Bonds between fibronectin and fibronectin-binding proteins on Staphylococcus aureus and Lactococcus lactis

Human Osteoblast-Conditioned Media Can Influence Staphylococcus aureus Biofilm Formation

Osteoblasts are bone-forming and highly active cells participating in bone homeostasis. In the case of osteomyelitis and more specifically prosthetic joint infections (PJI) for which Staphylococcus aureus (S. aureus) is mainly involved, the interaction between osteoblasts and S. aureus results in impaired bone homeostasis. If, so far, most of the studies of osteoblasts and S. aureus interactions were focused on osteoblast response following direct interactions with co-culture and/or internalization models, less is known about the effect of osteoblast factors on S. aureus biofilm formation. In the present study, we investigated the effect of human osteoblast culture supernatant on methicillin sensitive S. aureus (MSSA) SH1000 and methicillin resistant S. aureus (MRSA) USA300. Firstly, Saos-2 cell line was incubated with either medium containing TNF-α to mimic the inflammatory periprosthetic environment or with regular medium. Biofilm biomass was slightly increased for both strains in the presence of culture supernatant collected from Saos-2 cells, stimulated or not with TNF-α. In such conditions, SH1000 was able to develop microcolonies, suggesting a rearrangement in biofilm organization. However, the biofilm matrix and regulation of genes dedicated to biofilm formation were not substantially changed. Secondly, culture supernatant obtained from primary osteoblast culture induced varied response from SH1000 strain depending on the different donors tested, whereas USA300 was only slightly affected. This suggested that the sensitivity to bone cell secretions is strain dependent. Our results have shown the impact of osteoblast secretions on bacteria and further identification of involved factors will help to manage PJI.

The Role of Nanoscale Distribution of Fibronectin in the Adhesion of Staphylococcus aureus Studied by Protein Patterning and DNA-PAINT

Staphylococcus aureus is a widespread and highly virulent pathogen that can cause superficial and invasive infections. Interactions between S. aureus surface receptors and the extracellular matrix protein fibronectin mediate the bacterial invasion of host cells and is implicated in the colonization of medical implant surfaces. In this study, we investigate the role of distribution of both fibronectin and cellular receptors on the adhesion of S. aureus to interfaces as a model for primary adhesion at tissue interfaces or biomaterials. We present fibronectin in patches of systematically varied size (100-1000 nm) in a background of protein and bacteria rejecting chemistry based on PLL-g-PEG and studied S. aureus adhesion under flow. We developed a single molecule imaging assay for localizing fibronectin binding receptors on the surface of S. aureus via the super-resolution DNA points accumulation for imaging in nanoscale topography (DNA-PAINT) technique. Our results indicate that S. aureus adhesion to fibronectin biointerfaces is regulated by the size of available ligand patterns, with an adhesion threshold of 300 nm and larger. DNA-PAINT was used to visualize fibronectin binding receptor organization in situ at ∼7 nm localization precision and with a surface density of 38-46 μm^-2, revealing that the engagement of two or more receptors is required for strong S. aureus adhesion to fibronectin biointerfaces.

Host blood proteins as bridging ligand in bacterial aggregation as well as anchor point for adhesion in the molecular pathogenesis of Staphylococcus aureus infections

Fibronectin (Fn) and fibrinogen (Fg) are major host proteins present in the extracellular matrix, blood, and coatings on indwelling medical devices. The ability of Staphylococcus aureus to cause infections in humans depends on favorable interactions with these host ligands. Closely related bacterial adhesins, fibronectin-binding proteins A and B (FnBPA, FnBPB) were evaluated for two key steps in pathogenesis: clumping and adhesion. Experiments utilized optical spectrophotometry, flow cytometry, and atomic force microscopy to probe FnBPA/B alone or in combination in seven different strains of S. aureus and Lactococcus lactis, a Gram-positive surrogate that naturally lacks adhesins to mammalian ligands. In the absence of soluble ligands, both FnBPA and FnBPB were capable of interacting with adjacent FnBPs from neighboring bacteria to mediate clumping. In the presence of soluble host ligands, clumping was enhanced particularly under shear stress and with Fn present in the media. FnBPB exhibited greater ability to clump compared to FnBPA. The strength of adhesion was similar for immobilized Fn to FnBPA and FnBPB. These findings suggest that these two distinct but closely related bacterial adhesins, have different functional capabilities to interact with host ligands in different settings (e.g., soluble vs. immobilized). Survival and persistence of S. aureus in a human host may depend on complementary roles of FnBPA and FnBPB as they interact with different conformations of Fn or Fg (compact in solution vs. extended on a surface) present in different physiological spaces.

Staphylococcus aureus Strain-Dependent Biofilm Formation in Bone-Like Environment

Staphylococcus aureus species is an important threat for hospital healthcare because of frequent colonization of indwelling medical devices such as bone and joint prostheses through biofilm formations, leading to therapeutic failure. Furthermore, bacteria within biofilm are less sensitive to the host immune system responses and to potential antibiotic treatments. We suggested that the periprosthetic bone environment is stressful for bacteria, influencing biofilm development. To provide insights into S. aureus biofilm properties of three strains [including one methicillin-resistant S. aureus (MRSA)] under this specific environment, we assessed several parameters related to bone conditions and expected to affect biofilm characteristics. We reported that the three strains harbored different behaviors in response to the lack of oxygen, casamino acids and glucose starvation, and high concentration of magnesium. Each strain presented different biofilm biomass and live adherent cells proportion, or matrix production and composition. However, the three strains shared common responses in a bone-like environment: a similar production of extracellular DNA and engagement of the SOS response. This study is a step toward a better understanding of periprosthetic joint infections and highlights targets, which could be common among S. aureus strains and for future antibiofilm strategies.

Orthopedic Implant-Related Biofilm Pathophysiology: A Review of the Literature

Orthopedic implant-related infections remain a major problem even nowadays. Bacterial resistance through biofilm formation, in addition to the limited treatment options available, has resulted in an increased effort to better understand pathophysiology mechanisms. We performed a review of the literature in order to identify major biofilm formation pathways through which possible treatment strategies could arise.

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.

Binding Strength of Gram-Positive Bacterial Adhesins

Bacterial pathogens are equipped with specialized surface-exposed proteins that bind strongly to ligands on host tissues and biomaterials. These adhesins play critical roles during infection, especially during the early step of adhesion where the cells are exposed to physical stress. Recent single-molecule experiments have shown that staphylococci interact with their ligands through a wide diversity of mechanosensitive molecular mechanisms. Adhesin-ligand interactions are activated by tensile force and can be ten times stronger than classical non-covalent biological bonds. Overall these studies demonstrate that Gram-positive adhesins feature unusual stress-dependent molecular interactions, which play essential roles during bacterial colonization and dissemination. With an increasing prevalence of multidrug resistant infections caused by Staphylococcus aureus and Staphylococcus epidermidis, chemotherapeutic targeting of adhesins offers an innovative alternative to antibiotics.

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.

Probing ligand-receptor bonds in physiologically relevant conditions using AFM

Cell surface receptors, often called transmembrane receptors, are key cellular components as they control and mediate cell communication and signalling, converting extracellular signals into intracellular signals. Elucidating the molecular details of ligand binding (cytokine, growth factors, hormones, pathogens,...) to cell surface receptors and how this binding triggers conformational changes that initiate intracellular signalling is needed to improve our understanding of cellular processes and for rational drug design. Unfortunately, the molecular complexity and high hydrophobicity of membrane proteins significantly hamper their structural and functional characterization in conditions mimicking their native environment. With its piconewton force sensitivity and (sub)nanometer spatial resolution, together with the capability of operating in liquid environment and at physiological temperature, atomic force microscopy (AFM) has proven to be one of the most powerful tools to image and quantify receptor-ligand bonds in situ under physiologically relevant conditions. In this article, a brief overview of the rapid evolution of AFM towards quantitative biological mapping will be given, followed by selected examples highlighting the main advances that AFM-based ligand-receptor studies have brought to the fields of cell biology, immunology, microbiology, and virology, along with future prospects and challenges. Graphical abstract.

Mechanical Forces Guiding Staphylococcus aureus Cellular Invasion

Staphylococcus aureus can invade various types of mammalian cells, thereby enabling it to evade host immune defenses and antibiotics. The current model for cellular invasion involves the interaction between the bacterial cell surface located fibronectin (Fn)-binding proteins (FnBPA and FnBPB) and the α5β1 integrin in the host cell membrane. While it is believed that the extracellular matrix protein Fn serves as a bridging molecule between FnBPs and integrins, the fundamental forces involved are not known. Using single-cell and single-molecule experiments, we unravel the molecular forces guiding S. aureus cellular invasion, focusing on the prototypical three-component FnBPA-Fn-integrin interaction. We show that FnBPA mediates bacterial adhesion to soluble Fn via strong forces (∼1500 pN), consistent with a high-affinity tandem β-zipper, and that the FnBPA-Fn complex further binds to immobilized α5β1 integrins with a strength much higher than that of the classical Fn-integrin bond (∼100 pN). The high mechanical stability of the Fn bridge favors an invasion model in which Fn binding by FnBPA leads to the exposure of cryptic integrin-binding sites via allosteric activation, which in turn engage in a strong interaction with integrins. This activation mechanism emphasizes the importance of protein mechanobiology in regulating bacterial-host adhesion. We also find that Fn-dependent adhesion between S. aureus and endothelial cells strengthens with time, suggesting that internalization occurs within a few minutes. Collectively, our results provide a molecular foundation for the ability of FnBPA to trigger host cell invasion by S. aureus and offer promising prospects for the development of therapeutic approaches against intracellular pathogens.

Single Molecule Force Spectroscopy Reveals Two-Domain Binding Mode of Pilus-1 Tip Protein RrgA of Streptococcus pneumoniae to Fibronectin

For host cell adhesion and invasion, surface piliation procures benefits for bacteria. A detailed investigation of how pili adhere to host cells is therefore a key aspect in understanding their role during infection. Streptococcus pneumoniae TIGR 4, a clinical relevant serotype 4 strain, is capable of expressing pilus-1 with terminal RrgA, an adhesin interacting with host extracellular matrix (ECM) proteins. We used single molecule force spectroscopy to investigate the binding of full-length RrgA and single RrgA domains to fibronectin. Our results show that full-length RrgA and its terminal domains D3 and D4 bind to fibronectin with forces of 51.6 (full length), 52.8 (D3), and 46.2 pN (D4) at force-loading rates of around 1500 pN/s. Selective saturation of D3 and D4 binding sites on fibronectin showed that both domains can interact simultaneously with fibronectin, revealing a two-domain binding mechanism for the pilus-1 tip protein. The high off rates and the corresponding short lifetime of the RrgA Fn bond (τ = 0.26 s) may enable piliated pneumococci to form and maintain a transient contact to fibronectin-containing host surfaces and thus to efficiently scan the surface for specific receptors promoting host cell adhesion and invasion. These molecular properties could be essential for S. pneumoniae pili to mediate initial contact to the host cells and-shared with other piliated Gram-positive bacteria-favor host invasion.

Amino acid polymorphisms in the fibronectin-binding repeats of fibronectin-binding protein A affect bond strength and fibronectin conformation

The Staphylococcus aureus cell surface contains cell wall-anchored proteins such as fibronectin-binding protein A (FnBPA) that bind to host ligands (e.g. fibronectin; Fn) present in the extracellular matrix of tissue or coatings on cardiac implants. Recent clinical studies have found a correlation between cardiovascular infections caused by S. aureus and nonsynonymous SNPs in FnBPA. Atomic force microscopy (AFM), surface plasmon resonance (SPR), and molecular simulations were used to investigate interactions between Fn and each of eight 20-mer peptide variants containing amino acids Ala, Asn, Gln, His, Ile, and Lys at positions equivalent to 782 and/or 786 in Fn-binding repeat-9 of FnBPA. Experimentally measured bond lifetimes (1/k_off) and dissociation constants (K_d = k_off/k_on), determined by mechanically dissociating the Fn·peptide complex at loading rates relevant to the cardiovascular system, varied from the lowest-affinity H782A/K786A peptide (0.011 s, 747 μm) to the highest-affinity H782Q/K786N peptide (0.192 s, 15.7 μm). These atomic force microscopy results tracked remarkably well to metadynamics simulations in which peptide detachment was defined solely by the free-energy landscape. Simulations and SPR experiments suggested that an Fn conformational change may enhance the stability of the binding complex for peptides with K786I or H782Q/K786I (K_d^app = 0.2-0.5 μm, as determined by SPR) compared with the lowest-affinity double-alanine peptide (K_d^app = 3.8 μm). Together, these findings demonstrate that amino acid substitutions in Fn-binding repeat-9 can significantly affect bond strength and influence the conformation of Fn upon binding. They provide a mechanistic explanation for the observation of nonsynonymous SNPs in fnbA among clinical isolates of S. aureus that cause endovascular infections.

Forces guiding staphylococcal adhesion

Staphylococcus epidermidis and Staphylococcus aureus are two important nosocomial pathogens that form biofilms on indwelling medical devices. Biofilm infections are difficult to fight as cells within the biofilm show increased resistance to antibiotics. Our understanding of the molecular interactions driving bacterial adhesion, the first stage of biofilm formation, has long been hampered by the paucity of appropriate force-measuring techniques. In this minireview, we discuss how atomic force microscopy techniques have enabled to shed light on the molecular forces at play during staphylococcal adhesion. Specific highlights include the study of the binding mechanisms of adhesion molecules by means of single-molecule force spectroscopy, the measurement of the forces involved in whole cell interactions using single-cell force spectroscopy, and the probing of the nanobiophysical properties of living bacteria via multiparametric imaging. Collectively, these findings emphasize the notion that force and function are tightly connected in staphylococcal adhesion.

A detailed guideline for the fabrication of single bacterial probes used for atomic force spectroscopy

The atomic force microscope (AFM) evolved as a standard device in modern microbiological research. However, its capability as a sophisticated force sensor is not used to its full capacity. The AFM turns into a unique tool for quantitative adhesion research in bacteriology by using "bacterial probes". Thereby, bacterial probes are AFM cantilevers that provide a single bacterium or a cluster of bacteria as the contact-forming object. We present a step-by-step protocol for preparing bacterial probes, performing force spectroscopy experiments and processing force spectroscopy data. Additionally, we provide a general insight into the field of bacterial cell force spectroscopy.

Polymorphisms in Fibronectin Binding Proteins A and B among Staphylococcus aureus Bloodstream Isolates Are Not Associated with Arthroplasty Infection

Background

Nonsynonymous single nucleotide polymorphisms (SNPs) in fibronectin binding protein A (fnbA) of Staphylococcus aureus are associated with cardiac device infections. However, the role of fnbA SNPs in S. aureus arthroplasty infection is unknown.

Methods

Bloodstream S. aureus isolates from a derivation cohort of patients at a single U.S. medical center with S. aureus bacteremia (SAB) and prosthetic hip or knee arthroplasties that were infected (PJI, n = 27) or uninfected (PJU, n = 43) underwent sequencing of fnbA and fnbB. A validation cohort of S. aureus bloodstream PJI (n = 12) and PJU (n = 58) isolates from Germany also underwent fnbA and fnbB sequencing.

Results

Overall, none of the individual fnbA or fnbB SNPs were significantly associated with the PJI or PJU clinical groups within the derivation cohort. Similarly, none of the individual fnbA or fnbB SNPs were associated with PJI or PJU when the analysis was restricted to patients with either early SAB (i.e., bacteremia occurring <1 year after placement or manipulation of prostheses) or late SAB (i.e., bacteremia >1 year after placement or manipulation of prostheses).

Conclusions

In contrast to cardiac device infections, there is no association between nonsynonymous SNPs in fnbA or fnbB of bloodstream S. aureus isolates and arthroplasty infection. These results suggest that initial steps leading to S. aureus infection of cardiovascular and orthopedic prostheses may arise by distinct processes.

Endovascular infections caused by methicillin-resistant Staphylococcus aureus are linked to clonal complex-specific alterations in binding and invasion domains of fibronectin-binding protein A as well as the occurrence of fnbB

Endovascular infections caused by Staphylococcus aureus involve interactions with fibronectin present as extracellular matrix or surface ligand on host cells. We examined the expression, structure, and binding activity of the two major S. aureus fibronectin-binding proteins (FnBPA, FnBPB) in 10 distinct, methicillin-resistant clinical isolates from patients with either persistent or resolving bacteremia. The persistent bacteremia isolates (n = 5) formed significantly stronger bonds with immobilized fibronectin as determined by dynamic binding measurements performed with atomic force microscopy. Several notable differences were also observed when the results were grouped by clonal complex 5 (CC5) strains (n = 5) versus CC45 strains (n = 5). Fibronectin-binding receptors on CC5 formed stronger bonds with immobilized fibronectin (P < 0.001). The fnbA gene was expressed at higher levels in CC45, whereas fnbB was found in only CC5 isolates. The fnbB gene was not sequenced because all CC45 isolates lacked this gene. Instead, comparisons were made for fnbA, which was present in all 10 isolates. Sequencing of fnbA revealed discrete differences within high-affinity, fibronectin-binding repeats (FnBRs) of FnBPA that included (i) 5-amino-acid polymorphisms in FnBR-9, FnBR-10, and FnBR-11 involving charged or polar side chains, (ii) an extra, 38-amino-acid repeat inserted between FnBR-9 and FnBR-10 exclusively seen in CC45 isolates, and (iii) CC5 isolates had the SVDFEED epitope in FnBR-11 (a sequence shown to be essential for fibronectin binding), while this sequence was replaced in all CC45 isolates with GIDFVED (a motif known to favor host cell invasion at the cost of reduced fibronectin binding). These complementary sequence and binding data suggest that differences in fnbA and fnbB, particularly polymorphisms and duplications in FnBPA, give S. aureus two distinct advantages in human endovascular infections: (i) FnBPs similar to that of CC5 enhance ligand binding and foster initiation of disease, and (ii) CC45-like FnBPs promote cell invasion, a key attribute in persistent endovascular infections.

Nanoadhesion of Staphylococcus aureus onto Titanium Implant Surfaces

Adhesion of bacteria to dental implant surfaces is the critical initial step in the process of biofilm colonization; however, the specific nanoadhesive interactions occurring during the first contact between bacterial cells and biomaterial substrates remain poorly understood. In this report, we utilize single-cell force spectroscopy to characterize the dynamics of the initial interaction between living Staphylococcus aureus cells and machined titanium surfaces at the nanoscale. Values for maximum adhesion force were found to increase from 0-s (-0.27 ± 0.30 nN) to 60-s (-9.15 ± 0.78 nN) surface delays, with similar results observed for total adhesion work (7.39 ± 2.38 and 988.06 ± 117.08 aJ, respectively). Single unbinding events observed at higher surface delays were modeled according to the wormlike chain model, obtaining molecular contour-length predictions of 314.06 ± 9.27 nm. Average single-bond rupture forces of -0.95 ± 0.04 nN were observed at increased contact times. Short- and long-range force components of bacterial adhesion were obtained by Poisson analysis of single unbinding event peaks, yielding values of -0.75 ± 0.04 and -0.58 ± 0.15 nN, respectively. Addition of 2-mg/mL chlorhexidine to the buffer solution resulted in the inhibition of specific adhesive events but an increased overall adhesion force and work. These results suggest that initial attachment of S. aureus to smooth titanium is mostly mediated by short-range attractive forces observed at higher surface delays.

Staphylococcus aureus Fibronectin-Binding Protein A Mediates Cell-Cell Adhesion through Low-Affinity Homophilic Bonds

Staphylococcus aureus is an important opportunistic pathogen which is a leading cause of biofilm-associated infections on indwelling medical devices. The cell surface-located fibronectin-binding protein A (FnBPA) plays an important role in the accumulation phase of biofilm formation by methicillin-resistant S. aureus (MRSA), but the underlying molecular interactions are not yet established. Here, we use single-cell and single-molecule atomic force microscopy to unravel the mechanism by which FnBPA mediates intercellular adhesion. We show that FnBPA is responsible for specific cell-cell interactions that involve the FnBPA A domain and cause microscale cell aggregation. We demonstrate that the strength of FnBPA-mediated adhesion originates from multiple low-affinity homophilic interactions between FnBPA A domains on neighboring cells. Low-affinity binding by means of FnBPA may be important for biofilm dynamics. These results provide a molecular basis for the ability of FnBPA to promote cell accumulation during S. aureus biofilm formation. We speculate that homophilic interactions may represent a generic strategy among staphylococcal cell surface proteins for guiding intercellular adhesion. As biofilm formation by MRSA strains depends on proteins rather than polysaccharides, our approach offers exciting prospects for the design of drugs or vaccines to inhibit protein-dependent intercellular interactions in MRSA biofilms.

IMPORTANCE

Staphylococcus aureus is a human pathogen that forms biofilms on indwelling medical devices, such as central venous catheters and prosthetic joints. This leads to biofilm infections that are difficult to treat with antibiotics because many cells within the biofilm matrix are dormant. The fibronectin-binding proteins (FnBPs) FnBPA and FnBPB promote biofilm formation by clinically relevant methicillin-resistant S. aureus (MRSA) strains, but the molecular mechanisms involved remain poorly understood. We used atomic force microscopy techniques to demonstrate that FnBPA mediates cell-cell adhesion via multiple, low-affinity homophilic bonds between FnBPA A domains on adjacent cells. Therefore, FnBP-mediated homophilic interactions represent an interesting target to prevent MRSA biofilms. We propose that such homophilic mechanisms may be widespread among staphylococcal cell surface proteins, providing a means to guide intercellular adhesion and biofilm accumulation.

Atomic force microscopy in microbiology: new structural and functional insights into the microbial cell surface

Microbial cells sense and respond to their environment using their surface constituents. Therefore, understanding the assembly and biophysical properties of cell surface molecules is an important research topic. With its ability to observe living microbial cells at nanometer resolution and to manipulate single-cell surface molecules, atomic force microscopy (AFM) has emerged as a powerful tool in microbiology. Here, we survey major breakthroughs made in cell surface microbiology using AFM techniques, emphasizing the most recent structural and functional insights.

The binding force of the staphylococcal adhesin SdrG is remarkably strong

SdrG is a cell surface adhesin from Staphylococcus epidermidis which binds to the blood plasma protein fibrinogen (Fg). Ligand binding follows a 'dock, lock and latch' model involving dynamic conformational changes of the adhesin that result in a greatly stabilized adhesin-ligand complex. To date, the force and dynamics of this multistep interaction are poorly understood. Here we use atomic force microscopy (AFM) to unravel the binding strength and cell surface localization of SdrG at molecular resolution. Single-cell force spectroscopy shows that SdrG mediates time-dependent attachment to Fg-coated surfaces. Single-molecule force spectroscopy with Fg-coated AFM tips demonstrates that the adhesin forms nanoscale domains on the cell surface, which we believe contribute to strengthen cell adhesion. Notably, we find that the rupture force of single SdrG-Fg bonds is very large, ∼ 2 nN, equivalent to the strength of a covalent bond, and shows a low dissociation rate, suggesting that the bond is very stable. The strong binding force, slow dissociation and clustering of SdrG provide a molecular foundation for the ability of S. epidermidis to colonize implanted biomaterials and to withstand physiological shear forces.

Novel materials for biofilm reactors and their characterization

The application of adherently growing microorganisms for biotechnological production processes is established, but it is still a niche technology with only a small economic impact. However, novel approaches are under development for new types of biofilm reactors. In this context, increasingly more microstructured metal surfaces are being investigated, and they show positive effects on the bacterial growth and the biofilm establishment. However, for comparison of the data, the different surface materials have to correspond in their different characteristics, such as wettability and chemical composition. Also, new materials, such as plastic composite supports, were developed. To understand the interaction between these new materials and the biofilm-producing microorganisms, different surface science methods have to be applied to reveal a detailed knowledge of the surface characteristics. In conclusion, microstructured surfaces show a high potential for enhanced biofilm growth, probably accompanied by an enhanced productivity of the microorganisms.

Molecular characterization of endocarditis-associated Staphylococcus aureus

Infective endocarditis (IE) is a life-threatening infection of the heart endothelium and valves. Staphylococcus aureus is a predominant cause of severe IE and is frequently associated with infections in health care settings and device-related infections. Multilocus sequence typing (MLST), spa typing, and virulence gene microarrays are frequently used to classify S. aureus clinical isolates. This study examined the utility of these typing tools to investigate S. aureus epidemiology associated with IE. Ninety-seven S. aureus isolates were collected from patients diagnosed with (i) IE, (ii) bloodstream infection related to medical devices, (iii) bloodstream infection not related to medical devices, and (iv) skin or soft-tissue infections. The MLST clonal complex (CC) for each isolate was determined and compared to the CCs of members of the S. aureus population by eBURST analysis. The spa type of all isolates was also determined. A null model was used to determine correlations of IE with CC and spa type. DNA microarray analysis was performed, and a permutational analysis of multivariate variance (PERMANOVA) and principal coordinates analysis were conducted to identify genotypic differences between IE and non-IE strains. CC12, CC20, and spa type t160 were significantly associated with IE S. aureus. A subset of virulence-associated genes and alleles, including genes encoding staphylococcal superantigen-like proteins, fibrinogen-binding protein, and a leukocidin subunit, also significantly correlated with IE isolates. MLST, spa typing, and microarray analysis are promising tools for monitoring S. aureus epidemiology associated with IE. Further research to determine a role for the S. aureus IE-associated virulence genes identified in this study is warranted.

Infection of orthopedic implants with emphasis on bacterial adhesion process and techniques used in studying bacterial-material interactions

Staphylococcus comprises up to two-thirds of all pathogens in orthopedic implant infections and they are the principal causative agents of two major types of infection affecting bone: septic arthritis and osteomyelitis, which involve the inflammatory destruction of joint and bone. Bacterial adhesion is the first and most important step in implant infection. It is a complex process influenced by environmental factors, bacterial properties, material surface properties and by the presence of serum or tissue proteins. Properties of the substrate, such as chemical composition of the material, surface charge, hydrophobicity, surface roughness and the presence of specific proteins at the surface, are all thought to be important in the initial cell attachment process. The biofilm mode of growth of infecting bacteria on an implant surface protects the organisms from the host immune system and antibiotic therapy. The research for novel therapeutic strategies is incited by the emergence of antibiotic-resistant bacteria. This work will provide an overview of the mechanisms and factors involved in bacterial adhesion, the techniques that are currently being used studying bacterial-material interactions as well as provide insight into future directions in the field.

Dissociation rate constants of human fibronectin binding to fibronectin-binding proteins on living Staphylococcus aureus isolated from clinical patients

Staphylococcus aureus is part of the indigenous microbiota of humans. Sometimes, S. aureus bacteria enter the bloodstream, where they form infections on implanted cardiovascular devices. A critical, first step in such infections is a bond that forms between fibronectin-binding protein (FnBP) on S. aureus and host proteins, such as fibronectin (Fn), that coat the surface of implants in vivo. In this study, native FnBPs on living S. aureus were shown to form a mechanically strong conformational structure with Fn by atomic force microscopy. The tensile acuity of this bond was probed for 46 bloodstream isolates, each from a patient with a cardiovascular implant. By analyzing the force spectra with the worm-like chain model, we determined that the binding events were consistent with a multivalent, cluster bond consisting of ~10 or ~80 proteins in parallel. The dissociation rate constant (k(off), s(-1)) of each multibond complex was determined by measuring strength as a function of the loading rate, normalized by the number of bonds. The bond lifetime (1/k(off)) was two times longer for bloodstream isolates from patients with an infected device (1.79 or 69.47 s for the 10- or 80-bond clusters, respectively; n = 26 isolates) relative to those from patients with an uninfected device (0.96 or 34.02 s; n = 20 isolates). This distinction could not be explained by different amounts of FnBP, as confirmed by Western blots. Rather, amino acid polymorphisms within the Fn-binding repeats of FnBPA explain, at least partially, the statistically (p < 0.05) longer bond lifetime for isolates associated with an infected cardiovascular device.

Polymorphisms in fibronectin binding protein A of Staphylococcus aureus are associated with infection of cardiovascular devices

Medical implants, like cardiovascular devices, improve the quality of life for countless individuals but may become infected with bacteria like Staphylococcus aureus. Such infections take the form of a biofilm, a structured community of bacterial cells adherent to the surface of a solid substrate. Every biofilm begins with an attractive force or bond between bacterium and substratum. We used atomic force microscopy to probe experimentally forces between a fibronectin-coated surface (i.e., proxy for an implanted cardiac device) and fibronectin-binding receptors on the surface of individual living bacteria from each of 80 clinical isolates of S. aureus. These isolates originated from humans with infected cardiac devices (CDI; n = 26), uninfected cardiac devices (n = 20), and the anterior nares of asymptomatic subjects (n = 34). CDI isolates exhibited a distinct binding-force signature and had specific single amino acid polymorphisms in fibronectin-binding protein A corresponding to E652D, H782Q, and K786N. In silico molecular dynamics simulations demonstrate that residues D652, Q782, and N786 in fibronectin-binding protein A form extra hydrogen bonds with fibronectin, complementing the higher binding force and energy measured by atomic force microscopy for the CDI isolates. This study is significant, because it links pathogenic bacteria biofilms from the length scale of bonds acting across a nanometer-scale space to the clinical presentation of disease at the human dimension.

A tactile response in Staphylococcus aureus

It is well established that bacteria are able to respond to temporal gradients (e.g., by chemotaxis). However, it is widely held that prokaryotes are too small to sense spatial gradients. This contradicts the common observation that the vast majority of bacteria live on the surface of a solid substrate (e.g., as a biofilm). Herein we report direct experimental evidence that the nonmotile bacterium Staphylococcus aureus possesses a tactile response, or primitive sense of touch, that allows it to respond to spatial gradients. Attached cells recognize their substrate interface and localize adhesins toward that region. Braille-like avidity maps reflect a cell's biochemical sensory response and reveal ultrastructural regions defined by the actual binding activity of specific proteins.