Staphylococcus aureus adhesion via Spa, ClfA, and SdrCDE to immobilized platelets demonstrates shear-dependent behavior

Contribution of staphylococcal virulence factors in the pathogenesis of thrombosis

Staphylococci are responsible for many infections in humans, starting with skin and soft tissue infections and finishing with invasive diseases such as endocarditis, sepsis and pneumonia, which lead to high mortality. Patients with sepsis often demonstrate activated clotting pathways, decreased levels of anticoagulants, decreased fibrinolysis, activated endothelial surfaces and activated platelets. This results in disseminated intravascular coagulation and formation of a microthrombus, which can lead to a multiorgan failure. This review describes various staphylococcal virulence factors that contribute to vascular thrombosis, including deep vein thrombosis in infected patients. The article presents mechanisms of action of different factors released by bacteria in various host defense lines, which in turn can lead to formation of blood clots in the vessels.

Chondrocyte Invasion May Be a Mechanism for Persistent Staphylococcus Aureus Infection In Vitro

BACKGROUND

Recurrent bone and joint infection with Staphylococcus aureus is common. S. aureus can invade and persist in osteoblasts and fibroblasts, but little is known about this mechanism in chondrocytes. If S. aureus were able to invade and persist within chondrocytes, this could be a difficult compartment to treat.

QUESTION/PURPOSE

Can S. aureus infiltrate and persist intracellularly within chondrocytes in vitro?

METHODS

Cell lines were cultured in vitro and infected with S. aureus. Human chondrocytes (C20A4) were compared with positive controls of human osteoblasts (MG63) and mouse fibroblasts (NIH3T3), which have previously demonstrated S. aureus invasion and persistence (human fibroblasts were not available to us). Six replicates per cell type were followed for 6 days after infection. Cells were treated daily with antibiotic media for extracellular killing. To determine whether S. aureus can infiltrate chondrocytes, fluorescence microscopy was performed to qualitatively assess the presence of intracellular bacteria, and intracellular colony-forming units (CFU) were enumerated 2 hours after infection. To determine whether S. aureus can persist within chondrocytes, intracellular CFUs were enumerated from infected host cells each day postinfection.

RESULTS

S. aureus invaded human chondrocytes (C20A4) at a level (2.8 x 105 ± 5.5 x 104 CFUs/mL) greater than positive controls of human osteoblasts (MG63) (9.5 x 102 ± 2.5 x 102 CFUs/mL; p = 0.01) and mouse fibroblasts (NIH3T3) (9.1 x 104 ± 2.5 x 104 CFUs/mL; p = 0.02). S. aureus also persisted within human chondrocytes (C20A4) for 6 days at a level (1.4 x 103 ± 5.3 x 102 CFUs/mL) greater than that of human osteoblasts (MG63) (4.3 x 102 ± 3.5 x 101 CFUs/mL; p = 0.02) and mouse fibroblasts (NIH3T3) (0 CFUs/mL; p < 0.01). S. aureus was undetectable within mouse fibroblasts (NIH3T3) after 4 days. There were 0 CFUs yielded from cell media, confirming extracellular antibiotic treatment was effective.

CONCLUSION

S. aureus readily invaded human chondrocytes (C20A4) in vitro and persisted viably for 6 days after infection, evading extracellular antibiotics. Chondrocytes demonstrated a greater level of intracellular invasion and persistence by S. aureus than positive control human osteoblast (MG63) and mouse fibroblast (NIH3T3) cell lines.

CLINICAL RELEVANCE

Chondrocyte invasion and persistence may contribute to recurrent bone and joint infections. Additional research should assess longer periods of persistence and whether this mechanism is present in vivo.

Relationships of platelet glycoprotein specific antibody with therapeutic efficacy of short-term high-dose dexamethasone and bleeding score in the newly diagnosed adult patients with primary immune thrombocytopenia

Objectives: We aimed to investigate relationships of platelet glycoprotein (GP) specific antibody with therapeutic efficacy of high-dose dexamethasone (HD-DXM) and bleeding score in primary immune thrombocytopenia (ITP) adults. Methods: A retrospective study was carried out to analyze relationships of polymorphism of GP specific antibody with initial therapeutic efficacy of HD-DXM and bleeding score of newly diagnosed ITP adults between 1 June, 2016 and 31 January, 2020. Results: 59 patients were involved in the study, with 33 cases of responders and 26 cases of non-responders between June 2016 and January 2020. At admission, there were 31 (52.5%) GP antibody-positive patients. Initial therapy of HD-DXM was effective for 78.6% GP antibody-negative patients and 35.5% GP antibody-positive patients, with a better therapeutic efficacy in patients with anti-GP Ib/IX antibody or anti-GP IIb/IIIa antibody but not in those with anti-GP Ib/IX antibody plus anti-GP IIb/IIIa antibody. Notably, therapeutic efficacy is much worse for minority (Uyghur) patients compared with corresponding Han patients. Similarly, it was much lower in GP antibody-positive patients compared with corresponding negative ones at low and medium bleeding score, with no response in GP antibody-positive patients at high bleeding score. Furthermore, there was a moderate negative correlation between therapeutic efficacy and GP-specific antibody (p < 0.05), but no obvious linear relationship between clinical bleeding degree and GP-specific antibody (p > 0.05). Conclusion: Collectively, the newly diagnosed ITP adults with GP-specific antibody have a poor response to short-term HD-DXM, especially in minority (Uyghur) patients with GP-specific antibody in China.

Activation of Human Platelets by Staphylococcus aureus Secreted Protease Staphopain A

Infection by Staphylococcus aureus is the leading cause of infective endocarditis (IE). Activation of platelets by this pathogen results in their aggregation and thrombus formation which are considered to be important steps in the development and pathogenesis of IE. Here, we show that a secreted cysteine protease, staphopain A, activates human platelets and induces their aggregation. The culture supernatant of a scpA mutant deficient in staphopain A production was reduced in its ability to trigger platelet aggregation. The platelet agonist activity of purified staphopain A was inhibited by staphostatin A, a specific inhibitor, thus implicating its protease activity in the agonism. In whole blood, using concentrations of staphopain A that were otherwise insufficient to induce platelet aggregation, increased binding to collagen and thrombus formation was observed. Using antagonists specific to protease-activated receptors 1 and 4, we demonstrate their role in mediating staphopain A induced platelet activation.

α-hemolysin of Staphylococcus aureus impairs thrombus formation

BACKGROUND

Toxins are key virulence determinants of pathogens and can impair the function of host immune cells, including platelets. Insights into pathogen toxin interference with platelets will be pivotal to improve treatment of patients with bacterial bloodstream infections.

MATERIALS AND METHODS

In this study, we deciphered the effects of Staphylococcus aureus toxins α-hemolysin, LukAB, LukDE, and LukSF on human platelets and compared the effects with the pore forming toxin pneumolysin of Streptococcus pneumoniae. Activation of platelets and loss of platelet function were investigated by flow cytometry, aggregometry, platelet viability, fluorescence microscopy, and intracellular calcium release. Thrombus formation was assessed in whole blood.

RESULTS

α-hemolysin (Hla) is known to be a pore-forming toxin. Hla-induced calcium influx initially activates platelets as indicated by CD62P and αIIbβ3 integrin activation, but also induces finally alterations in the phenotype of platelets. In contrast to Hla and pneumolysin, S. aureus bicomponent pore-forming leukocidins LukAB, LukED, and LukSF do not bind to platelets and had no significant effect on platelet activation and viability. The presence of small amounts of Hla (0.2 µg/ml) in whole blood abrogates thrombus formation indicating that in systemic infections with S. aureus the stability of formed thrombi is impaired. Damage of platelets by Hla was not neutralized by intravenous immune globulins.

CONCLUSION

Our findings might be of clinical relevance for S. aureus induced endocarditis. Stabilizing the aortic-valve thrombi by inhibiting Hla-induced impairment of platelets might reduce the risk for septic (micro-)embolization.

A global transcriptomic analysis of Staphylococcus aureus biofilm formation across diverse clonal lineages

A key characteristic of Staphylococcus aureus infections, and one that also varies phenotypically between clones, is that of biofilm formation, which aids in bacterial persistence through increased adherence and immune evasion. Though there is a general understanding of the process of biofilm formation - adhesion, proliferation, maturation and dispersal - the tightly orchestrated molecular events behind each stage, and what drives variation between S. aureus strains, has yet to be unravelled. Herein we measure biofilm progression and dispersal in real-time across the five major S. aureus CDC-types (USA100-USA500) revealing adherence patterns that differ markedly amongst strains. To gain insight into this, we performed transcriptomic profiling on these isolates at multiple timepoints, compared to planktonically growing counterparts. Our findings support a model in which eDNA release, followed by increased positive surface charge, perhaps drives initial abiotic attachment. This is seemingly followed by cooperative repression of autolysis and activation of poly-N-acetylglucosamine (PNAG) production, which may indicate a developmental shift in structuring the biofilm matrix. As biofilms mature, diminished translational capacity was apparent, with 53 % of all ribosomal proteins downregulated, followed by upregulation of anaerobic respiration enzymes. These findings are noteworthy because reduced cellular activity and an altered metabolic state have been previously shown to contribute to higher antibiotic tolerance and bacterial persistence. In sum, this work is, to our knowledge, the first study to investigate transcriptional regulation during the early, establishing phase of biofilm formation, and to compare global transcriptional regulation both temporally and across multiple clonal lineages.

A global transcriptomic analysis of Staphylococcus aureus biofilm formation across diverse clonal lineages

A key characteristic of Staphylococcus aureus infections, and one that also varies phenotypically between clones, is that of biofilm formation, which aids in bacterial persistence through increased adherence and immune evasion. Though there is a general understanding of the process of biofilm formation - adhesion, proliferation, maturation and dispersal - the tightly orchestrated molecular events behind each stage, and what drives variation between S. aureus strains, has yet to be unravelled. Herein we measure biofilm progression and dispersal in real-time across the five major S. aureus CDC-types (USA100-USA500) revealing adherence patterns that differ markedly amongst strains. To gain insight into this, we performed transcriptomic profiling on these isolates at multiple timepoints, compared to planktonically growing counterparts. Our findings support a model in which eDNA release, followed by increased positive surface charge, perhaps drives initial abiotic attachment. This is seemingly followed by cooperative repression of autolysis and activation of poly-N-acetylglucosamine (PNAG) production, which may indicate a developmental shift in structuring the biofilm matrix. As biofilms mature, diminished translational capacity was apparent, with 53 % of all ribosomal proteins downregulated, followed by upregulation of anaerobic respiration enzymes. These findings are noteworthy because reduced cellular activity and an altered metabolic state have been previously shown to contribute to higher antibiotic tolerance and bacterial persistence. In sum, this work is, to our knowledge, the first study to investigate transcriptional regulation during the early, establishing phase of biofilm formation, and to compare global transcriptional regulation both temporally and across multiple clonal lineages.

Prostaglandin E2 promotes Staphylococcus aureus infection via EP4 receptor in bovine endometrium

Prostaglandin E2 (PGE₂) enhances Staphylococcus aureus infection but its mechanism is not well understood. Here, we examined the effect of PGE₂ on Staphylococcal Protein A (SPA) expression in bovine endometrium and determined the role of select PGE₂ receptors (i.e., EP2 and EP4) in adhesion and internalization of S. aureus. S. aureus isolate SA113 was used for in vitro infection of bovine endometrial tissues and epithelial cells, with treatment conditions consisting of untreated control, SA113 treatment, SA113 + PGE₂, SA113 + PGE₂ + EP2 receptor antagonist (AH-6809), and SA113 + PGE₂ + EP4 receptor antagonist (AH-23848). Immunofluorescence assay revealed that PGE₂ could promote SPA expression in S. aureus-infected bovine endometrial tissues. PGE₂ also enhanced the adhesion and internalization of S. aureus in bovine endometrial cells. The addition of EP4 antagonist, but not the EP2 antagonist, abrogated the ability of PGE₂ to promote S. aureus SPA expression, adhesion, and internalization in endometrial cells. Our findings suggest that S. aureus infection in the endometrium is enhanced by PGE₂ through the EP4 receptor. This result is essential for the development of new approach to treating S. aureus infection, such as the application of EP4 antagonist as an adjunct drug treatment.

Intracellular Staphylococcus aureus in bone and joint infections: A mechanism of disease recurrence, inflammation, and bone and cartilage destruction

Bone and joint infections are devastating afflictions. Although medical interventions and advents have improved their care, bone and joint infections still portend dismal outcomes. Indeed, bone and joint infections are associated with extremely high mortality and morbidity rates and, generally, occur secondary to the aggressive pathogen Staphylococcus aureus. The consequences of bone and joint infections are further compounded by the fact that although they are aggressively treated, they frequently recur and result in massive bone and articular cartilage loss. Here, we review the literature and chronicle the fact that the fundamental cellular components of the musculoskeletal system can be internally infected with Staphylococcus aureus, which explains the ready recurrence of bone and joint infections even after extensive administration of antibiotic therapy and debridement and offer potential treatment solutions for further study. Moreover, we review the ramifications of intracellular infection and expound that the massive bone and articular cartilage loss is caused by the sustained proinflammatory state induced by infection and offer potential combination therapies for further study to protect bone and cartilage.

Native aortic versus mitral valve infective endocarditis: a nationwide registry study

Background

Native aortic and mitral valve infective endocarditis (AVE and MVE, respectively) are usually grouped together as left-sided native valve infective endocarditis (LNVE), while the differences between AVE and MVE have not yet been properly investigated. We aimed to compare AVE and MVE in regard to patient characteristics, microbiology and determinants of survival.

Methods

We conducted a retrospective study using the Swedish national registry on infective endocarditis, which contains nationwide patient data. The study period was 2007‒2017, and included cases were patients who had either AVE or MVE.

Results

We included 649 AVE and 744 MVE episodes. Staphylococcus aureus was more often the causative pathogen in MVE (41% vs 31%, p<0.001), whereas enterococci were more often the causative pathogen in AVE (14% vs 7.4%, p<0.001). Perivalvular involvement occurred more frequently in AVE (8.5% vs 3.5%, p<0.001) and brain emboli more frequently in MVE (21% vs 13%, p<0.001). Surgery for IE was performed more often (35% vs 27%, p<0.001) and sooner after diagnosis (6.5 days vs 9 days, p=0.012) in AVE than in MVE. Several risk predictors differed between the two groups.

Conclusions

The microbiology seems to differ between AVE and MVE. The causative pathogen was not associated with mortality in AVE. The between-group differences regarding clinical presentation and predictors of survival indicate that it may be important to differentiate AVE from MVE in the treatment of LNVE.

Adhesion of Staphylococcus aureus to Corneocytes from Atopic Dermatitis Patients Is Controlled by Natural Moisturizing Factor Levels

The bacterial pathogen Staphylococcus aureus plays an important role in atopic dermatitis (AD), a chronic disorder that mostly affects children. Colonization of the skin of AD patients by S. aureus exacerbates the disease, but the molecular determinants of the bacterium-skin adhesive interactions are poorly understood. Specifically, reduced levels of natural moisturizing factor (NMF) in the stratum corneum have been shown to be associated with more severe AD symptoms, but whether this is directly related to S. aureus adhesion is still an open question. Here, we demonstrate a novel relationship between NMF expression in AD skin and strength of bacterial adhesion. Low-NMF corneocytes, unlike high-NMF ones, are covered by a dense layer of nanoscale villus protrusions. S. aureus bacteria isolated from AD skin bind much more strongly to corneocytes when the NMF level is reduced. Strong binding forces originate from a specific interaction between the bacterial adhesion clumping factor B (ClfB) and skin ligands. Remarkably, mechanical tension dramatically strengthens ClfB-mediated adhesion, as observed with catch bonds, demonstrating that physical stress plays a role in promoting colonization of AD skin by S. aureus Collectively, our findings demonstrate that patient NMF levels regulate the strength of S. aureus-corneocyte adhesion, the first step in skin colonization, and suggest that the ClfB binding mechanism could represent a potential target for new therapeutic treatments.IMPORTANCE Bacterium-skin interactions play important roles in skin disorders, yet their molecular details are poorly understood. In this study, we decipher the molecular forces at play during adhesion of Staphylococcus aureus to skin corneocytes in the clinically important context of atopic dermatitis (AD), also known as eczema. We identify a unique relationship between the level of natural moisturizing factor (NMF) in the skin and the strength of bacterium-corneocyte adhesion. Bacterial adhesion is primarily mediated by the surface protein clumping factor B (ClfB) and is enhanced by physical stress, highlighting the role of protein mechanobiology in skin colonization. Similar to a catch bond behavior, this mechanism represents a promising target for the development of novel antistaphylococcal agents.

Phylogenetic analysis and virulence determinant of the host-adapted Staphylococcus aureus lineage ST188 in China

Staphylococcus aureus (S. aureus) is an important pathogen of humans and livestock species, but an understanding of the clonal distribution of S. aureus causing different host-species infections in the same geographical environment and within the same period is lacking. By characterizing infections caused by S. aureus in bovine, pediatric, and adult patients in Shanghai, China, between 2012 and 2014, we identified methicillin-sensitive S. aureus (MSSA) ST188 as the major lineage causing infections in multiple host species. Whole-genome sequencing and phenotypic analyses demonstrated that ST188 might evolve from livestock, and there was no significant genomic or virulence difference between ST188 isolated from livestock and humans. The virulence of ST188 is related to its adhesion and nasal colonization ability. This result is in accord with the strong epithelial cell adhesion and biofilm formation properties of ST188. Furthermore, the adhesion- and biofilm-formation-related genes are present in multiple copies and exhibit significantly increased expression in ST188. In conclusion, S. aureus ST188 is the major lineage causing human and livestock infections in Shanghai, China. Due to its high expression of the factors associated with bacterial adhesion and biofilm formation, ST188 has the ability to colonize and infect different host species.

Staphylococcus aureus, master manipulator of the human hemostatic system

The coagulation system does not only offer protection against bleeding, but also aids in our defense against invading microorganisms. The hemostatic system and innate immunity are strongly entangled, which explains why so many infections are complicated by either bleeding or thrombosis. Staphylococcus aureus (S. aureus), currently the most deadly infectious agent in the developed world, causes devastating intravascular infections such as sepsis and infective endocarditis. During these infections S. aureus comes in close contact with the host hemostatic system and proves to be a master in manipulating coagulation. The coagulases of S. aureus directly induce coagulation by activating prothrombin. S. aureus also manipulates fibrinolysis by triggering plasminogen activation via staphylokinase. Furthermore, S. aureus binds and activates platelets and interacts with key coagulation proteins such as fibrin(ogen), fibronectin and von Willebrand factor. By manipulating the coagulation system S. aureus gains a significant advantage over the host defense mechanisms. Studying the interplay between S. aureus and the hemostatic system can therefore lead to new innovative therapies for battling S. aureus infections.

Force-Induced Strengthening of the Interaction between Staphylococcus aureus Clumping Factor B and Loricrin

Bacterial pathogens that colonize host surfaces are subjected to physical stresses such as fluid flow and cell surface contacts. How bacteria respond to such mechanical cues is an important yet poorly understood issue. Staphylococcus aureus uses a repertoire of surface proteins to resist shear stress during the colonization of host tissues, but whether their adhesive functions can be modulated by physical forces is not known. Here, we show that the interaction of S. aureus clumping factor B (ClfB) with the squamous epithelial cell envelope protein loricrin is enhanced by mechanical force. We find that ClfB mediates S. aureus adhesion to loricrin through weak and strong molecular interactions both in a laboratory strain and in a clinical isolate. Strong forces (~1,500 pN), among the strongest measured for a receptor-ligand bond, are consistent with a high-affinity "dock, lock, and latch" binding mechanism involving dynamic conformational changes in the adhesin. Notably, we demonstrate that the strength of the ClfB-loricrin bond increases as mechanical force is applied. These findings favor a two-state model whereby bacterial adhesion to loricrin is enhanced through force-induced conformational changes in the ClfB molecule, from a weakly binding folded state to a strongly binding extended state. This force-sensitive mechanism may provide S. aureus with a means to finely tune its adhesive properties during the colonization of host surfaces, helping cells to attach firmly under high shear stress and to detach and spread under low shear stress.IMPORTANCEStaphylococcus aureus colonizes the human skin and the nose and can cause various disorders, including superficial skin lesions and invasive infections. During nasal colonization, the S. aureus surface protein clumping factor B (ClfB) binds to the squamous epithelial cell envelope protein loricrin, but the molecular interactions involved are poorly understood. Here, we unravel the molecular mechanism guiding the ClfB-loricrin interaction. We show that the ClfB-loricrin bond is remarkably strong, consistent with a high-affinity "dock, lock, and latch" binding mechanism. We discover that the ClfB-loricrin interaction is enhanced under tensile loading, thus providing evidence that the function of an S. aureus surface protein can be activated by physical stress.

Microbially and phytofabricated AgNPs with different mode of bactericidal action were identified to have comparable potential for surface fabrication of central venous catheters to combat Staphylococcus aureus biofilm

In spite of newer innovations and process improvements, catheter related infections still pose serious threat to hospitalized patients. Silver nanoparticles (AgNPs) are well demonstrated to have antibacterial properties and also have been implemented for surface fabrication of many indwelling medical devices. So, herein we sought to compare the performance of AgNPs generated through biogenic routes using bacteria and plant extract for their antibacterial and antibiofilm potential against biofilm forming Staphylococcus aureus. The biosynthesized AgNPs were characterized by UV- Visible spectroscopy, HR-TEM and EDS analysis. The antibacterial efficiency of the nanoparticles was detected by Disc diffusion assay, MIC and MBC analysis. The antibiofilm properties of the nanoparticles were also investigated. The antibacterial mode of interaction of both nanoparticles on the bacterium was analyzed by HR-TEM. Insight into mode of interaction and mechanism of antibacterial activity of both AgNPs showed them to have promises for surface fabrication of central venous catheters. No study has been conducted so far to compare the efficiency of two different biogenic AgNPs and this highlights the novelty of the current work. Though both AgNPs were observed to exhibit comparable activity in terms of bactericidal and antibiofilm, the mode of bacterial interaction and degree of damage caused was entirely different.

Mechanical forces regulate the reactivity of a thioester bond in a bacterial adhesin

Bacteria must withstand large mechanical shear forces when adhering to and colonizing hosts. Recent structural studies on a class of Gram-positive bacterial adhesins have revealed an intramolecular Cys-Gln thioester bond that can react with surface-associated ligands to covalently anchor to host surfaces. Two other examples of such internal thioester bonds occur in certain anti-proteases and in the immune complement system, both of which react with the ligand only after the thioester bond is exposed by a proteolytic cleavage. We hypothesized that mechanical forces in bacterial adhesion could regulate thioester reactivity to ligand analogously to such proteolytic gating. Studying the pilus tip adhesin Spy0125 of Streptococcus pyogenes, we developed a single molecule assay to unambiguously resolve the state of the thioester bond. We found that when Spy0125 was in a folded state, its thioester bond could be cleaved with the small-molecule nucleophiles methylamine and histamine, but when Spy0125 was mechanically unfolded and subjected to forces of 50-350 piconewtons, thioester cleavage was no longer observed. For folded Spy0125 without mechanical force exposure, thioester cleavage was in equilibrium with spontaneous thioester reformation, which occurred with a half-life of several minutes. Functionally, this equilibrium reactivity allows thioester-containing adhesins to sample potential substrates without irreversible cleavage and inactivation. We propose that such reversible thioester reactivity would circumvent potential soluble inhibitors, such as histamine released at sites of inflammation, and allow the bacterial adhesin to selectively associate with surface-bound ligands.

Comparison of Biofilm Formation between Methicillin-Resistant and Methicillin-Susceptible Isolates of Staphylococcus aureus

Background:

The aim of this study was to compare the biofilm formation and the prevalence of biofilm-associated genes between the isolates of methicillin-resistant (MRSA) and methicillin-susceptible (MSSA) Staphylococcus aureus.

Methods:

In total, 209 S. aureus isolates were collected. The antibiotic susceptibility test was conducted using nine antibiotics according to the guidelines of Clinical and Laboratory Standards Institute. Phenotypic biofilm formation was performed with microtiter plate assay. The polymerase chain reaction was employed to detect icaA, icaD, icaB, icaC, clfA, clfB, fnbA, fnbB, fib, cna,eno, ebps, bbp,mecA, and SCCmec types as well as agr group genes with specific primers.

Results:

Sixty-four (30.62%) isolates were resistant to methicillin, and 54 (83%) MRSA harbored SCCmec III. Furthermore, 122 (58.3%) isolates belonged to agr group I. Twenty-six (36.1%) MRSA and 42 (28.9%) MSSA isolates were strong biofilm producers (no significant difference). The prevalence of icaA, icaD, icaB, and icaC genes in MSSA isolates was 71, 41, 76, and 72%, respectively. The frequency of clfA, clfB, fnbA, fnbB, fib, cna, eno, ebps, and bbp in MSSA was 100, 100, 56, 46, 74, 54, 78, 11, and 1%, respectively. However, in MRSA isolates, the frequency was 97, 97, 64, 51, 76, 56, 79, and 12% with no track of bbp, respectively.

Conclusion:

Statistical difference between MSSA and MRSA regarding biofilm formation and the frequency of all biofilm-encoding genes was not significant. The majority of the S. aureus isolates harbored clfA, clfB, eno, fib, icaA, and icaD genes.

The Landscape of Realized Homologous Recombination in Pathogenic Bacteria

Recombination enhances the adaptive potential of organisms by allowing genetic variants to be tested on multiple genomic backgrounds. Its distribution in the genome can provide insight into the evolutionary forces that underlie traits, such as the emergence of pathogenicity. Here, we examined landscapes of realized homologous recombination of 500 genomes from ten bacterial species and found all species have "hot" regions with elevated rates relative to the genome average. We examined the size, gene content, and chromosomal features associated with these regions and the correlations between closely related species. The recombination landscape is variable and evolves rapidly. For example in Salmonella, only short regions of around 1 kb in length are hot whereas in the closely related species Escherichia coli, some hot regions exceed 100 kb, spanning many genes. Only Streptococcus pyogenes shows evidence for the positive correlation between GC content and recombination that has been reported for several eukaryotes. Genes with function related to the cell surface/membrane are often found in recombination hot regions but E. coli is the only species where genes annotated as "virulence associated" are consistently hotter. There is also evidence that some genes with "housekeeping" functions tend to be overrepresented in cold regions. For example, ribosomal proteins showed low recombination in all of the species. Among specific genes, transferrin-binding proteins are recombination hot in all three of the species in which they were found, and are subject to interspecies recombination.

Proteome Analyses of Staphylococcus aureus Biofilm at Elevated Levels of NaCl

Our studies demonstrate that sodium chloride (NaCl) induces changes in biofilm, mediated by increased production of polysaccharides intercellular adhesion (PIA). We identified 12 proteins that showed higher abundance in increased level of NaCl. This includes one important protein (IsaA) known to be associated with biofilm stability. In addition, we also found higher abundance of a cold shock protein, CspA, at higher NaCl. We have also identified several other proteins that are differentially expressed to the elevated levels of NaCl and mapped them in the regulatory pathways of PIA. The majority of proteins are involved with various aspects bacterial metabolic function. Our results demonstrated that NaCl influences gene regulatory networks controlling exopolysaccharide expression.

Controlled Transcription of Exogenous mRNA in Platelets Using Protocells

Transcribing exogenous RNA in eukaryotic cells requires delivering DNA to their nuclei and changing their genome. Nuclear delivery is often inefficient, limiting the potential scope of gene therapy and synthetic biology. These challenges may be overcome by techniques that allow for extranucleate transcription within eukaryotic cells. Protocells have been developed that enable transcription inside of liposomes; however, it has not yet been demonstrated whether this technology can be extended for use within eukaryotic cells. Here we show RNA-synthesizing nanoliposomes allow transcription of exogenous RNA inside anucleate cells. To accomplish this, components of transcription were encapsulated into liposomes and delivered to platelets. These liposomes were capable of light-induced transcription in platelets, providing proof-of-concept that protocell technology can be adapted for use within mammalian cells.

Inhibition and Reversal of Microbial Attachment by an Antibody with Parasteric Activity against the FimH Adhesin of Uropathogenic E. coli

Attachment proteins from the surface of eukaryotic cells, bacteria and viruses are critical receptors in cell adhesion or signaling and are primary targets for the development of vaccines and therapeutic antibodies. It is proposed that the ligand-binding pocket in receptor proteins can shift between inactive and active conformations with weak and strong ligand-binding capability, respectively. Here, using monoclonal antibodies against a vaccine target protein - fimbrial adhesin FimH of uropathogenic Escherichia coli, we demonstrate that unusually strong receptor inhibition can be achieved by antibody that binds within the binding pocket and displaces the ligand in a non-competitive way. The non-competitive antibody binds to a loop that interacts with the ligand in the active conformation of the pocket but is shifted away from ligand in the inactive conformation. We refer to this as a parasteric inhibition, where the inhibitor binds adjacent to the ligand in the binding pocket. We showed that the receptor-blocking mechanism of parasteric antibody differs from that of orthosteric inhibition, where the inhibitor replaces the ligand or allosteric inhibition where the inhibitor binds at a site distant from the ligand, and is very potent in blocking bacterial adhesion, dissolving surface-adherent biofilms and protecting mice from urinary bladder infection.

A common approach in the development of selective inhibitors for ligand-receptor interactions is targeting the receptor binding site with the expectation that inhibitors will sterically interfere with ligand binding and thus block receptor function via a competitive (orthosteric) mechanism. However, using monoclonal antibodies specific for the mannose-binding Escherichia coli adhesin, FimH, we demonstrate that the binding site epitopes allow for non-competitive inhibition that is more effective than orthosteric blocking. FimH, similar to other binding proteins, exhibits conformational flexibility of the ligand-binding pocket shifting between open (inactive) and tight (active) conformations, with relatively low- and high- affinity towards mannose. We show that an antibody that binds just one of the mannose-binding pocket loops prevents the shift from the inactive to the active conformation and hence blocks formation of high-affinity ligand-receptor complexes. This antibody type was more effective in inhibition of bacterial adhesion than anti-FimH antibodies competitively blocking mannose binding, and unlike the latter or a soluble ligand, showed the ability to detach an established bacterial biofilm from a ligand-coated surface. As the newly described antibody can bind the FimH pocket simultaneously with ligand, we refer to it as a parasteric (next-to-ligand) inhibitor that exhibits non-competitive inhibition from within the binding-pocket of the receptor.

Staphylococcus aureus gene expression in a rat model of infective endocarditis

Background

Diabetes mellitus is a frequent underlying comorbidity in patients with Staphylococcus aureus endocarditis, and it represents a risk factor for complications and a negative outcome. The pathogenesis of staphylococcal endocardial infections in diabetic hosts has been poorly characterized, and little is known about S. aureus gene expression in endocardial vegetations.

Methods

We utilized a rat model of experimental S. aureus endocarditis to compare the pathogenesis of staphylococcal infection in diabetic and nondiabetic hosts and to study the global S. aureus transcriptome in endocardial vegetations in vivo.

Results

Diabetic rats had higher levels of bacteremia and larger endocardial vegetations than nondiabetic control animals. Microarray analyses revealed that 61 S. aureus genes were upregulated in diabetic rats, and the majority of these bacterial genes were involved in amino acid and carbohydrate metabolism. When bacterial gene expression in vivo (diabetic or nondiabetic endocardial vegetations) was compared to in vitro growth conditions, higher in vivo expression of genes encoding toxins and proteases was observed. Additionally, genes involved in the production of adhesins, capsular polysaccharide, and siderophores, as well as in amino acid and carbohydrate transport and metabolism, were upregulated in endocardial vegetations. To test the contribution of selected upregulated genes to the pathogenesis of staphylococcal endocarditis, isogenic deletion mutants were utilized. A mutant defective in production of the siderophore staphyloferrin B was attenuated in the endocarditis model, whereas the virulence of a surface adhesin (ΔsdrCDE) mutant was similar to that of the parental S. aureus strain.

Conclusions

Our results emphasize the relevance of diabetes mellitus as a risk factor for infectious endocarditis and provide a basis for understanding gene expression during staphylococcal infections in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-014-0093-3) contains supplementary material, which is available to authorized users.

Proteomic analysis of Staphylococcus aureus biofilm cells grown under physiologically relevant fluid shear stress conditions

BACKGROUND

The biofilm forming bacterium Staphylococcus aureus is responsible for maladies ranging from severe skin infection to major diseases such as bacteremia, endocarditis and osteomyelitis. A flow displacement system was used to grow S. aureus biofilms in four physiologically relevant fluid shear rates (50, 100, 500 and 1000 s(-1)) to identify proteins that are associated with biofilm.

RESULTS

Global protein expressions from the membrane and cytosolic fractions of S. aureus biofilm cells grown under the above shear rate conditions are reported. Sixteen proteins in the membrane-enriched fraction and eight proteins in the cytosolic fraction showed significantly altered expression (p < 0.05) under increasing fluid shear. These 24 proteins were identified using nano-LC-ESI-MS/MS. They were found to be associated with various metabolic functions such as glycolysis / TCA pathways, protein synthesis and stress tolerance. Increased fluid shear stress did not influence the expression of two important surface binding proteins: fibronectin-binding and collagen-binding proteins.

CONCLUSIONS

The reported data suggest that while the general metabolic function of the sessile bacteria is minimal under high fluid shear stress conditions, they seem to retain the binding capacity to initiate new infections.

A microfluidic platform for rapid, stress-induced antibiotic susceptibility testing of Staphylococcus aureus

The emergence and spread of bacterial resistance to ever increasing classes of antibiotics intensifies the need for fast phenotype-based clinical tests for determining antibiotic susceptibility. Standard susceptibility testing relies on the passive observation of bacterial growth inhibition in the presence of antibiotics. In this paper, we present a novel microfluidic platform for antibiotic susceptibility testing based on stress-activation of biosynthetic pathways that are the primary targets of antibiotics. We chose Staphylococcus aureus (S. aureus) as a model system due to its clinical importance, and we selected bacterial cell wall biosynthesis as the primary target of both stress and antibiotic. Enzymatic and mechanical stresses were used to damage the bacterial cell wall, and a β-lactam antibiotic interfered with the repair process, resulting in rapid cell death of strains that harbor no resistance mechanism. In contrast, resistant bacteria remained viable under the assay conditions. Bacteria, covalently-bound to the bottom of the microfluidic channel, were subjected to mechanical shear stress created by flowing culture media through the microfluidic channel and to enzymatic stress with sub-inhibitory concentrations of the bactericidal agent lysostaphin. Bacterial cell death was monitored via fluorescence using the Sytox Green dead cell stain, and rates of killing were measured for the bacterial samples in the presence and absence of oxacillin. Using model susceptible (Sanger 476) and resistant (MW2) S. aureus strains, a metric was established to separate susceptible and resistant staphylococci based on normalized fluorescence values after 60 min of exposure to stress and antibiotic. Because this ground-breaking approach is not based on standard methodology, it circumvents the need for minimum inhibitory concentration (MIC) measurements and long wait times. We demonstrate the successful development of a rapid microfluidic-based and stress-activated antibiotic susceptibility test by correctly designating the phenotypes of 16 additional clinically relevant S. aureus strains in a blinded study. In addition to future clinical utility, this method has great potential for studying the effects of various stresses on bacteria and their antibiotic susceptibility.

The sortase A substrates FnbpA, FnbpB, ClfA and ClfB antagonize colony spreading of Staphylococcus aureus

Staphylococcus aureus is an important human pathogen that is renowned both for its rapid transmission within hospitals and the community, and for the formation of antibiotic resistant biofilms on medical implants. Recently, it was shown that S. aureus is able to spread over wet surfaces. This motility phenomenon is promoted by the surfactant properties of secreted phenol-soluble modulins (PSMs), which are also known to inhibit biofilm formation. The aim of the present studies was to determine whether any cell surface-associated S. aureus proteins have an impact on colony spreading. To this end, we analyzed the spreading capabilities of strains lacking non-essential components of the protein export and sorting machinery. Interestingly, our analyses reveal that the absence of sortase A (SrtA) causes a hyper-spreading phenotype. SrtA is responsible for covalent anchoring of various proteins to the staphylococcal cell wall. Accordingly, we show that the hyper-spreading phenotype of srtA mutant cells is an indirect effect that relates to the sortase substrates FnbpA, FnbpB, ClfA and ClfB. These surface-exposed staphylococcal proteins are known to promote biofilm formation, and cell-cell interactions. The hyper-spreading phenotype of srtA mutant staphylococcal cells was subsequently validated in Staphylococcus epidermidis. We conclude that cell wall-associated factors that promote a sessile lifestyle of S. aureus and S. epidermidis antagonize the colony spreading motility of these bacteria.

Fibrin formation by staphylothrombin facilitates Staphylococcus aureus-induced platelet aggregation

Interactions of Staphylococcus aureus (S. aureus) and platelets play an important role in the pathogenesis of intravascular infections such as infective endocarditis (IE). A typical feature of S. aureus is the ability to generate thrombin activity through the secretion of two prothrombin activating molecules, staphylocoagulase and von Willebrand factor-binding protein (vWbp), which bind to human prothrombin to form the enzymatically active staphylothrombin complex. The role of staphylothrombin in the interaction between S. aureus and platelets has not yet been studied. We found that in contrast with thrombin, staphylothrombin did not directly activate human platelets. However, the staphylothrombin-mediated conversion of fibrinogen to fibrin initiated platelet aggregation and secondary activation and facilitated S. aureus-platelet interactions. Both the genetic absence of staphylocoagulase and vWbp and pharmacological inhibition of staphylothrombin increased the lag time to aggregation, and reduced platelet trapping by S. aureus in high shear stress conditions. The combined inhibition of staphylothrombin and immunoglobulin binding to platelets completely abolished the ability of S. aureus to aggregate platelets in vitro. In conclusion, although staphylothrombin did not directly activate platelets, the formation of a fibrin scaffold facilitated bacteria-platelet interaction, and the inhibition of staphylothrombin resulted in a reduced activation of platelets by S. aureus.

Deep venous thrombosis in children with musculoskeletal infections: the clinical evidence

BACKGROUND

Clinical manifestations of deep venous thrombosis (DVT) tend to overlap with those of deep-seated musculoskeletal infections (MSIs). Consequently, the incidence of DVT as a complication of MSI may be underestimated. The objective of this study was to evaluate the incidence, clinical features, and outcomes of MSI-related DVT in children.

METHODS

We systematically reviewed relevant studies retrieved from PubMed and Scopus databases.

RESULTS

Overall, 93 children with MSIs who developed DVT were identified from 28 retrospective studies. The majority were boys. Osteomyelitis was the most frequent MSI (69/74, 93%). Staphylococcus aureus was the predominant pathogen (83/93, 89%); 61% of these isolates were methicillin-resistant S. aureus (MRSA). Pulmonary involvement, presumably due to septic emboli, was observed in 65% of the included children. Four children died due to multiple organ failure and two due to respiratory distress. In two of the three studies providing comparative data, MRSA infections were observed significantly more frequently in children who developed DVT compared to those who did not. Yet, the respective differences observed for methicillin-susceptible S. aureus (MSSA) infections were non-significant in these three studies.

CONCLUSIONS

Despite the inclusion of many case reports and the retrospective design of the evaluated studies, our findings suggest that boys seem to be more frequently affected by MSIs complicated by DVT. Moreover, MRSA seems to be more frequently associated with DVT compared to MSSA. Pulmonary involvement appears to be a frequent complication. Prospective studies are needed in order to further clarify this issue.

Oxidized von Willebrand factor is efficiently cleaved by serine proteases from primary granules of leukocytes: divergence from ADAMTS-13

BACKGROUND

The leukocyte serine proteases (LSPs) elastase, proteinase 3 and cathepsin G cleave von Willebrand factor (VWF) near or at the same cleavage site (Tyr1605-Met1606) as ADAMTS-13, the metalloprotease that specifically controls the proteolytic processing of VWF. Recent studies have shown that oxidation of VWF at Met1606 with formation of methionine sulfoxide (MetSO) severely impairs its proteolysis by ADAMTS-13.

METHODS

This study was aimed at assessing whether or not oxidation of VWF by reactive oxygen species (ROS) can also affect its cleavage by elastase, proteinase 3, and cathepsin G. In this study, the catalytic specificity of hydrolysis by LSPs of the VWF peptide substrate VWF74 and full-length VWF, both unaltered and in the oxidized form, was measured by RP-HPLC, electrophoretic and mass spectrometry methods.

RESULTS

LSPs cleaved both VWF multimers and VWF74 near or at the same peptide bond as is cleaved by ADAMTS-13, with k(cat)/K(m) values similar to those of the metalloprotease. However, unlike ADAMTS-13, cathepsin G cleaved VWF74 containing a MetSO residue at position 1606 with a k(cat)/K(m) value higher than that for VWF74, whereas the catalytic efficiencies of both elastase and proteinase 3 were unaffected by the replacement of Met1606 with MetSO. Likewise, oxidation of VWF multimers by hypochlorous acid and ROS, produced by activated leukocytes, improved their hydrolysis by LSPs.

CONCLUSIONS

Oxidation by leukocyte ROS has a net positive effect on the cleavage of VWF multimers by LSPs, under conditions where high concentrations of oxidant species would severely reduce the proteolytic efficiency of ADAMTS-13.

The periodontal anaerobe Porphyromonas gingivalis induced platelet activation and increased aggregation in whole blood by rat model

INTRODUCTION

More and more evidence show that periodontal anaerobes contribute to pathogenesis of peripheral artery diseases. As a typical oral anaerobe that results in periodontitis, P.gingivalis aggregates platelets in PRP in vitro and participated in artery thrombosis. However, in vivo effect on platelet activation and aggregation remains unclear. This study aimed to clarify its role on platelets activation on more physiological environment, that is, on whole blood and systemic circulation.

MATERIALS AND METHODS

To fully estimate platelet activation, CD62P(P-selectin) expression on platelet surface and fibrinogen binding of platelet via conjugated glycoprotein GPIIb/IIIa in whole blood were assayed by flow cytometry, and platelet aggregation was measured on an impedance aggregometor. As primary study, platelet reactivity was assessed after in vitro rat whole blood incubation with P.gingivalis strain 381 in tubes, followed or not followed by ADP and arachidonic acid stimulation. In addition, PBS solution of P.gingivalis was infused into rat to produce transient bacteremia model for 5 minutes and blood samples were subjected to analysis for platelet activation in vivo.

RESULTS

P.gingivalis could not induce rat platelet aggregation in whole blood in vitro, but increased aggregation when irritated by collagen stimulation. Flow cytometric study showed that incubation with P.gingivalis increased CD62P expression and fibrinogen binding of platelet. Moreover, further stress by 10 μmol/L ADP and 260 mmlol/L arachidonic acid yielded additional expression. As in vivo study, after P.gingivalis solution challenged, rat platelet aggregability was enhanced, and CD62P positive percentage of platelets and further reactivity to ADP stimulation improved.

CONCLUSION

In whole blood and in systemic circulation, P.gingivalis could induce rat platelet activation and increase aggregability transiently. The results helped to understand the mechanism underlining which P.gingivalis promoted arteriosclerosis and thrombo-embolic disorders. Further study about chronic infection with P.gingivalis on platelet activity is expected.

Bacteriophage lysin mediates the binding of streptococcus mitis to human platelets through interaction with fibrinogen

The binding of bacteria to human platelets is a likely central mechanism in the pathogenesis of infective endocarditis. We have previously found that platelet binding by Streptococcus mitis SF100 is mediated by surface components encoded by a lysogenic bacteriophage, SM1. We now demonstrate that SM1-encoded lysin contributes to platelet binding via its direct interaction with fibrinogen. Far Western blotting of platelets revealed that fibrinogen was the major membrane-associated protein bound by lysin. Analysis of lysin binding with purified fibrinogen in vitro confirmed that these proteins could bind directly, and that this interaction was both saturable and inhibitable. Lysin bound both the Aα and Bβ chains of fibrinogen, but not the γ subunit. Binding of lysin to the Bβ chain was further localized to a region within the fibrinogen D fragment. Disruption of the SF100 lysin gene resulted in an 83±3.1% reduction (mean ± SD) in binding to immobilized fibrinogen by this mutant strain (PS1006). Preincubation of this isogenic mutant with purified lysin restored fibrinogen binding to wild type levels. When tested in a co-infection model of endocarditis, loss of lysin expression resulted in a significant reduction in virulence, as measured by achievable bacterial densities (CFU/g) within vegetations, kidneys, and spleens. These results indicate that bacteriophage-encoded lysin is a multifunctional protein, representing a new class of fibrinogen-binding proteins. Lysin appears to be cell wall-associated through its interaction with choline. Once on the bacterial surface, lysin can bind fibrinogen directly, which appears to be an important interaction for the pathogenesis of endocarditis.

The binding of bacteria to human platelets is thought to be a central event in the development of endocarditis (a life-threatening cardiovascular infection). We have previously found that platelet binding by Streptococcus mitis is mediated by surface components encoded by a bacteriophage contained within the host bacterium. We now show that lysin (an enzyme of bacteriophage origin) contributes to platelet binding via its direct interaction with fibrinogen on the platelet surface. Lysin bound to purified fibrinogen in vitro, and this interaction specifically involved the Aα and Bβ chains of fibrinogen. Binding of lysin to the Bβ chain was further localized to a region within the fibrinogen D fragment. Disruption of the gene encoding lysin gene resulted in a significant reduction in binding to fibrinogen by S. mitis, as well as a major reduction in virulence, as measured by a rat model of endocarditis. These results indicate that lysin is a multifunctional protein, representing a new class of fibrinogen-binding molecules. Lysin is localized to the bacterial surface via its interaction with cell wall choline, where it then can bind fibrinogen directly. Cell surface lysin apparently also contributes to the development of endovascular infections via its previously unrecognized fibrinogen binding activity.

Characterization, crystallization and preliminary X-ray analysis of the adhesive domain of SdrE from Staphylococcus aureus

The adhesive domain of SdrE from Staphylococcus aureus was recombinantly expressed in Escherichia coli. The purified protein was identified by SDS-PAGE and MALDI-TOF MS. The protein was crystallized using the vapour-diffusion method in hanging-drop mode with PEG 8000 as the primary precipitating agent. X-ray diffraction data were collected to 1.8 A resolution from a single crystal of the protein. Preliminary X-ray analysis indicated that the crystal belonged to space group P1, with unit-cell parameters a = 40.714, b = 66.355, c = 80.827 A, alpha = 111.19, beta = 93.99, gamma = 104.39 degrees.

Platelet-bacterial interactions

Many bacteria are capable of interacting with platelets and inducing platelet aggregation. This interaction may be a direct interaction between a bacterial surface protein and a platelet receptor or may be an indirect interaction where plasma proteins bind to the bacterial surface and subsequently bind to a platelet receptor. However, these interactions usually do not trigger platelet activation as a secondary co-signal is also required. This is usually due to specific antibody bound to the bacteria interacting with FcgammaRIIa on the platelet surface. Secreted bacterial products such as gingipains and lipopolysaccharide may also be capable of triggering platelet activation.

Molecular basis for Staphylococcus aureus-mediated platelet aggregate formation under arterial shear in vitro

OBJECTIVE

Staphylococcus aureus is the most frequent causative organism of infective endocarditis (IE) and is characterized by thrombus formation on a cardiac valve that can embolize to a distant site. Previously, we showed that S. aureus clumping factor A (ClfA) and fibronectin-binding protein A (FnBPA) can stimulate rapid platelet aggregation.

METHODS AND RESULTS

In this study we investigate their relative roles in mediating aggregate formation under physiological shear conditions. Platelets failed to interact with immobilized wild-type S. aureus (Newman) at shear rates <500 s(-1) but rapidly formed an aggregate at shear rates >800 s(-1). Inactivation of the ClfA gene eliminated aggregate formation at any shear rate. Using surrogate hosts that do not interact with platelets bacteria overexpressing ClfA supported rapid aggregate formation under high shear with a similar profile to Newman whereas bacteria overexpressing FnBPA did not. Fibrinogen binding to ClfA was found to be essential for aggregate formation although fibrinogen-coated surfaces only allowed single-platelets to adhere under all shear conditions. Blockade of the platelet immunoglobulin receptor Fc gammaRIIa inhibited aggregate formation.

CONCLUSIONS

Thus, fibrinogen and IgG binding to ClfA is essential for aggregate formation under arterial shear conditions and may explain why S. aureus is the major cause of IE.