SdrG, a fibrinogen-binding bacterial adhesin of the microbial surface components recognizing adhesive matrix molecules subfamily from Staphylococcus epidermidis, targets the thrombin cleavage site in the Bbeta chain

Host Soluble Factors Cause Changes in Staphylococcus epidermidis Antibiotic Susceptibility and Biofilm Formation Ability

Staphylococcus epidermidis is a major nosocomial pathogen with a remarkable ability to adhere to the surfaces of indwelling medical devices and form biofilms. Unlike other nosocomial pathogens, the interaction of S. epidermidis with host factors has not been the focus of substantial research. This study aimed to assess the alterations in the antibiotic susceptibility and biofilm formation ability of S. epidermidis in the presence of host serum factors. S. epidermidis strain RP62A was cultured in a laboratory culture medium with or without human serum/plasma, and changes in antibiotic susceptibility, biofilm formation, and gene expression were evaluated. The data obtained revealed that exposure to host serum factors increased the susceptibility of S. epidermidis to glycopeptide antibiotics and was also detrimental to biofilm formation. Gene expression analysis revealed downregulation of both dltA and fmtC genes shortly after human serum/plasma exposure. The importance of transferrin-mediated iron sequestration as a host anti-biofilm strategy against S. epidermidis was also emphasized. We have demonstrated that serum factors play a pivotal role as part of the host's anti-infective strategy against S. epidermidis infections, highlighting the importance of incorporating such factors during in vitro studies with this pathogen.

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.

Convergent behavior of extended stalk regions from staphylococcal surface proteins with widely divergent sequence patterns

Staphylococcus epidermidis and S. aureus are highly problematic bacteria in hospital settings. This stems, at least in part, from strong abilities to form biofilms on abiotic or biotic surfaces. Biofilms are well-organized multicellular aggregates of bacteria, which, when formed on indwelling medical devices, lead to infections that are difficult to treat. Cell wall-anchored (CWA) proteins are known to be important players in biofilm formation and infection. Many of these proteins have putative stalk-like regions or regions of low complexity near the cell wall-anchoring motif. Recent work demonstrated the strong propensity of the stalk region of the S. epidermidis accumulation-associated protein (Aap) to remain highly extended under solution conditions that typically induce compaction or other significant conformational changes. This behavior is consistent with the expected function of a stalk-like region that is covalently attached to the cell wall peptidoglycan and projects the adhesive domains of Aap away from the cell surface. In this study, we evaluate whether the ability to resist compaction is a common theme among stalk regions from various staphylococcal CWA proteins. Circular dichroism spectroscopy was used to examine secondary structure changes as a function of temperature and cosolvents along with sedimentation velocity analytical ultracentrifugation and SAXS to characterize structural characteristics in solution. All stalk regions tested are intrinsically disordered, lacking secondary structure beyond random coil and polyproline type II helix, and they all sample highly extended conformations. Remarkably, the Ser-Asp dipeptide repeat region of SdrC exhibited nearly identical behavior in solution when compared to the Aap Pro/Gly-rich region, despite highly divergent sequence patterns, indicating conservation of function by various distinct staphylococcal CWA protein stalk regions.

A Review on Host-Leptospira Interactions: What We Know and Future Expectations

Leptospirosis is a widespread zoonosis caused by pathogenic Leptospira spp. It is considered a neglected infectious disease of human and veterinary concern. Our group has been investigating proteins annotated as hypothetical, predicted to be located on the leptospiral surface. Because of their location, these proteins may have the ability to interact with various host components, which could allow establishment of the infection. These proteins act as adherence factors by binding to host receptor molecules, such as the extracellular matrix (ECM) components laminin and glycosaminoglycans to help bacterial colonization. Leptospira also interacts with the host fibrinolytic system, which has been demonstrated to be a powerful tool for invasion mechanisms. The interaction with fibrinogen and thrombin has been shown to reduce fibrin clot formation. Additionally, the degradation of coagulation cascade components by secreted proteases or by acquired surface plasmin could also play a role in reducing clot formation, hence facilitating dissemination during infection. Interaction with host complement system regulators also plays a role in helping bacteria to evade the immune system, facilitating invasion. Interaction of Leptospira to cell receptors, such as cadherins, can contribute to investigate molecules that participate in virulence. To achieve a better understanding of the host-pathogen interaction, leptospiral mutagenesis tools have been developed and explored. This work presents several proteins that mediate binding to components of the ECM, plasma, components of the complement system and cells, to gather research achievements that can be helpful in better understanding the mechanisms of leptospiral-host interactions and discuss genetic manipulation for Leptospira spp. aimed at protein function validation.

The Role of Fibrin(ogen) in Wound Healing and Infection Control

Fibrinogen, one of the most abundant plasma proteins playing a key role in hemostasis, is an important modulator of wound healing and host defense against microbes. In the current review, we address the role of fibrin(ogen) throughout the process of wound healing and subsequent tissue repair. Initially fibrin(ogen) acts as a provisional matrix supporting incoming leukocytes and acting as reservoir for growth factors. It later goes on to support re-epithelialization, angiogenesis, and fibroplasia. Importantly, removal of fibrin(ogen) from the wound is essential for wound healing to progress. We also discuss how fibrin(ogen) functions through several mechanisms to protect the host against bacterial infection by providing a physical barrier, entrapment of bacteria in fibrin(ogen) networks, and by directing immune cell function. The central role of fibrin(ogen) in defense against bacterial infection has made it a target of bacterial proteins, evolved to interact with fibrin(ogen) to manipulate clot formation and degradation for the purpose of promoting microbial virulence and survival. Further understanding of the dual roles of fibrin(ogen) in wound healing and infection could provide novel means of therapy to improve recovery from surgical or chronic wounds and help to prevent infection from highly virulent bacterial strains, including those resistant to antibiotics.

Identification of Three Clf-Sdr Subfamily Proteins in Staphylococcus warneri, and Comparative Genomics Analysis of a Locus Encoding CWA Proteins in Staphylococcus Species

Coagulase-negative Staphylococcus warneri is an opportunistic pathogen that is capable of causing several infections, especially in patients with indwelling medical devices. Here, we determined the complete genome sequence of a clinical S. warneri strain isolated from the blood culture of a 1-year-old nursling patient with acute upper respiratory infection. Genome-wide phylogenetic analysis confirmed the phylogenetic relationships between S. warneri and other Staphylococcus species. Using comparative genomics, we identified three cell wall-anchored (CWA) proteins at the same locus (sdr), named SdrJ, SdrK, and SdrL, on the chromosome sequences of different S. warneri strains. Structural predictions showed that SdrJ/K/L have structural features characteristic of Sdr proteins but exceptionally contained an unusual N-terminal repeat region. However, the C-terminal repetitive (R) region of SdrJ contains a significantly larger proportion of alanine (142/338, 42.01%) than the previously reported SdrI (37.00%). Investigation of the genetic organization revealed that the sdrJ/K/L genes were always followed by one or two glycosyltransferase genes, gtfA and gtfB and were present in an ∼56 kb region bordered by a pair of 8 bp identical direct repeats, named Sw-Sdr. This region was further found to be located on a 160-kb region subtended by a pair of 160-bp direct repeats along with other virulence genes and resistance genes. Sw-Sdr contained a putative integrase that was probably a remnant of a functional integrase. Evidence suggests that Sw-Sdr is improbably an efficient pathogenicity island. A large-scale investigation of Staphylococcus genomes showed that sdr loci were a potential hotspot of insertion sequences (ISs), which could lead to intraspecific diversity at these loci. Our work expanded the repository of Staphylococcus Sdr proteins, and for the first time, we established the connection between sdr loci and phylogenetic relationships and compared the sdr loci in different Staphylococcus species, which provided large insights into the genetic environment of CWA genes in Staphylococcus.

Inhibition of Fibrinolysis by Streptococcal Phage LysinSM1

Expression of bacteriophage lysin_SM1 by Streptococcus oralis strain SF100 is thought to be important for the pathogenesis of infective endocarditis, due to its ability to mediate bacterial binding to fibrinogen. To better define the lysin_SM1 binding site on fibrinogen Aα, and to investigate the impact of binding on fibrinolysis, we examined the interaction of lysin_SM1 with a series of recombinant fibrinogen Aα variants. These studies revealed that lysin_SM1 binds the C-terminal region of fibrinogen Aα spanned by amino acid residues 534 to 610, with an affinity of equilibrium dissociation constant (K_D) of 3.23 × 10^-5 M. This binding site overlaps the known binding site for plasminogen, an inactive precursor of plasmin, which is a key protease responsible for degrading fibrin polymers. When tested in vitro, lysin_SM1 competitively inhibited plasminogen binding to the αC region of fibrinogen Aα. It also inhibited plasminogen-mediated fibrinolysis, as measured by thromboelastography (TEG). These results indicate that lysin_SM1 is a bi-functional virulence factor for streptococci, serving as both an adhesin and a plasminogen inhibitor. Thus, lysin_SM1 may facilitate the attachment of bacteria to fibrinogen on the surface of damaged cardiac valves and may also inhibit plasminogen-mediated lysis of infected thrombi (vegetations) on valve surfaces. IMPORTANCE The interaction of streptococci with human fibrinogen and platelets on damaged endocardium is a central event in the pathogenesis of infective endocarditis. Streptococcus oralis can bind platelets via the interaction of bacteriophage lysin_SM1 with fibrinogen on the platelet surface, and this process has been associated with increased virulence in an animal model of endocarditis. We now report that lysin_SM1 binds to the αC region of the human fibrinogen Aα chain. This interaction blocks plasminogen binding to fibrinogen and inhibits fibrinolysis. In vivo, this inhibition could prevent the lysis of infected vegetations, thereby promoting bacterial persistence and virulence.

Staphylococcus epidermidis Has Growth Phase Dependent Affinity for Fibrinogen and Resulting Fibrin Clot Elasticity

Bacterial infection and thrombosis are highly correlated, especially in patients with indwelling medical devices. Coagulase-negative staphylococci, typified by Staphylococcus epidermidis, are a common cause of medical device infections owing to their biofilm forming capacity which provides protection from antibiotics and host immune response. Attention has been drawn to the interaction between S. epidermidis and host proteins, specifically fibrinogen. However, little is known regarding the impact of the transition from planktonic to biofilm forming phenotype on this interaction. Here we investigate the growth phase dependence of bacteria-fibrinogen interaction and the resulting effect on fibrin clot formation, structure, and mechanics. Flow cytometry demonstrated growth phase dependent affinity for fibrinogen. To mimic intravascular device seeding, we quantified the adhesion of S. epidermidis to a fibrinogen coated surface under continuous flow conditions in vitro. The bacterial deposition rate onto fibrinogen was significantly greater for stationary (5,360 ± 1,776 cells/cm²s) versus exponential phase (2,212 ± 264, cells/cm² s). Furthermore, the expression of sdrG–a cell surface adhesion protein with specificity for fibrinogen–was upregulated ∼twofold in the stationary versus the exponential phase. Rheometry and confocal microscopy demonstrated that stationary phase S. epidermidis slows clot formation and generates a more heterogeneous fibrin network structure with greater elasticity (G′ = 5.7 ± 1.0 Pa) compared to sterile fibrinogen (G′ = l.5 ± 0.2 Pa), while exponential phase cells had little effect. This work contributes to the current understanding of the growth phase dependent regulation of bacterial virulence factors and the correlation between bacterial infection and thrombosis.

Adsorption of Fibrinogen on Silica Surfaces-The Effect of Attached Nanoparticles

When a biomaterial is inserted into the body, proteins rapidly adsorb onto its surface, creating a conditioning protein film that functions as a link between the implant and adhering cells. Depending on the nano-roughness of the surface, proteins will adsorb in different amounts, with different conformations and orientations, possibly affecting the subsequent attachment of cells to the surface. Thus, modifications of the surface nanotopography of an implant may prevent biomaterial-associated infections. Fibrinogen is of particular importance since it contains adhesion epitopes that are recognized by both eukaryotic and prokaryotic cells, and can therefore influence the adhesion of bacteria. The aim of this study was to model adsorption of fibrinogen to smooth or nanostructured silica surfaces in an attempt to further understand how surface nanotopography may affect the orientation of the adsorbed fibrinogen molecule. We used a coarse-grained model, where the main body of fibrinogen (visible in the crystal structure) was modeled as rigid and the flexible α C-chains (not visible in the crystal structure) were modeled as completely disordered. We found that the elongated fibrinogen molecule preferably adsorbs in such a way that it protrudes further into solution on a nanostructured surface compared to a flat one. This implicates that the orientation on the flat surface increases its bio-availability.

Staphylococcus epidermidis MSCRAMM SesJ Is Encoded in Composite Islands

Staphylococcus epidermidis is a leading cause of nosocomial infections in patients with a compromised immune system and/or an implanted medical device. Seventy to 90% of S. epidermidis clinical isolates are methicillin resistant and carry the mecA gene, present in a mobile genetic element (MGE) called the staphylococcal cassette chromosome mec (SCCmec) element. Along with the presence of antibiotic and heavy metal resistance genes, MGEs can also contain genes encoding secreted or cell wall-anchored virulence factors. In our earlier studies of S. epidermidis clinical isolates, we discovered S. epidermidis surface protein J (SesJ), a prototype of a recently discovered subfamily of the microbial surface component recognizing adhesive matrix molecule (MSCRAMM) group. MSCRAMMs are major virulence factors of pathogenic Gram-positive bacteria. Here, we report that the sesJ gene is always accompanied by two glycosyltransferase genes, gtfA and gtfB, and is present in two MGEs, called the arginine catabolic mobile element (ACME) and the staphylococcal cassette chromosome (SCC) element. The presence of the sesJ gene was associated with the left-hand direct repeat DR_B or DR_E. When inserted via DR_E, the sesJ gene was encoded in the SCC element. When inserted via DR_B, the sesJ gene was accompanied by the genes for the type 1 restriction modification system and was encoded in the ACME. Additionally, the SCC element and ACME carry different isoforms of the SesJ protein. To date, the genes encoding MSCRAMMs have been seen to be located in the bacterial core genome. Here, we report the presence of an MSCRAMM in an MGE in S. epidermidis clinical isolates.IMPORTANCES. epidermidis is an opportunistic bacterium that has established itself as a successful nosocomial pathogen. The modern era of novel therapeutics and medical devices has extended the longevity of human life, but at the same time, we also witness the evolution of pathogens to adapt to newly available niches in the host. Increasing antibiotic resistance among pathogens provides an example of such pathogen adaptation. With limited opportunities to modify the core genome, most of the adaptation occurs by acquiring new genes, such as virulence factors and antibiotic resistance determinants present in MGEs. In this study, we describe that the sesJ gene, encoding a recently discovered cell wall-anchored protein in S. epidermidis, is present in both ACME and the SCC element. The presence of virulence factors in MGEs can influence the virulence potential of a specific strain. Therefore, it is critical to study the virulence factors found in MGEs in emerging pathogenic bacteria or strains to understand the mechanisms used by these bacteria to cause infections.

Time-variable expression levels of mazF, atlE, sdrH, and bap genes during biofilm formation in Staphylococcus epidermidis

Staphylococcus epidermidis is an opportunistic pathogen causing infections related to the usage of implants and medical devices. Pathogenicity of this microorganism is mainly linked to its capability to form biofilm structures. Biofilm formation vastly depends on several factors including different proteins. We studied the expression levels of three proteins including SdrH, Bap, AtlE, and MazF at different time intervals during the course of biofilm formation. In this study, a catheter-derived S. epidermidis isolate with strong ability of biofilm formation was selected. PCR assay was used to detect sdrH, bap, atlE, and mazF genes in this isolate. Real-time PCR was used to determine the expression levels of these genes after 4, 8, and 20 h during the course of biofilm formation. The studied genes showed different expression levels at different time intervals during biofilm formation by real-time PCR method. Expression levels of atlE and sdrH genes were the highest at 4 h, whereas bap gene showed the highest expression level at 8 h during the course of biofilm formation. In addition, the expression level of mazF gene peaked at 4 h and then progressively decreased at 8 and 20 h. Our results suggest the importance of AtlE, SdrH, and MazF proteins in the establishment and development of the biofilm structure. In addition, our results showed the important role of protein Bap in the accumulation of biofilm structure. Future studies are required to understand the exact role of MazF in the process of biofilm formation.

Inoculation Pneumonia Caused by Coagulase Negative Staphylococcus

Rationale

Although frequently retrieved in tracheal secretions of critically ill patients on mechanical ventilation, the existence of pneumonia caused by coagulase-negative staphylococci (CoNS) remains controversial.

Objective

To assess whether Staphylococcus haemolyticus (S. haemolyticus) inoculated in mice’s trachea can infect normal lung parenchyma, increasing concentrations of S. haemolyticus were intratracheally administered in 221 immunocompetent mice.

Methods

Each animal received intratracheally phosphate-buffered saline (PBS) (n = 43) or live (n = 141) or inactivated (n = 37) S. haemolyticus at increasing load: 1.0 × 10⁶, 1.0 × 10⁷, and 1.0 × 10⁸ colony forming units (CFU). Forty-three animals were sacrificed at 12 h and 178 were sacrificed at 36 h; 64 served for post-mortem lung histology, 157 served for pre-mortem bronchoalveolar lavage (BAL) analysis, and 42 served for post-mortem quantitative bacteriology of lung tissue. The distribution of biofilm-associated genes was investigated in the S. haemolyticus strain used in our in vivo experiment as well as among 19 other clinical S. haemolyticus strains collected from hospitals or nursing houses.

Measurements and Main Results

Intratracheal inoculation of 1.0 × 10⁸ CFU live S. haemolyticus caused macroscopic and histological confluent pneumonia with significant increase in BAL white cell count, tumor necrosis factor-α (TNF-α), and macrophage inflammatory protein (MIP)-2. At 12 h, high concentrations of S. haemolyticus were identified in BAL. At 36 h, lung injury and BAL inflammation were less severe than at 12 h and moderate concentrations of species belonging to the oropharyngeal flora were identified in lung tissue. The inoculation of 1.0 × 10⁶ and 1.0 × 10⁷ CFU live S. haemolyticus caused histologic interstitial pneumonia and moderate BAL inflammation. Similar results were observed after inoculation of inactivated S. haemolyticus. Moreover, biofilm formation was a common phenotype in S. haemolyticus isolates. The low prevalence of the ica operon in our clinical S. haemolyticus strain collection indicated icaA and icaD independent-biofilm formation.

Conclusion

In immunocompetent spontaneously breathing mice, inoculation of S. haemolyticus causes concentration-dependent lung infection that spontaneously recovers over time. icaA and icaD independent biofilm formation is a common phenotype in S. haemolyticus isolates.

Sodium bicarbonate catheter lock solution reduces hemodialysis catheter loss due to catheter-related thrombosis and blood stream infection: an open-label clinical trial

BACKGROUND

There is no ideal lock solution that prevents hemodialysis (HD) catheter loss due to catheter-related thrombosis (CRT) and catheter-related bloodstream infection (CRBSI). Catheter loss is associated with increased hospitalization and high inpatient costs. Sodium bicarbonate (NaHCO3) demonstrates anti-infective and anticoagulation properties with a good safety profile, making it an ideal lock solution development target.The objective of this study was to determine the safety and efficacy of using sodium bicarbonate catheter lock solution (SBCLS) as a means of preventing HD catheter loss due to CRT and CRBSI.

METHODS

The study took place in a community hospital in Brooklyn, NY, USA. All admitted patients ≥18 years of age who needed HD treatment through CVC were included in the study. 451 patients included in the study were provided SBCLS or NSCLS post-dialysis. Catheter loss due to CRT or CRBSI was evaluated over a period of 546 days.

RESULTS

A total of 452 patients met the criteria; 1 outlier was excluded, 226 were in the NSCLS group and 225 were in the SBCLS group. There were no significant differences between groups in comorbidities at the outset. The NSCLS group had CRT and CRBSI rates of 4.1 and 2.6/1000 catheter days (CD), respectively, compared with 0.17/1000 CD for both outcomes in the SBCLS group. SBCLS patients had a significantly reduced catheter loss rate due to CRT (P < 0.0001) and CRBSI (P = 0.0004). NSCLS patients had higher odds of losing their catheter due to CRT {odds ratio [OR] 26.6 [95% confidence interval (CI) 3.57-198.52]} and CRBSI [OR 15.9 (95% CI 2.09-121.61)] during the study period.

CONCLUSION

The novel approach of using SBCLS was found to be safe and was statistically superior to normal saline in preventing HD catheter loss due to CRT and CRBSI. NaHCO3 solution is inexpensive, readily available in various settings and holds the potential to decrease hospitalization, length of stay and dialysis-related costs.

TRIAL REGISTRATION

Maimonides Medical Center Investigational Review Board, Study IRB 2015-06-25-CIH. ClinicalTrials.gov identifier: NCT03627884.

Crosstalk between Platelet and Bacteria: A Therapeutic Prospect

Platelets are typically recognized for their roles in the maintenance of hemostasis and vascular wall repair to reduce blood loss. Beyond hemostasis, platelets also play a critical role in pathophysiological conditions like atherosclerosis, stroke, thrombosis, and infections. During infection, platelets interact directly and indirectly with bacteria through a wide range of cellular and molecular mechanisms. Platelet surface receptors such as GPIbα, FcγRIIA, GPIIbIIIa, and TLRs, etc. facilitate direct interaction with bacterial cells. Besides, the indirect interaction between platelet and bacteria involves host plasma proteins such as von Willebrand Factor (vWF), fibronectin, IgG, and fibrinogen. Bacterial cells induce platelet activation, aggregation, and thrombus formation in the microvasculature. The activated platelets induce the Neutrophil Extracellular Traps (NETs) formation, which further contribute to thrombosis. Thus, platelets are extensively anticipated as vital immune modulator cells during infection, which may further lead to cardiovascular complications. In this review, we cover the interaction mechanisms between platelets and bacteria that may lead to the development of thrombotic disorders. Platelet receptors and other host molecules involved in such interactions can be used to develop new therapeutic strategies to combat against infection-induced cardiovascular complications. In addition, we highlight other receptor and enzyme targets that may further reduce infection-induced platelet activation and various pathological conditions.

Biofilm formation by staphylococci in health-related environments and recent reports on their control using natural compounds

Staphylococci are Gram-positive bacteria that are ubiquitous in the environment and able to form biofilms on a range of surfaces. They have been associated with a range of human health issues such as medical device-related infection, localized skin infection, or direct infection caused by toxin production. The extracellular material produced by these bacteria resists antibiotics and host defence mechanism which complicates the treatment process. The commonly reported Staphylococcus species are Staphylococcus aureus and S. epidermidis as they inhabit human bodies. However, the emergence of other staphylococci, such as S. haemolyticus, S. lugdunensis, S. saprophyticus, S. capitis, S. saccharolyticus, S. warneri, S. cohnii, and S. hominis, is also of concern and they have been associated with biofilm formation. This review critically assesses recent cases on the biofilm formation by S. aureus, S. epidermidis, and other staphylococci reported in health-related environments. The control of biofilm formation by staphylococci using natural compounds is specifically discussed as they represent potential anti-biofilm agents which may reduce the burden of antibiotic resistance.

Staphylococcus aureus: setting its sights on the human innate immune system

Staphylococcus aureus has colonized humans for at least 10 000 years, and today inhabits roughly a third of the population. In addition, S. aureus is a major pathogen that is responsible for a significant disease burden, ranging in severity from mild skin and soft-tissue infections to life-threatening endocarditis and necrotizing pneumonia, with treatment often hampered by resistance to commonly available antibiotics. Underpinning its versatility as a pathogen is its ability to evade the innate immune system. S. aureus specifically targets innate immunity to establish and sustain infection, utilizing a large repertoire of virulence factors to do so. Using these factors, S. aureus can resist phagosomal killing, impair complement activity, disrupt cytokine signalling and target phagocytes directly using proteolytic enzymes and cytolytic toxins. Although most of these virulence factors are well characterized, their importance during infection is less clear, as many display species-specific activity against humans or against animal hosts, including cows, horses and chickens. Several staphylococcal virulence factors display species specificity for components of the human innate immune system, with as few as two amino acid changes reducing binding affinity by as much as 100-fold. This represents a major issue for studying their roles during infection, which cannot be examined without the use of humanized infection models. This review summarizes the major factors S. aureus uses to impair the innate immune system, and provides an in-depth look into the host specificity of S. aureus and how this problem is being approached.

Biofilm Formation by ica-Negative Ocular Isolates of Staphylococcus haemolyticus

Staphylococcus haemolyticus is the second most frequently isolated CoNS from ocular infections and human blood cultures. In this study, we examined 18 ocular S. haemolyticus isolates for their capacity to form biofilm and conducted detachment assay to determine the composition of the biofilm matrix and involvement of various elements in cell lysis. PCR identified the presence of biofilm-associated genes, and ica operon and CLSM visualized the components of the biofilm matrix. We found that PIA-independent biofilm formation is the characteristic feature of S. haemolyticus isolates, irrespective of the sources of isolation, and protein or DNA or both are the major components of the biofilm matrix. Cell lysis enabling DNA release was an essential step for biofilm attachment during the initial stages of biofilm development. The srtA transcript expression study indicates its role in the early stages of biofilm development. We found the presence of antibiotic resistance genes in the eDNA and gDNA thus suggesting the possible role of biofilm in horizontal gene transfer of antibiotic resistance determinants. The overall study indicates that S. haemolyticus formed the biofilm comprising of protein or DNA or both and srtA play a role in the initial development of biofilm.

Molecular mechanism of extreme mechanostability in a pathogen adhesin

High resilience to mechanical stress is key when pathogens adhere to their target and initiate infection. Using atomic force microscopy-based single-molecule force spectroscopy, we explored the mechanical stability of the prototypical staphylococcal adhesin SdrG, which targets a short peptide from human fibrinogen β. Steered molecular dynamics simulations revealed, and single-molecule force spectroscopy experiments confirmed, the mechanism by which this complex withstands forces of over 2 nanonewtons, a regime previously associated with the strength of a covalent bond. The target peptide, confined in a screwlike manner in the binding pocket of SdrG, distributes forces mainly toward the peptide backbone through an intricate hydrogen bond network. Thus, these adhesins can attach to their target with exceptionally resilient mechanostability, virtually independent of peptide side chains.

Identification and screening of effective protective antigens for channel catfish against Streptococcus iniae

Vaccination is a potential approach for prevention and control of disease in fish. The use of genetically engineered vaccines is an effective method and a green intervention to control bacterial infection in aquaculture. However, efforts to develop these vaccines are limited by the lack of conserved protective antigens. In this study, three candidate immunogens (Srr, NeuA, and Hsp) of the pathogenic Streptococcus iniae strain DGX07 isolated from diseased channel catfish were identified and analyzed. Molecular cloning, expression, and purification of candidate antigen genes were carried out to obtain the candidate immunogens in the form of recombinant subunit vaccines. Western blotting was performed to evaluate immunogenicity in vitro and channel catfish were vaccinated by intraperitoneal injection and the specific antibody titers and relative percent of survival were determined to evaluate immune protection in vivo. The results showed that these three candidate immunogens were expressed correctly as recombinant proteins fused with His tags, with molecular weights of 70 kDa for Srr, 86 kDa for NeuA, and 51 kDa for Hsp, respectively. Moreover, each immunogen was predicted to be located either extracellularly or on the surface of S. iniae, and were able to offer protection against S. iniae infection in the form of recombinant subunit vaccines with adjuvant ISA763, especially Srr, with a relative percent of survival of 70% for Srr, 55% for NeuA, and 50% for Hsp, respectively.

Staphylococcal Osteomyelitis: Disease Progression, Treatment Challenges, and Future Directions

Osteomyelitis is an inflammatory bone disease that is caused by an infecting microorganism and leads to progressive bone destruction and loss. The most common causative species are the usually commensal staphylococci, with Staphylococcus aureus and Staphylococcus epidermidis responsible for the majority of cases. Staphylococcal infections are becoming an increasing global concern, partially due to the resistance mechanisms developed by staphylococci to evade the host immune system and antibiotic treatment. In addition to the ability of staphylococci to withstand treatment, surgical intervention in an effort to remove necrotic and infected bone further exacerbates patient impairment. Despite the advances in current health care, osteomyelitis is now a major clinical challenge, with recurrent and persistent infections occurring in approximately 40% of patients. This review aims to provide information about staphylococcus-induced bone infection, covering the clinical presentation and diagnosis of osteomyelitis, pathophysiology and complications of osteomyelitis, and future avenues that are being explored to treat osteomyelitis.

Molecular Genetics of Staphylococcus Epidermidis Biofilms on Indwelling Medical Devices

Staphylococcus epidermidis is an opportunistic pathogen associated with foreign body infections and nosocomial sepsis. The pathogenicity of S. epidermidis is mostly due to its ability to colonize indwelling polymeric devices and form a thick, multilayered biofilm. Biofilm formation is a major problem in treating S. epidermidis infection as biofilms provide significant resistance to antibiotics and to components of the innate host defenses. Various cell surface associated bacterial factors play a role in adherence and accumulation of the biofilm such as the polysaccharide intercellular adhesin and the autolysin AtlE. Furthermore, recent studies have shown that global regulators such as the agr quorum sensing system, the transcriptional regulator sarA and the alternative sigma factor sigB have an important function in the regulation of biofilm formation. Understanding the many complex mechanisms involved in biofilm formation is a key factor in the search for new anti-staphylococcal therapeutics.

Staphylococcus Biofilm Components as Targets for Vaccines and Drugs

Staphylococci have become the most common cause of nosocomial infections, especially in patients with predisposing factors such as indwelling or implanted foreign polymer bodies. The pathogenesis of foreign-body associated infections with S. aureus and S. epidermidis is mainly related to the ability of these bacteria to form thick, adherent multilayered biofilms. In a biofilm, staphylococci are protected against antibiotic treatment and attack from the immune system, thus making eradication of the infections problematic. This necessitates the discovery of novel prophylactic and therapeutic strategies to treat these infections. In this review, we provide an overview of staphylococcal biofilm components and discuss new possible approaches to controlling these persistent biofilm-dwelling bacteria.

Leptospira Immunoglobulin-Like Protein B (LigB) Binds to Both the C-Terminal 23 Amino Acids of Fibrinogen αC Domain and Factor XIII: Insight into the Mechanism of LigB-Mediated Blockage of Fibrinogen α Chain Cross-Linking

The coagulation system provides a primitive but effective defense against hemorrhage. Soluble fibrinogen (Fg) monomers, composed of α, β and γ chains, are recruited to provide structural support for the formation of a hemostatic plug. Fg binds to platelets and is processed into a cross-linked fibrin polymer by the enzymatic clotting factors, thrombin and Factor XIII (FXIII). The newly formed fibrin-platelet clot can act as barrier to protect against pathogens from entering the bloodstream. Further, injuries caused by bacterial infections can be confined to the initial wound site. Many pathogenic bacteria have Fg-binding adhesins that can circumvent the coagulation pathway and allow the bacteria to sidestep containment. Fg expression is upregulated during lung infection providing an attachment surface for bacteria with the ability to produce Fg-binding adhesins. Fg binding by leptospira might play a crucial factor in Leptospira-associated pulmonary hemorrhage, the main factor contributing to lethality in severe cases of leptospirosis. The 12^th domain of Leptospira immunoglobulin-like protein B (LigB12), a leptospiral adhesin, interacts with the C-terminus of FgαC (FgαCC). In this study, the binding site for LigB12 was mapped to the final 23 amino acids at the C-terminal end of FgαCC (FgαCC8). The association of FgαCC8 with LigB12 (ELISA, K_D = 0.76 μM; SPR, K_D = 0.96 μM) was reduced by mutations of both charged residues (R608, R611 and H614 from FgαCC8; D1061 from LigB12) and hydrophobic residues (I613 from FgαCC8; F1054 and A1065 from LigB12). Additionally, LigB12 bound strongly to FXIII and also inhibited fibrin formation, suggesting that LigB can disrupt coagulation by suppressing FXIII activity. Here, the detailed binding mechanism of a leptospiral adhesin to a host hemostatic factor is characterized for the first time and should provide better insight into the pathogenesis of leptospirosis.

Leptospirosis, caused by pathogenic Leptospira spp., has been increasingly recognized as an emerging zoonosis worldwide. In human cases, clinical presentation can vary from a mild flu-like syndrome to severe multi-organ failure including hepatitis, nephritis and occasionally meningitis. Particularly, pulmonary hemorrhage has become one of the major factors leading to fatality. The host coagulation system normally can be activated to confine damage caused by bacteria. However, this spirochete has developed several virulence proteins to manipulate hemostatic factors including fibrinogen (Fg). Previously, we had observed that Leptospira immunoglobulin-like protein B (LigB) can bind to Fg and inhibit fibrin clot formation. In this study, the LigB binding site on fibrinogen was fine-mapped. The key amino acids contributing to this strong pathogen-host interaction were also identified. In addition, LigB bound to factor XIII and further interfered with the cross-linking of Fg. For the first time, a potential mechanism of leptospiral adhesin binding to fibrinogen was revealed, which should provide a better understanding of the pathogenesis of leptospirosis.

Staphylococcus aureus Aggregation and Coagulation Mechanisms, and Their Function in Host-Pathogen Interactions

The human commensal bacterium Staphylococcus aureus can cause a wide range of infections ranging from skin and soft tissue infections to invasive diseases like septicemia, endocarditis, and pneumonia. Muticellular organization almost certainly contributes to S. aureus pathogenesis mechanisms. While there has been considerable focus on biofilm formation and its role in colonizing prosthetic joints and indwelling devices, less attention has been paid to nonsurface-attached group behavior like aggregation and clumping. S. aureus is unique in its ability to coagulate blood, and it also produces multiple fibrinogen-binding proteins that facilitate clumping. Formation of clumps, which are large, tightly packed groups of cells held together by fibrin(ogen), has been demonstrated to be important for S. aureus virulence and immune evasion. Clumps of cells are able to avoid detection by the host's immune system due to a fibrin(ogen) coat that acts as a shield, and the size of the clumps facilitates evasion of phagocytosis. In addition, clumping could be an important early step in establishing infections that involve tight clusters of cells embedded in host matrix proteins, such as soft tissue abscesses and endocarditis. In this review, we discuss clumping mechanisms and regulation, as well as what is known about how clumping contributes to immune evasion.

Novel polymeric nanoparticles targeting the lipopolysaccharides of Pseudomonas aeruginosa

Considering outburst of various infectious diseases globally, nanoparticle assisted targeted drug delivery has emerged as a promising strategy that can enhance the therapeutic efficacy and minimize the undesirable side effects of an antimicrobial agents. Molecular imprinting is a newly developed strategy that can synthesize a drug carrier with highly stable ligand-like 'cavity', may serve as a new platform of ligand-free targeted drug delivery systems. In this study, we use the amphiphilic lipopolysaccharides, derived from Pseudomonas aeruginosa as imprinting template and obtained an evenly distributed sub-40 nm polymeric nanoparticles by using inverse emulsion method. These molecularly imprinted nanoparticles (MIPNPs) showed specific binding to the lipopolysaccharide as determined by fluorescence polarization and microscale thermophoresis. MIPNPs showed selective recognition of target bacteria as detected by flow cytometry. Additionally, MIPNPs exhibited the in vivo targeting capabilities in both the keratitis model and meningitis model. Moreover, the photosensitizer methylene blue-loaded MIPNPs presented significantly strong inhibition of bacterial Growth, compared to non-imprinted controls for in vitro model of the photodynamic therapy. Our study shows an attempt to design a magic bullet by molecular imprinting that may provide a novel approach to generate synthetic carrier for targeting pathogen and treatment for a variety of infectious human diseases.

Prevalence of Virulence-Related Determinants in Clinical Isolates of Staphylococcus epidermidis

BACKGROUND

Staphylococcus epidermidis, a member of the human flora, is recognized as an opportunistic pathogen and cause of nosocomial infections. Staphylococcus epidermidis surface components are able to establish bacteria on the host surface, and cause infection.

OBJECTIVES

The frequency of icaA, IS256, aap, fbe and bhp in clinical isolates of S. epidermidis were investigated in this study.

MATERIALS AND METHODS

Fifty-nine S. epidermidis isolates were collected from blood (50), wound (1), urine (4) and tracheal (4) samples (Tehran, Iran). Staphylococcus epidermidis isolates were identified with conventional bacteriological tests. Virulence-associated genes were detected by specific polymerase chain reactions (PCRs).

RESULTS

Of the 59 S. epidermidis, fbe was found in 89.8%, while aap and bhp were observed in 64.4% and 15.3% of the samples, respectively. Coexistence of aap and fbe was found in 32 isolates, while coexistence of bhp and fbe was observed in five isolates. Two isolates were negative for the investigated genes.

CONCLUSIONS

Prevalence of fbe and aap was significantly different from similar studies, yet frequency of bhp was in accordance with other studies. Prevalence of icaA and IS256 was not significantly different from some studies while a significant difference was observed when results were compared with some other studies.

The Possible Role of Staphylococcus epidermidis LPxTG Surface Protein SesC in Biofilm Formation

Staphylococcus epidermidis is the most common cause of device-associated infections. It has been shown that active and passive immunization in an animal model against protein SesC significantly reduces S. epidermidis biofilm-associated infections. In order to elucidate its role, knock-out of sesC or isolation of S. epidermidis sesC-negative mutants were attempted, however, without success. As an alternative strategy, sesC was introduced into Staphylococcus aureus 8325–4 and its isogenic icaADBC and srtA mutants, into the clinical methicillin-sensitive S. aureus isolate MSSA4 and the MRSA S. aureus isolate BH1CC, which all lack sesC. Transformation of these strains with sesC i) changed the biofilm phenotype of strains 8325–4 and MSSA4 from PIA-dependent to proteinaceous even though PIA synthesis was not affected, ii) converted the non-biofilm-forming strain 8325–4 ica::tet to a proteinaceous biofilm-forming strain, iii) impaired PIA-dependent biofilm formation by 8325–4 srtA::tet, iv) had no impact on protein-mediated biofilm formation of BH1CC and v) increased in vivo catheter and organ colonization by strain 8325–4. Furthermore, treatment with anti-SesC antibodies significantly reduced in vitro biofilm formation and in vivo colonization by these transformants expressing sesC. These findings strongly suggest that SesC is involved in S. epidermidis attachment to and subsequent biofilm formation on a substrate.

A Novel MSCRAMM Subfamily in Coagulase Negative Staphylococcal Species

Coagulase negative staphylococci (CoNS) are important opportunistic pathogens. Staphylococcus epidermidis, a coagulase negative staphylococcus, is the third leading cause of nosocomial infections in the US. Surface proteins like Microbial Surface Components Recognizing Adhesive Matrix Molecules (MSCRAMMs) are major virulence factors of pathogenic gram positive bacteria. Here, we identified a new chimeric protein in S. epidermidis, that we call SesJ. SesJ represents a prototype of a new subfamily of MSCRAMMs. Structural predictions show that SesJ has structural features characteristic of a MSCRAMM along with a N-terminal repeat region and an aspartic acid containing C-terminal repeat region, features that have not been previously observed in staphylococcal MSCRAMMs but have been found in other surface proteins from gram positive bacteria. We identified and analyzed structural homologs of SesJ in three other CoNS. These homologs of SesJ have an identical structural organization but varying sequence identities within the domains. Using flow cytometry, we also show that SesJ is expressed constitutively on the surface of a representative S. epidermidis strain, from early exponential to stationary growth phase. Thus, SesJ is positioned to interact with protein targets in the environment and plays a role in S. epidermidis virulence.

Leptospira spp.: Novel insights into host-pathogen interactions

Leptospirosis is a widespread zoonosis caused by pathogenic Leptospira spp. It is an important infectious disease that affects humans and animals. The disease causes economic losses as it affects livestock, with decreased milk production and death. Our group is investigating the genome sequences of L. interrogans targeting surface-exposed proteins because, due to their location, these proteins are capable to interact with several host components that could allow establishment of the infection. These interactions may involve adhesion of the bacteria to extracellular matrix (ECM) components and, hence, help bacterial colonization. The bacteria could also react with the host fibrinolytic system and/or with the coagulation cascade components, such as, plasminogen (PLG) and fibrinogen (Fg), respectively. The binding with the first system generates plasmin (PLA), increasing the proteolytic power of the bacteria, while the second interferes with clotting in a thrombin-catalyzed reaction, which may promote hemorrhage foci and increase bacterial dissemination. Interaction with the complement system negative regulators may help bacteria to evade the host immune system, facilitating the invasion. This work compiles the main described leptospiral proteins that could act as adhesins, as PLG and fibrinogen receptors and as complement regulator binding proteins. We present models in which we suggest possible mechanisms of how leptospires might colonize and invade host tissues, causing the disease. Understanding leptospiral pathogenesis will help to identify antigen candidates that would contribute to the development of more effective vaccines and diagnostic tests.

Interaction of fibrinogen and muramidase-released protein promotes the development of Streptococcus suis meningitis

Muramidase-released protein (MRP) is as an important virulence marker of Streptococcus suis (S. suis) serotype 2. Our previous works have shown that MRP can bind human fibrinogen (hFg); however, the function of this interaction in S. suis meningitis is not known. In this study, we found that the deletion of mrp significantly impairs the hFg-mediated adherence and traversal ability of S. suis across human cerebral microvascular endothelial cells (hCMEC/D3). Measurement of the permeability to Lucifer yellow in vitro and Evans blue extravasation in vivo show that the MRP-hFg interaction significantly increases the permeability of the blood–brain barrier (BBB). In the mouse meningitis model, wild type S. suis caused higher bacterial loads in the brain and more severe histopathological signs of meningitis than the mrp mutant at day 3 post-infection. Western blot analysis and immunofluorescence observations reveal that the MRP-hFg interaction can destroy the cell adherens junction protein p120-catenin of hCMEC/D3. These results indicate that the MRP-hFg interaction is important in the development of S. suis meningitis.

Crystal structures of Bbp from Staphylococcus aureus reveal the ligand binding mechanism with Fibrinogen α

Bone sialoprotein-binding protein (Bbp), a MSCRAMMs (Microbial Surface Components Recognizing Adhesive Matrix Molecules) family protein expressed on the surface of Staphylococcus aureus (S. aureus), mediates adherence to fibrinogen α (Fg α), a component in the extracellular matrix of the host cell and is important for infection and pathogenesis. In this study, we solved the crystal structures of apo-Bbp^273−598 and Bbp^273−598-Fg α^561−575 complex at a resolution of 2.03 Å and 1.45 Å, respectively. Apo-Bbp^273−598 contained the ligand binding region N2 and N3 domains, both of which followed a DE variant IgG fold characterized by an additional D1 strand in N2 domain and D1′ and D2′ strands in N3 domain. The peptide mapped to the Fg α^561−575 bond to Bbp^273−598 on the open groove between the N2 and N3 domains. Strikingly, the disordered C-terminus in the apo-form reorganized into a highly-ordered loop and a β-strand G′′ covering the ligand upon ligand binding. Bbp^{Ala298–Gly301} in the N2 domain of the Bbp^273−598-Fg α^561−575 complex, which is a loop in the apo-form, formed a short α-helix to interact tightly with the peptide. In addition, Bbp^{Ser547–Gln561} in the N3 domain moved toward the binding groove to make contact directly with the peptide, while Bbp^{Asp338–Gly355} and Bbp^{Thr365–Tyr387} in N2 domain shifted their configurations to stabilize the reorganized C-terminus mainly through strong hydrogen bonds. Altogether, our results revealed the molecular basis for Bbp-ligand interaction and advanced our understanding of S. aureus infection process.

Biofilm formation and fibrinogen and fibronectin binding activities by Corynebacterium pseudodiphtheriticum invasive strains

Biofilm-related infections are considered a major cause of morbidity and mortality in hospital environments. Biofilms allow microorganisms to exchange genetic material and to become persistent colonizers and/or multiresistant to antibiotics. Corynebacterium pseudodiphtheriticum (CPS), a commensal bacterium that colonizes skin and mucosal sites has become progressively multiresistant and responsible for severe nosocomial infections. However, virulence factors of this emergent pathogen remain unclear. Herein, we report the adhesive properties and biofilm formation on hydrophilic (glass) and hydrophobic (plastic) abiotic surfaces by CPS strains isolated from patients with localized (ATCC10700/Pharyngitis) and systemic (HHC1507/Bacteremia) infections. Adherence to polystyrene attributed to hydrophobic interactions between bacterial cells and this negatively charged surface indicated the involvement of cell surface hydrophobicity in the initial stage of biofilm formation. Attached microorganisms multiplied and formed microcolonies that accumulated as multilayered cell clusters, a step that involved intercellular adhesion and synthesis of extracellular matrix molecules. Further growth led to the formation of dense bacterial aggregates embedded in the exopolymeric matrix surrounded by voids, typical of mature biofilms. Data also showed CPS recognizing human fibrinogen (Fbg) and fibronectin (Fn) and involvement of these sera components in formation of "conditioning films". These findings suggested that biofilm formation may be associated with the expression of different adhesins. CPS may form biofilms in vivo possibly by an adherent biofilm mode of growth in vitro currently demonstrated on hydrophilic and hydrophobic abiotic surfaces. The affinity to Fbg and Fn and the biofilm-forming ability may contribute to the establishment and dissemination of infection caused by CPS.

Proteomics identification of novel fibrinogen-binding proteins of Streptococcus suis contributing to antiphagocytosis

Streptococcus suis serotype 2 (SS2) induced sepsis and meningitis are often accompanied by bacteremia. However, the mechanism whereby it helps S. suis to evade PMN-mediated phagocytosis remain unclear. Because of the central roles of bacteria-human fibrinogen (hFg) interaction in innate immunity, here, a proteomics based Far-western blotting (PBFWB) was developed to identify the fibrinogen-binding surface proteins of S. suis (SsFBPs) on a large-scale. And then thirteen potential SsFBPs were identified by PBFWB and we selected seven potential surface proteins to further confirm their binding ability to hFg, of which the gene mutant strains of MRP displayed significantly decrease in binding to immobilized hFg. Additionally, the polyclonal antibodies against Enolase were found to significantly inhibit the binding of SS2 to hFg. Strikingly, MRP and Enolase were found to improve the antiphagocytic ability of SS2 to PMNs by interacting with hFg and enhance the survival of SS2 in human blood. Taken together, the PBFWB method provides useful clues to the bacteria-host interactions. These studies firstly disclose MRP and Enolase were involved in immune evasion of SS2 at least in part by binding to Fg, which make them potential targets for therapies for SS2 infection.

The expanding field of platelet-bacterial interconnections

Given their small size, platelets are emerging as being one of the most important entities in the bloodstream. Not only do they play a key role in maintaining thrombosis and haemostasis, platelets also play a critical role in orchestrating the immune response. Being the first cell at the site of injury, they are perfectly placed to assess the extent of the damage and recruit immune cells as is necessary. As a first line of defence, platelets can act as primitive immune cells themselves by interacting with invading pathogens. A number of platelet receptors have been shown to interact with bacteria either directly or indirectly, involving a plasma protein bridge. This review will discuss the molecular mechanisms that exist between platelets and bacteria and the functional response to the interaction. We will also discuss the importance of considering animal models of disease and the use of physiological shear when studying platelet-bacterial interactions.

The molecular basis of leptospiral pathogenesis

The mechanisms of disease pathogenesis in leptospirosis are poorly defined. Recent developments in the application of genetic tools in the study of Leptospira have advanced our understanding by allowing the assessment of mutants in animal models. As a result, a small number of essential virulence factors have been identified, though most do not have a clearly defined function. Significant advances have also been made in the in vitro characterization of leptospiral interaction with host structures, including extracellular matrix proteins (such as laminin, elastin, fibronectin, collagens), proteins related to hemostasis (fibrinogen, plasmin), and soluble mediators of complement resistance (factor H, C4b-binding protein), although none of these in vitro findings has been translated to the host animal. Binding to host structures may permit colonization of the host, prevention of blood clotting may contribute to hemorrhage, while interaction with complement resistance mediators may contribute to survival in serum. While not a classical intracellular pathogen, the interaction of leptospires and phagocytic cells appears complex, with bacteria surviving uptake and promoting apoptosis; mutants relating to these processes (such as cell invasion and oxidative stress resistance) are attenuated in vivo. Another feature of leptospiral biology is the high degree of functional redundancy and the surprising lack of attenuation of mutants in what appear to be certain virulence factors, such as LipL32 and LigB. While many advances have been made, there remains a lack of understanding of how Leptospira causes tissue pathology. It is likely that leptospires have many novel pathogenesis mechanisms that are yet to be identified.

Coagulase-negative staphylococci

The definition of the heterogeneous group of coagulase-negative staphylococci (CoNS) is still based on diagnostic procedures that fulfill the clinical need to differentiate between Staphylococcus aureus and those staphylococci classified historically as being less or nonpathogenic. Due to patient- and procedure-related changes, CoNS now represent one of the major nosocomial pathogens, with S. epidermidis and S. haemolyticus being the most significant species. They account substantially for foreign body-related infections and infections in preterm newborns. While S. saprophyticus has been associated with acute urethritis, S. lugdunensis has a unique status, in some aspects resembling S. aureus in causing infectious endocarditis. In addition to CoNS found as food-associated saprophytes, many other CoNS species colonize the skin and mucous membranes of humans and animals and are less frequently involved in clinically manifested infections. This blurred gradation in terms of pathogenicity is reflected by species- and strain-specific virulence factors and the development of different host-defending strategies. Clearly, CoNS possess fewer virulence properties than S. aureus, with a respectively different disease spectrum. In this regard, host susceptibility is much more important. Therapeutically, CoNS are challenging due to the large proportion of methicillin-resistant strains and increasing numbers of isolates with less susceptibility to glycopeptides.

Physical stress and bacterial colonization

Bacterial surface colonizers are subject to a variety of physical stresses. During the colonization of human epithelia such as on the skin or the intestinal mucosa, bacteria mainly have to withstand the mechanical stress of being removed by fluid flow, scraping, or epithelial turnover. To that end, they express a series of molecules to establish firm attachment to the epithelial surface, such as fibrillar protrusions (pili) and surface-anchored proteins that bind to human matrix proteins. In addition, some bacteria--in particular gut and urinary tract pathogens--use internalization by epithelial cells and other methods such as directed inhibition of epithelial turnover to ascertain continued association with the epithelial layer. Furthermore, many bacteria produce multilayered agglomerations called biofilms with a sticky extracellular matrix, providing additional protection from removal. This review will give an overview over the mechanisms human bacterial colonizers have to withstand physical stresses with a focus on bacterial adhesion.

Evidence for steric regulation of fibrinogen binding to Staphylococcus aureus fibronectin-binding protein A (FnBPA)

The adjacent fibrinogen (Fg)- and fibronectin (Fn)-binding sites on Fn-binding protein A (FnBPA), a cell surface protein from Staphylococcus aureus, are implicated in the initiation and persistence of infection. FnBPA contains a single Fg-binding site (that also binds elastin) and multiple Fn-binding sites. Here, we solved the structure of the N2N3 domains containing the Fg-binding site of FnBPA in the apo form and in complex with a Fg peptide. The Fg binding mechanism is similar to that of homologous bacterial proteins but without the requirement for "latch" strand residues. We show that the Fg-binding sites and the most N-terminal Fn-binding sites are nonoverlapping but in close proximity. Although Fg and a subdomain of Fn can form a ternary complex on an FnBPA protein construct containing a Fg-binding site and single Fn-binding site, binding of intact Fn appears to inhibit Fg binding, suggesting steric regulation. Given the concentrations of Fn and Fg in the plasma, this mechanism might result in targeting of S. aureus to fibrin-rich thrombi or elastin-rich tissues.

Adhesion, invasion and evasion: the many functions of the surface proteins of Staphylococcus aureus

Staphylococcus aureus is an important opportunistic pathogen and persistently colonizes about 20% of the human population. Its surface is 'decorated' with proteins that are covalently anchored to the cell wall peptidoglycan. Structural and functional analysis has identified four distinct classes of surface proteins, of which microbial surface component recognizing adhesive matrix molecules (MSCRAMMs) are the largest class. These surface proteins have numerous functions, including adhesion to and invasion of host cells and tissues, evasion of immune responses and biofilm formation. Thus, cell wall-anchored proteins are essential virulence factors for the survival of S. aureus in the commensal state and during invasive infections, and targeting them with vaccines could combat S. aureus infections.

Staphylococcus epidermidis pathogenesis

Staphylococcus epidermidis is the most frequently encountered member of the coagulase-negative staphylococci on human epithelial surfaces. It has emerged as an important nosocomial pathogen, especially in infections of indwelling medical devices. The mechanisms that S. epidermidis uses to survive during infection are in general of a passive nature, reflecting their possible origin in the commensal life of this bacterium. Most importantly, S. epidermidis excels in forming biofilms, sticky agglomerations that inhibit major host defense mechanisms. Furthermore, S. epidermidis produces a series of protective surface polymers and exoenzymes. Moreover, S. epidermidis has the capacity to secrete strongly cytolytic members of the phenol-soluble modulin (PSM) family, but PSMs in S. epidermidis overall appear to participate primarily in biofilm development. Finally, there is evidence for a virulence gene reservoir function of S. epidermidis, as it appears to have transferred important immune evasion and antibiotic resistance factors to Staphylococcus aureus. Conversely, S. epidermidis also has a beneficial role in balancing the microflora on human epithelial surfaces by controlling outgrowth of harmful bacteria such as in particular S. aureus. Recent research yielded detailed insight into key S. epidermidis virulence determinants and their regulation, in particular as far as biofilm formation is concerned, but we still have a serious lack of understanding of the in vivo relevance of many pathogenesis mechanisms and the factors that govern the commensal life of S. epidermidis.

Nitric oxide-flux dependent bacterial adhesion and viability at fibrinogen-coated surfaces

Nitric oxide (NO) is an endogenous antibacterial agent produced by immune cells in response to pathogens. Herein, the NO fluxes necessary to reduce bacterial adhesion of different bacteria (S. aureus, methicillin-resistant S. aureus, S. epidermidis, E. faecalis, E. coli, and P. aeruginosa) were investigated to ascertain the sensitivity of these bacteria to NO. S-nitrosothiol NO donor-modified xerogels were selected as a model NO-release surface due to their extended NO-release kinetics relative to other NO donor systems. The xerogels were coated with poly(vinyl chloride) (PVC) to achieve consistent surface energy between NO-releasing and control substrates. Fibrinogen was pre-adsorbed to these materials to more accurately mimic conditions encountered in blood and promote bacteria adhesion. Nitric oxide fluxes ranging from 20-50 pmol cm-2 s-1 universally inhibited the bacterial adhesion by >80% for each strain studied. Maximum bacteria killing activity (reduced viability by 85-98%) was observed at the greatest NO payload (1700 nmol cm-2).

Characterization of fibrinogen binding by glycoproteins Srr1 and Srr2 of Streptococcus agalactiae

The serine-rich repeat glycoproteins of Gram-positive bacteria comprise a large family of cell wall proteins. Streptococcus agalactiae (group B streptococcus, GBS) expresses either Srr1 or Srr2 on its surface, depending on the strain. Srr1 has recently been shown to bind fibrinogen, and this interaction contributes to the pathogenesis of GBS meningitis. Although strains expressing Srr2 appear to be hypervirulent, no ligand for this adhesin has been described. We now demonstrate that Srr2 also binds human fibrinogen and that this interaction promotes GBS attachment to endothelial cells. Recombinant Srr1 and Srr2 bound fibrinogen in vitro, with affinities of K_D = 2.1 × 10⁻⁵ and 3.7 × 10⁻⁶m, respectively, as measured by surface plasmon resonance spectroscopy. The binding site for Srr1 and Srr2 was localized to tandem repeats 6–8 of the fibrinogen Aα chain. The structures of both the Srr1 and Srr2 binding regions were determined and, in combination with mutagenesis studies, suggest that both Srr1 and Srr2 interact with a segment of these repeats via a “dock, lock, and latch” mechanism. Moreover, properties of the latch region may account for the increased affinity between Srr2 and fibrinogen. Together, these studies identify how greater affinity of Srr2 for fibrinogen may contribute to the increased virulence associated with Srr2-expressing strains.

Adhesins of Leptospira interrogans mediate the interaction to fibrinogen and inhibit fibrin clot formation in vitro

We report in this work that Leptospira strains, virulent L. interrogans serovar Copenhageni, attenuated L. interrogans serovar Copenhageni and saprophytic L. biflexa serovar Patoc are capable of binding fibrinogen (Fg). The interaction of leptospires with Fg inhibits thrombin- induced fibrin clot formation that may affect the haemostatic equilibrium. Additionally, we show that plasminogen (PLG)/plasmin (PLA) generation on the surface of Leptospira causes degradation of human Fg. The data suggest that PLA-coated leptospires were capable to employ their proteolytic activity to decrease one substrate of the coagulation cascade. We also present six leptospiral adhesins and PLG- interacting proteins, rLIC12238, Lsa33, Lsa30, OmpL1, rLIC11360 and rLIC11975, as novel Fg-binding proteins. The recombinant proteins interact with Fg in a dose-dependent and saturable fashion when increasing protein concentration was set to react to a fix human Fg concentration. The calculated dissociation equilibrium constants (K_D) of these reactions ranged from 733.3±276.8 to 128±89.9 nM for rLIC12238 and Lsa33, respectively. The interaction of recombinant proteins with human Fg resulted in inhibition of fibrin clot by thrombin-catalyzed reaction, suggesting that these versatile proteins could mediate Fg interaction in Leptospira. Our data reveal for the first time the inhibition of fibrin clot by Leptospira spp. and presents adhesins that could mediate these interactions. Decreasing fibrin clot would cause an imbalance of the coagulation cascade that may facilitate bleeding and help bacteria dissemination

Leptospirosis is probably the most widespread zoonosis in the world. Caused by spirochaetes of the genus Leptospira, it has greater incidence in tropical and subtropical regions. The disease has become prevalent in cities with sanitation problems and a large population of urban rodent reservoirs, which contaminate the environment through their urine. Understanding the mechanisms involved in pathogenesis of leptospirosis should contribute to new strategies that would help fight the disease. We show in this work that Leptospira strains, virulent, attenuated or saprophytic are capable of binding fibrinogen (Fg). The interaction of leptospires with Fg inhibits the formation of fibrin clot that may result of an imbalance in the haemostatic equilibrium. In addition, we show that plasminogen (PLG)/plasmin (PLA) generation on the surface of leptospires can lead to Fg degradation, showing evidence of possible route of fibrinolysis in leptospirosis. We also present six leptospiral proteins, as novel Fg-binding proteins, capable of inhibiting fibrin clot formation by thrombin-catalyzed reaction, suggesting that in Leptospira these multifunctional proteins could mediate Fg interaction. Our data suggest possible mechanisms that leptospires could employ to affect the coagulation cascade and fibrinolytic system that might lead to bacteria spreading.

Degradation of MSCRAMM target macromolecules in VLU slough by Lucilia sericata chymotrypsin 1 (ISP) persists in the presence of tissue gelatinase activity

Venous leg ulcer slough is unpleasant to the patient and difficult to manage clinically. It harbours infection, also preventing wound management materials and dressings from supporting the underlying viable tissues. In other words, slough has significant nuisance value in the tissue viability clinic. In this study, we have sought to increase our knowledge of slough by building upon a previous but limited analysis of this necrotic tissue. In particular, slough has been probed using Western blotting for the presence of proteins with the capacity to engage microbial surface components recognising adhesive matrix macromolecules. Although the samples were difficult to resolve, we detected fibrinogen, fibronectin, IgG, collagen, human serum albumin and matrix metalloproteinase-9. Furthermore, the effect of a maggot-derived debridement enzyme, chymotrypsin 1 on macromolecules in slough was confirmed across seven patient samples. The effect of chymotrypsin 1 on slough confirms our thesis that this potential debridement enzyme could be effective in removing slough along with its associated bacteria, given its observed resistance to intrinsic gelatinase activity. In summary, we believe that the data provide scientists and clinicians with further insights into the potential molecular interactions between bacteria, wound tissue and Lucilia sericata in a clinically problematic yet scientifically interesting wound ecosystem.

Antimicrobial activity of fibrinogen and fibrinogen-derived peptides--a novel link between coagulation and innate immunity

Fibrinogen is a key player in the blood coagulation system, and is upon activation with thrombin converted into fibrin that subsequently forms a fibrin clot. In the present study, we investigated the role of fibrinogen in the early innate immune response. Here we show that the viability of fibrinogen-binding bacteria is affected in human plasma activated with thrombin. Moreover, we found that the peptide fragment GHR28 released from the β-chain of fibrinogen has antimicrobial activity against bacteria that bind fibrinogen to their surface, whereas non-binding strains are unaffected. Notably, bacterial killing was detected in Group A Streptococcus bacteria entrapped in a fibrin clot, suggesting that fibrinogen and coagulation is involved in the early innate immune system to quickly wall off and neutralise invading pathogens.

Binding of glycoprotein Srr1 of Streptococcus agalactiae to fibrinogen promotes attachment to brain endothelium and the development of meningitis

The serine-rich repeat glycoprotein Srr1 of Streptococcus agalactiae (GBS) is thought to be an important adhesin for the pathogenesis of meningitis. Although expression of Srr1 is associated with increased binding to human brain microvascular endothelial cells (hBMEC), the molecular basis for this interaction is not well defined. We now demonstrate that Srr1 contributes to GBS attachment to hBMEC via the direct interaction of its binding region (BR) with human fibrinogen. When assessed by Far Western blotting, Srr1 was the only protein in GBS extracts that bound fibrinogen. Studies using recombinant Srr1-BR and purified fibrinogen in vitro confirmed a direct protein-protein interaction. Srr1-BR binding was localized to amino acids 283–410 of the fibrinogen Aα chain. Structural predictions indicated that the conformation of Srr1-BR is likely to resemble that of SdrG and other related staphylococcal proteins that bind to fibrinogen through a “dock, lock, and latch” mechanism (DLL). Deletion of the predicted latch domain of Srr1-BR abolished the interaction of the BR with fibrinogen. In addition, a mutant GBS strain lacking the latch domain exhibited reduced binding to hBMEC, and was significantly attenuated in an in vivo model of meningitis. These results indicate that Srr1 can bind fibrinogen directly likely through a DLL mechanism, which has not been described for other streptococcal adhesins. This interaction was important for the pathogenesis of GBS central nervous system invasion and subsequent disease progression.

Streptococcus agalactiae (Group B streptococcus, GBS) is a leading cause of meningitis in newborns and infants. This life-threatening infection of the brain and surrounding tissues continues to result in a high incidence of morbidity and mortality, despite antibiotic therapy. A key factor in disease production is the ability of this organism to invade the central nervous system, via the bloodstream. We now report that a GBS surface protein called Srr1 binds fibrinogen, a major protein in human blood. This interaction enhances the attachment of GBS to brain vascular endothelial cells, and contributes to the development of meningitis. A mutation in Srr1 that specifically disrupted binding to fibrinogen significantly reduced GBS attachment to brain endothelium, and markedly reduced virulence in an in vivo model of GBS disease. These studies have identified a new mechanism by which Srr1 contributes to GBS invasion of the central nervous system and may provide a basis for novel therapies targeting Srr1 binding.

Targeted delivery of vancomycin to Staphylococcus epidermidis biofilms using a fibrinogen-derived peptide

This study reports on the use of a fibrinogen-derived peptide for the specific targeting and delivery of vancomycin to Staphylococcus epidermidis biofilms. One method by which S. epidermidis initially adheres to biomaterials uses the plasma protein fibrinogen as an intermediary, where the S. epidermidis surface protein SdrG binds to a short amino acid sequence near the amino terminus of the Bβ chain of fibrinogen. We mimicked this binding interaction and demonstrated the use of a synthetic fibrinogen-based β6-20 peptide to target and deliver vancomycin to S. epidermidis in vitro. The β6-20 peptide was synthesized and labeled with a Nanogold probe, and its targeting capabilities were examined through the use of scanning electron microscopy. The Nanogold component was then replaced by vancomycin, utilizing a flexible, variable length poly(ethylene glycol) linker between the peptide and antibiotic to create the targeted vancomycin products, β6-20-PEG(x) -VAN. Initial binding to surface adherent S. epidermidis was increased in a concentration-dependent manner relative to vancomycin for all equivalent concentrations ≥4 μg/mL, with targeted vancomycin content up to 22.9 times that of vancomycin alone. Retention of the targeted antibiotics was measured after an additional 24-h incubation period, revealing levels 1.3 times that of vancomycin. The results demonstrate the improved targeting and retention of vancomycin within a biofilm due to the incorporation of a specific targeting motif.