for a non-covalent biological interaction. The transition from weak to strong binding correlates with an increase in molecular stiffness but, surprisingly, with a decrease in molecular extension. This unanticipated mechanical behavior indicates that the adhesin is engaged in two distinct interaction mechanisms. Our results emphasize the crucial role of protein nanomechanics in the adhesion of staphylococci, and illustrate their wide diversity of force-dependent ligand-binding activities. These single-molecule mechanical experiments may contribute to the development of antiadhesion approaches to treat infections caused by S. pseudintermedius and other bacterial pathogens engaged in DLL interactions.

Bacterial Cell Mechanics Beyond Peptidoglycan

The bacterial cell envelope plays essential roles in controlling cell shape, division, pathogenicity, and resistance against external stresses. In Escherichia coli, peptidoglycan (PG) has long been thought to be the primary component that conveys mechanical strength to the envelope. But a recent publication demonstrates the key contribution of the lipoprotein Lpp in defining the stiffness of the cell envelope and its sensitivity to drugs.

What makes bacterial pathogens so sticky?

Pathogenic bacteria use a variety of cell surface adhesins to promote binding to host tissues and protein-coated biomaterials, as well as cell-cell aggregation. These cellular interactions represent the first essential step that leads to host colonization and infection. Atomic force microscopy (AFM) has greatly contributed to increase our understanding of the specific interactions at play during microbial adhesion, down to the single-molecule level. A key asset of AFM is that adhesive interactions are studied under mechanical force, which is highly relevant as surface-attached pathogens are often exposed to physical stresses in the human body. These studies have identified sophisticated binding mechanisms in adhesins, which represent promising new targets for antiadhesion therapy.

Large variability in the motility of spiroplasmas in media of different viscosities

Spiroplasmas are bacteria that do not possess flagella and their motility is linked to kink propagation coupled to changes in the cell body helicity. While the motility of bacteria with flagellar motion has been studied extensively, less work has been devoted to the motility of spiroplasmas. We first show that the motility of such bacteria has large variability from individual to individual as well as large fluctuations in time. The Brownian motion of such bacteria both in orientation and translation is also highlighted. We propose a simple model to disentangle the different components of this motility by examining trajectories of single bacteria in different viscosity solvents. The mean velocity of the bacteria turns out to depend on the viscosity of the medium as it increases with viscosity. Further, the temporal fluctuations of the bacteria motility turn out to be very strong with a direct link to tumbling events particular to this bacteria.

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

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

Influence of oxidized lipids on palmitoyl-oleoyl-phosphatidylcholine organization, contribution of Langmuir monolayers and Langmuir-Blodgett films

In this work, we studied the interaction of two oxidized lipids, PoxnoPC and PazePC, with POPC phospholipid. Mean molecular areas obtained from (π-A) isotherms of mixed PoxnoPC-POPC and PazePC-POPC monolayers revealed different behaviors of these two oxidized lipids: the presence of PoxnoPC in the monolayers induces their expansion, mean molecular areas being higher than those expected in the case of ideal mixtures. PazePC-POPC behave on the whole ideally. This difference can be explained by a different conformation of oxidized lipids. Moreover the carboxylic function of PazePC is protonated under our experimental conditions, as shown by (π-A) isotherms of PazePC at different pH values. Both oxidized lipids induce also an increase of the monolayer elasticity, PoxnoPC being slightly more efficient than PazePC. These monolayers were transferred from the air-water interface onto mica supports for a study by AFM. AFM images are on the whole homogenous, suggesting the presence of only one lipid phase in both cases. However, in the case of PazePC-POPC monolayers, AFM images show also the presence of areas thicker of 7nm to 10nm than the surrounding lipid phase, probably due to the local formation of multilayer systems induced by compression.