Clumping factor B (ClfB), a new surface-located fibrinogen-binding adhesin of Staphylococcus aureus

Sortases, Surface Proteins, and Their Roles in Staphylococcus aureus Disease and Vaccine Development

Sortases cleave short peptide motif sequences at the C-terminal end of secreted surface protein precursors and either attach these polypeptides to the peptidoglycan of Gram-positive bacteria or promote their assembly into pilus structures that are also attached to peptidoglycan. Sortase A, the enzyme first identified in the human pathogen Staphylococcus aureus, binds LPXTG motif sorting signals, cleaves between threonine (T) and glycine (G) residues, and forms an acyl enzyme between its active-site cysteine thiol and the carboxyl group of threonine (T). Sortase A acyl enzyme is relieved by the nucleophilic attack of the cross bridge amino group within lipid II, thereby generating surface protein linked to peptidoglycan precursor. Such products are subsequently incorporated into the cell wall envelope by enzymes of the peptidoglycan synthesis pathway. Surface proteins linked to peptidoglycan may be released from the bacterial envelope to diffuse into host tissues and fulfill specific biological functions. S. aureus sortase A is essential for host colonization and for the pathogenesis of invasive diseases. Staphylococcal sortase-anchored surface proteins fulfill key functions during the infectious process, and vaccine-induced antibodies targeting surface proteins may provide protection against S. aureus. Alternatively, small-molecule inhibitors of sortase may be useful agents for the prevention of S. aureus colonization and invasive disease.

Transcriptional regulation of virulence factors Spa and ClfB by the SpoVG-Rot cascade in Staphylococcus aureus

Staphylococcus aureus can produce numerous surface proteins involved in the adhesion and internalization of host cells, immune evasion, and inflammation initiation. Among these surface proteins, the microbial surface components recognizing adhesive matrix molecules contain many crucial cell wall-anchored virulence factors. The Sar-family regulatory protein Rot has been reported to regulate many important extracellular virulence factors at the transcriptional level, including Spa and clumping factor B. SpoVG, a global regulator in S. aureus, is known to control the expression of numerous genes. Here, we demonstrate that SpoVG can positively regulate the transcription of rot by directly binding to its promoter. SpoVG can also positively regulate the transcription of spa and clfB through direct-binding to their promoters and in a Rot-mediated manner. Furthermore, SpoVG can positively modulate the human fibrinogen-binding ability of S. aureus. In addition, phosphorylation of SpoVG by the serine/threonine kinase, Stk1, can positively regulate its binding to the promoters of rot, spa, and clfB. The human cell infection assay showed that the adhesion and internalization abilities were reduced in the spoVG mutant strain in comparison to those in the wild-type strain. Collectively, our data reveal a SpoVG-Rot regulatory cascade and novel molecular mechanisms in the virulence control in S. aureus.

A complex interplay between the extracellular matrix and the innate immune response to microbial pathogens

The role of the host extracellular matrix (ECM) in infection tends to be neglected. However, the complex interactions between invading pathogens, host tissues and immune cells occur in the context of the ECM. On the pathogen side, a variety of surface and secreted molecules, including microbial surface components recognizing adhesive matrix molecules and tissue-degrading enzymes, are employed that interact with different ECM proteins to effectively establish an infection at specific sites. Microbial pathogens can also hijack or misuse host proteolytic systems to modify the ECM, evade immune responses or process biologically active molecules such as cell surface receptors and cytokines that direct cell behaviour and immune defence. On the host side, the ECM composition and three-dimensional ultrastructure undergo significant modifications, which have a profound impact on the specific signals that the ECM conveys to immune cells at the forefront of infection. Unexpectedly, activated immune cells participate in the remodelling of the local ECM by synthesizing ECM glycoproteins, proteoglycans and collagen molecules. The close interplay between the ECM and the innate immune response to microbial pathogens ultimately affects the outcome of infection. This review explores and discusses recent data that implicate an active role for the ECM in the immune response to infection, encompassing antimicrobial activities, microbial recognition, macrophage activation, phagocytosis, leucocyte population balance, and transcriptional and post-transcriptional regulation of inflammatory networks, and may foster novel antimicrobial approaches.

Staphylococcus aureus Evasion of Host Immunity in the Setting of Prosthetic Joint Infection: Biofilm and Beyond

PURPOSE OF REVIEW

The incidence of complications from prosthetic joint infection (PJI) is increasing, and treatment failure remains high. We review the current literature with a focus on Staphylococcus aureus pathogenesis and biofilm, as well as treatment challenges, and novel therapeutic strategies.

RECENT FINDINGS

S. aureus biofilm creates a favorable environment that increases antibiotic resistance, impairs host immunity, and increases tolerance to nutritional deprivation. Secreted proteins from bacterial cells within the biofilm and the quorum-sensing agr system contribute to immune evasion. Additional immunoevasive properties of S. aureus include the formation of staphylococcal abscess communities (SACs) and canalicular invasion. Novel approaches to target biofilm and increase resistance to implant colonization include novel antibiotic therapy, immunotherapy, and local implant treatments. Challenges remain given the diverse mechanisms developed by S. aureus to alter the host immune responses. Further understanding of these processes should provide novel therapeutic mechanisms to enhance eradication after PJI.

Guanine Limitation Results in CodY-Dependent and -Independent Alteration of Staphylococcus aureus Physiology and Gene Expression

In Staphylococcus aureus, the global transcriptional regulator CodY modulates the expression of hundreds of genes in response to the availability of GTP and the branched-chain amino acids isoleucine, leucine, and valine (ILV). CodY DNA-binding activity is high when GTP and ILV are abundant. When GTP and ILV are limited, CodY's affinity for DNA drops, altering expression of CodY-regulated targets. In this work, we investigated the impact of guanine nucleotides (GNs) on S. aureus physiology and CodY activity by constructing a guaA null mutant (ΔguaA strain). De novo biosynthesis of guanine monophosphate is abolished due to the guaA mutation; thus, the mutant cells require exogenous guanosine for growth. We also found that CodY activity was reduced when we knocked out guaA, activating the Agr two-component system and increasing secreted protease activity. Notably, in a rich, complex medium, we detected an increase in alternative sigma factor B activity in the ΔguaA mutant, which results in a 5-fold increase in production of the antioxidant pigment staphyloxanthin. Under biologically relevant flow conditions, ΔguaA cells failed to form robust biofilms when limited for guanine or guanosine. Transcriptome sequencing (RNA-Seq) analysis of the S. aureus transcriptome during growth in guanosine-limited chemostats revealed substantial CodY-dependent and -independent alterations of gene expression profiles. Importantly, these changes increase production of proteases and δ-toxin, suggesting that S. aureus exhibits a more invasive lifestyle when limited for guanosine. Further, gene products upregulated under GN limitation, including those necessary for lipoic acid biosynthesis and sugar transport, may prove to be useful drug targets for treating Gram-positive infections.IMPORTANCE Staphylococcus aureus infections impose a serious economic burden on health care facilities and patients because of the emergence of strains resistant to last-line antibiotics. Understanding the physiological processes governing fitness and virulence of S. aureus in response to environmental cues is critical for developing efficient diagnostics and treatments. De novo purine biosynthesis is essential for both fitness and virulence in S. aureus since inhibiting production cripples S. aureus's ability to cause infection. Here, we corroborate these findings and show that blocking guanine nucleotide synthesis severely affects S. aureus fitness by altering metabolic and virulence gene expression. Characterizing pathways and gene products upregulated in response to guanine limitation can aid in the development of novel adjuvant strategies to combat S. aureus infections.

Human platelet antigens in disease

Platelets have various functions and participate in primary hemostasis, inflammation, and immune responses. Human platelet antigens (HPAs) are alloantigens expressed on the platelet membrane. Each HPA represent one of six platelet glycoproteins GPIIb, GPIIIa, GPIa, GPIbα, GPIbβ, and CD109, and six biallelic systems are grouped. A single nucleotide polymorphism (SNP) in the gene sequence causes a single amino acid substitution of relevant platelet glycoprotein with the exception of HPA-14bw. High-throughput next-generation sequencing-based method has been developed, which enable accurately identification of HPA polymorphisms. The roles of HPA in disease were reviewed. HPAs mediate platelet-microorganism and platelet-malignant cell interactions, and they also participate in pathogenesis of hemorrhagic fever with renal syndrome and infective endocarditis. The exploration of HPA polymorphisms in association with disease susceptibility of individuals will benefit prevention or management of disease.

Immunoproteomics to identify Staphylococcus aureus antigens expressed in bovine milk during mastitis

Staphylococcus aureus is an opportunistic pathogen affecting both human and animal species. An effective vaccine to prevent S. aureus bovine disease and transmission would have positive effects on animal well-being, food production, and human health. The objective of this study was to identify multiple antigens that are immunoreactive during udder colonization and disease for exploration as vaccine antigens to prevent bovine mastitis. Staphylococcus aureus produces several cell wall-anchored and surface-associated virulence factors that play key roles in the pathogenesis of mastitis. Many of these proteins are conserved between different strains of S. aureus and represent promising vaccine candidates. We used an immunoproteomics approach to identify antigenic proteins from the surface of S. aureus. The expression of cell wall and surface proteins from S. aureus was induced under low iron conditions, followed by trypsin extraction and separation by 2-dimensional electrophoresis. The separated proteins were blotted with antibodies from mastitic bovine milk and identified by liquid chromatography-mass spectrometry. Thirty-eight unique proteins were identified, of which 8 were predicted to be surface exposed and involved in S. aureus virulence. Two surface proteins, iron-regulated surface determinant protein C (IsdC) and ESAT-6 secretion system extracellular protein (EsxA), were cloned, expressed, and purified from Escherichia coli for confirmation of immune reactivity by ELISA. A PCR of 37 bovine S. aureus isolates indicated that the presence of esxA and isdC is conserved, and amino acid alignments revealed that IsdC and EsxA sequences are highly conserved. The immunoproteomics technique used in this study generated reproducible results and identified surface exposed and reactive antigens for further characterization.

Genomic evolution of Staphylococcus aureus isolates colonizing the nares and progressing to bacteremia

Background

Nasal colonization by Staphylococcus aureus is a key risk factor for bacteremia. The objective of this study is to identify genomic modifications occurring in nasal carriage strains of S. aureus as they progress to bacteremia in a cohort of hospitalized patients.

Methods

Eight patients with S. aureus bacteremia were identified. Genomic sequences of the bloodstream isolates were compared with 57 nasal isolates collected longitudinally prior to the occurrence of bacteremia, which covered a timespan of up to 326 days before bacteremia.

Results

Within each subject, nasal colonizing strains were closely related to bacteremia strains. Within a subject, the number of single nucleotide polymorphisms (SNPs) observed between time points was greater than within a single time point. Co-colonization and strain replacement were observed in one case. In all cases colonization progressed to bacteremia without addition of new virulence genes. In one case, a mutation in the accessory gene regulator gene caused abrogation of agr function.

Conclusion

S. aureus evolves in the human nares at a variable rate. Progression of S. aureus nasal colonization to nosocomial infection is seldom associated with acquisition of new virulence determinants. Mutation in the agr gene with abrogation of function was associated with progression to bacteremia in one case.

Staphylococcal Protein A Contributes to Persistent Colonization of Mice with Staphylococcus aureus

Staphylococcus aureus persistently colonizes the nasopharynx in humans, which increases the risk for invasive diseases, such as skin infection and bacteremia. Nasal colonization triggers IgG responses against staphylococcal surface antigens; however, these antibodies cannot prevent subsequent colonization or disease. Here, we describe S. aureus WU1, a multilocus sequence type 88 (ST88) isolate that persistently colonizes the nasopharynx in mice. We report that staphylococcal protein A (SpA) is required for persistence of S. aureus WU1 in the nasopharynx. Compared to animals colonized by wild-type S. aureus, mice colonized with the Δspa variant mount increased IgG responses against staphylococcal colonization determinants. Immunization of mice with a nontoxigenic SpA variant, which cannot cross-link B cell receptors and divert antibody responses, elicits protein A-neutralizing antibodies that promote IgG responses against colonizing S. aureus and diminish pathogen persistence.IMPORTANCE Staphylococcus aureus persistently colonizes the nasopharynx in about one-third of the human population, thereby promoting community- and hospital-acquired infections. Antibiotics are currently used for decolonization of individuals at increased risk of infection. However, the efficacy of antibiotics is limited by recolonization and selection for drug-resistant strains. Here, we propose a model of how staphylococcal protein A (SpA), a B cell superantigen, modifies host immune responses during colonization to support continued persistence of S. aureus in the nasopharynx. We show that this mechanism can be thwarted by vaccine-induced anti-SpA antibodies that promote IgG responses against staphylococcal antigens and diminish colonization.

Human Immunoglobulin G Cannot Inhibit Fibrinogen Binding by the Genetically Diverse A Domain of Staphylococcus aureus Fibronectin-Binding Protein A

The fibronectin-binding protein A (FnBPA) is a cell surface-associated protein of Staphylococcus aureus which mediates adherence to the host extracellular matrix and is important for bacterial virulence. Previously, substantial sequence diversity was found among strains in the fibrinogen-binding A domain of this protein, and 7 different isotypes were described. The effect of this sequence diversity on the human antibody response, in terms of both antibody production and antibody function, remains unclear. In this study, we identify five different FnBPA A domain isotypes based on the sequence results of 22 clinical S. aureus isolates, obtained from the same number of patients suffering from bacteremia. Using a bead-based Luminex technique, we measure the patients' total immunoglobulin G (IgG) against the 7 FnBPA isotypes at the onset and during the time course of bacteremia (median of 10 serum samples per patient over a median of 35 days). A significant increase in IgG against the FnBPA A domain, including the isotype carried by the infecting strain, is observed in only three out of 22 patients (14%) after the onset of bacteremia. Using a Luminex-based FnBPA-fibrinogen-binding assay, we find that preincubation of recombinant FnBPA isotypes with IgG from diverse patients does not interfere with binding to fibrinogen. This observation is confirmed using an alternative Luminex-based assay and enzyme-linked immunosorbent assay (ELISA). IMPORTANCE Despite the many in vitro and murine in vivo studies involving FnBPA, the actual presence of this virulence factor during human infection is less well established. Furthermore, it is currently unknown to what extent sequence variation in such a virulence factor affects the human antibody response and the ability of antibodies to interfere with FnBPA function. This study sheds new light on these issues. First, the uniform presence of a patient's IgG against FnBPA indicates the presence and importance of this virulence factor during S. aureus pathogenesis. Second, the absence of an increase in antibody production in most patients following bacteremia indicates the complexity of S. aureus-host interactions, possibly involving immune evasion or lack of expression of FnBPA during invasive infection. Finally, we provide new insights into the inability of human antibodies to interfere with FnBPA-fibrinogen binding. These observations should be taken into account during the development of novel vaccination approaches.

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.

Perspectives on DNA Vaccines. Targeting Staphylococcal Adhesins to Prevent Implant Infections

DNA vaccines consist of a plasmid DNA genetically engineered to produce one or more proteins able to elicit protective immune responses against virulence factors of infectious pathogens. Once introduced into the cells of the host, a DNA vaccine induces a high production of antigens by the endogenous presence of the peptide codifying gene; improves antigen processing and presentation; may be able to simultaneously co-express multiple antigenic molecules; and, lastly, switches on both humoral and cellular immune responses. In this mini-review, we underscore the advantageous characteristics of DNA vaccines compared with traditional ones and provide summaries of some of the more recent studies on them, mainly focusing the possibility of their use in targeting the staphylococcal adhesins that play a key role in the first adhesive phase of implant infections.

Methicillin-Resistant Staphylococcus aureus Sequence Type (ST) 5 Isolates from Health Care and Agricultural Sources Adhere Equivalently to Human Keratinocytes

Staphylococcus aureus is part of the nasal microbiome of many humans and has become a significant public health burden due to infections with antibiotic-resistant strains, including methicillin-resistant S. aureus (MRSA) strains. Several lineages of S. aureus, including MRSA, are found in livestock species and can be acquired by humans through contact with animals. These livestock-associated MRSA (LA-MRSA) isolates raise public health concerns because of the potential for livestock to act as reservoirs for MRSA outside the hospital setting. In the United States, swine harbor a mixed population of LA-MRSA isolates, with the sequence type 398 (ST398), ST9, and ST5 lineages being detected. LA-MRSA ST5 isolates are particularly concerning to the public health community because, unlike the isolates in the ST398 and ST9 lineages, isolates in the ST5 lineage are a significant cause of human disease in both the hospital and community settings globally. The ability of swine-associated LA-MRSA ST5 isolates to adhere to human keratinocytes in vitro was investigated, and the adherence genes harbored by these isolates were evaluated and compared to those in clinical MRSA ST5 isolates from humans with no swine contact. The two subsets of isolates adhered equivalently to human keratinocytes in vitro and contained an indistinguishable complement of adherence genes that possessed a high degree of sequence identity. Collectively, our data indicate that, unlike LA-MRSA ST398 isolates, LA-MRSA ST5 isolates do not exhibit a reduced genotypic or phenotypic capacity to adhere to human keratinocytes.IMPORTANCE Our data indicate that swine-associated livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) ST5 isolates are as capable of adhering to human skin and have the same genetic potential to adhere as clinical MRSA ST5 isolates from humans. This suggests that humans in contact with livestock have the potential to become colonized with LA-MRSA ST5 isolates; however, the genes that contribute to the persistence of S. aureus on human skin were absent in LA-MRSA ST5 isolates. The data presented here are important evidence in evaluating the potential risks that LA-MRSA ST5 isolates pose to humans who come into contact with livestock.

Kaempferol Inhibits the Primary Attachment Phase of Biofilm Formation in Staphylococcus aureus

The ability to form biofilms on surfaces makes Staphylococcus aureus the main pathogenic factor in implanted medical device infections. The aim of this study was to discover a biofilm inhibitor distinct from the antibiotics used to prevent infections resulting from S. aureus biofilms. Here, we describe kaempferol, a small molecule with anti-biofilm activity that specifically inhibited the formation of S. aureus biofilms. Crystal violet (CV) staining and fluorescence microscopy clearly showed that 64 μg/ml kaempferol inhibited biofilm formation by 80%. Meanwhile, the minimum inhibitory concentration (MIC) and growth curve results indicated that kaempferol had no antibacterial activity against the tested bacterial strain. Kaempferol inhibited the primary attachment phase of biofilm formation, as determined by a fibrinogen-binding assay. Moreover, a fluorescence resonance energy transfer (FRET) assay and quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) analyses revealed that kaempferol reduced the activity of S. aureus sortaseA (SrtA) and the expression of adhesion-related genes. Based on these results, kaempferol provides a starting point for the development of novel anti-biofilm drugs, which may decrease the risk of bacterial drug resistance, to prevent S. aureus biofilm-related infections.

Covalent Grafting of Antifouling Phosphorylcholine-Based Copolymers with Antimicrobial Nitric Oxide Releasing Polymers to Enhance Infection-Resistant Properties of Medical Device Coatings

Medical device coatings that resist protein adhesion and bacterial contamination are highly desirable in the healthcare industry. In this work, an antifouling zwitterionic terpolymer, 2-methacryloyloxyethyl phosphorylcholine-co-butyl methacrylate-co-benzophenone (BPMPC), is covalently grafted to a nitric oxide (NO) releasing antimicrobial biomedical grade copolymer of silicone-polycarbonate-urethane, CarboSil, to significantly enhance the biocompatibility, nonspecific protein repulsion and infection-resistant properties. The NO donor embedded into CarboSil is S-nitroso-N-acetylpenicillamine (SNAP) and covalent grafting of the BPMPC is achieved through rapid UV-cross-linking, providing a stable, hydrophilic coating that has excellent durability over a period of several weeks under physiological conditions. The protein adsorption test results indicate a significant reduction (∼84-93%) of protein adhesion on the test samples compared to the control samples. Bacteria tests were also performed using the common nosocomial pathogen, Staphylococcus aureus. Test samples containing both NO donor and BPMPC show a 99.91 ± 0.06% reduction of viable bacteria when compared to control samples. This work demonstrates a synergistic combination of both antimicrobial and antifouling properties in medical devices using NO donors and zwitterionic copolymers that can be covalently grafted to any polymer surface.

Catheterization alters bladder ecology to potentiate Staphylococcus aureus infection of the urinary tract

Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging cause of catheter-associated urinary tract infection (CAUTI), which frequently progresses to more serious invasive infections. We adapted a mouse model of CAUTI to investigate how catheterization increases an individual's susceptibility to MRSA UTI. This analysis revealed that catheterization was required for MRSA to achieve high-level, persistent infection in the bladder. As shown previously, catheter placement induced an inflammatory response resulting in the release of the host protein fibrinogen (Fg), which coated the bladder and implant. Following infection, we showed that MRSA attached to the urothelium and implant in patterns that colocalized with deposited Fg. Furthermore, MRSA exacerbated the host inflammatory response to stimulate the additional release and accumulation of Fg in the urinary tract, which facilitated MRSA colonization. Consistent with this model, analysis of catheters from patients with S. aureus-positive cultures revealed colocalization of Fg, which was deposited on the catheter, with S. aureus Clumping Factors A and B (ClfA and ClfB) have been shown to contribute to MRSA-Fg interactions in other models of disease. We found that mutants in clfA had significantly greater Fg-binding defects than mutants in clfB in several in vitro assays. Paradoxically, only the ClfB^- strain was significantly attenuated in the CAUTI model. Together, these data suggest that catheterization alters the urinary tract environment to promote MRSA CAUTI pathogenesis by inducing the release of Fg, which the pathogen enhances to persist in the urinary tract despite the host's robust immune response.

Rewiring of the FtsH regulatory network by a single nucleotide change in saeS of Staphylococcus aureus

In the Gram-positive pathogen Staphylococcus aureus, the membrane-bound ATP-dependent metalloprotease FtsH plays a critical role in resistance to various stressors. However, the molecular mechanism of the FtsH functions is not known. Here, we identified core FtsH target proteins in S. aureus. In the strains Newman and USA300, the abundance of 33 proteins were altered in both strains, of which 11 were identified as core FtsH substrate protein candidates. In the strain Newman and some other S. aureus strains, the sensor histidine kinase SaeS has an L18P (T53C in saeS) substitution, which transformed the protein into an FtsH substrate. Due to the increase of SaeS L18P in the ftsH mutant, Eap, a sae-regulon protein, was also increased in abundance, causing the Newman-specific cell-aggregation phenotype. Regardless of the strain background, however, the ftsH mutants showed lower virulence and survival in a murine infection model. Our study illustrates the elasticity of the bacterial regulatory network, which can be rewired by a single substitution mutation.

Characterization of a Propionibacterium acnes Surface Protein as a Fibrinogen-Binding Protein

Propionibacterium acnes (P. acnes) is a major skin-associated bacterium that was long considered commensal, until several studies revealed it to be an opportunistic pathogen. We investigated the ability of P. acnes surface proteins to recognize ECM proteins and showed that a 58 kDa P. acnes surface protein was specifically recognized by human fibrinogen (hFg). The 58 kDa protein was further characterized by two-dimensional (2-D) electrophoresis and MALDI-ToF as a P. acnes host cell-surface attachment protein, PA25957, recognizing dermatan sulfate (DsA1). This protein sequence contains 432 amino acids with the presence of three structurally different domains: an N-terminal signal peptide, a C-terminal LPXTG motif, and a PT repeat region. DsA1 is mostly produced during stationary phase. It appears to be highly glycosylated, containing GalNAc residues. Purified DsA1 strongly recognizes the Aα and Bβ subunits of hFg, and specific enzymatic deglycosylation of hFg demonstrated the involvement of the protein backbone in the recognition process. The Bβ subunit of hFg was cloned in four peptide fractions (Fg1-Fg4). The N-terminal Fg1 peptide of hFg was recognized by DsA1, and priming DsA1 with Fg1 inhibited DsA1/hFg recognition. We describe here for the first time, the characterization of a P. acnes surface glycoprotein recognizing human fibrinogen.

The Staphylococcal Biofilm: Adhesins, Regulation, and Host Response

The staphylococci comprise a diverse genus of Gram-positive, nonmotile commensal organisms that inhabit the skin and mucous membranes of humans and other mammals. In general, staphylococci are benign members of the natural flora, but many species have the capacity to be opportunistic pathogens, mainly infecting individuals who have medical device implants or are otherwise immunocompromised. Staphylococcus aureus and Staphylococcus epidermidis are major sources of hospital-acquired infections and are the most common causes of surgical site infections and medical device-associated bloodstream infections. The ability of staphylococci to form biofilms in vivo makes them highly resistant to chemotherapeutics and leads to chronic diseases. These biofilm infections include osteomyelitis, endocarditis, medical device infections, and persistence in the cystic fibrosis lung. Here, we provide a comprehensive analysis of our current understanding of staphylococcal biofilm formation, with an emphasis on adhesins and regulation, while also addressing how staphylococcal biofilms interact with the immune system. On the whole, this review will provide a thorough picture of biofilm formation of the staphylococcus genus and how this mode of growth impacts the host.

Enhanced antibacterial efficacy of nitric oxide releasing thermoplastic polyurethanes with antifouling hydrophilic topcoats

Surface fouling is one of the leading causes of infection associated with implants, stents, catheters, and other medical devices. The surface chemistry of medical device coatings is important in controlling and/or preventing fouling. In this study, we have shown that a combination of nitric oxide releasing hydrophobic polymer with a hydrophilic polymer topcoat can significantly reduce protein attachment and subsequently reduce bacterial adhesion as a result of the synergistic effect. Nitric oxide (NO) is a well-known potent antibacterial agent due to its adverse reactions on microbial cell components. Owing to the surface chemistry of hydrophilic polymers, they are suitable as antifouling topcoats. In this study, four biomedical grade polymers were compared for protein adhesion and NO-release behavior: CarboSil 2080A, silicone rubber, SP60D60, and SG80A. SP60D60 was found to resist protein adsorption up to 80% when compared to the other polymers while CarboSil 2080A maintained a steady NO flux even after 24 hours (∼0.50 × 10-10 mol cm-2 min-1) of soaking in buffer solution with a loss of less than 3% S-nitroso-N-acetylpenicillamine (SNAP), the NO donor molecule, in the leaching analysis. Therefore, CarboSil 2080A incorporated with SNAP and top-coated with SP60D60 was tested for antibacterial efficacy after exposure to fibrinogen, an abundantly found protein in blood. The NO-releasing CarboSil 2080A with the SP60D60 top-coated polymer showed a 96% reduction in Staphylococcus aureus viable cell count compared to the control samples. Hence, the study demonstrated that a hydrophilic polymer topcoat, when applied to a polymer with sustained NO release from an underlying SNAP incorporated hydrophobic polymer, can reduce bacterial adhesion and be used as a highly efficient antifouling, antibacterial polymer for biomedical applications.

Clumping Factor B Promotes Adherence of Staphylococcus aureus to Corneocytes in Atopic Dermatitis

Staphylococcus aureus skin infection is a frequent and recurrent problem in children with the common inflammatory skin disease atopic dermatitis (AD). S. aureus colonizes the skin of the majority of children with AD and exacerbates the disease. The first step during colonization and infection is bacterial adhesion to the cornified envelope of corneocytes in the outer layer, the stratum corneum. Corneocytes from AD skin are structurally different from corneocytes from normal healthy skin. The objective of this study was to identify bacterial proteins that promote the adherence of S. aureus to AD corneocytes. S. aureus strains from clonal complexes 1 and 8 were more frequently isolated from infected AD skin than from the nasal cavity of healthy children. AD strains had increased ClfB ligand binding activity compared to normal nasal carriage strains. Adherence of single S. aureus bacteria to corneocytes from AD patients ex vivo was studied using atomic force microscopy. Bacteria expressing ClfB recognized ligands distributed over the entire corneocyte surface. The ability of an isogenic ClfB-deficient mutant to adhere to AD corneocytes compared to that of its parent clonal complex 1 clinical strain was greatly reduced. ClfB from clonal complex 1 strains had a slightly higher binding affinity for its ligand than ClfB from strains from other clonal complexes. Our results provide new insights into the first step in the establishment of S. aureus colonization in AD patients. ClfB is a key adhesion molecule for the interaction of S. aureus with AD corneocytes and represents a target for intervention.

Adaptive Upregulation of Clumping Factor A (ClfA) by Staphylococcus aureus in the Obese, Type 2 Diabetic Host Mediates Increased Virulence

Obesity and associated type 2 diabetes (T2D) are important risk factors for infection following orthopedic implant surgery. Staphylococcus aureus, the most common pathogen in bone infections, adapts to multiple environments to survive and evade host immune responses. Whether adaptation of S. aureus to the unique environment of the obese/T2D host accounts for its increased virulence and persistence in this population is unknown. Thus, we assessed implant-associated osteomyelitis in normal versus high-fat-diet obese/T2D mice and found that S. aureus infection was more severe, including increases in bone abscesses relative to nondiabetic controls. S. aureus isolated from bone of obese/T2D mice displayed marked upregulation of four adhesion genes (clfA, clfB, bbp, and sdrC), all with binding affinity for fibrin(ogen). Immunostaining of infected bone revealed increased fibrin deposition surrounding bacterial abscesses in obese/T2D mice. In vitro coagulation assays demonstrated a hypercoagulable state in obese/T2D mice that was comparable to that of diabetic patients. S. aureus with an inactivating mutation in clumping factor A (clfA) showed a reduction in bone infection severity that eliminated the effect of obesity/T2D, while infections in control mice were unchanged. In infected mice that overexpress plasminogen activator inhibitor-1 (PAI-1), S. aureusclfA expression and fibrin-encapsulated abscess communities in bone were also increased, further linking fibrin deposition to S. aureus expression of clfA and infection severity. Together, these results demonstrate an adaptation by S. aureus to obesity/T2D with increased expression of clfA that is associated with the hypercoagulable state of the host and increased virulence of S. aureus.

The S. aureus 4-oxalocrotonate tautomerase SAR1376 enhances immune responses when fused to several antigens

A persistent goal of vaccine development is the enhancement of the immunogenicity of antigens while maintaining safety. One strategy involves alteration of the presentation of the antigen by combining antigens with a multimeric scaffold. Multi-antigen vaccines are under development, and there are presently far more candidate antigens than antigen scaffolding strategies. This is potentially problematic, since prior immunity to a scaffold may inhibit immune responses to the antigen-scaffold combination. In this study, a series of domains from S. aureus which have been shown to crystallise into multimeric structures have been examined for their scaffolding potential. Of these domains, SAR1376, a 62 amino acid member of the 4-oxalocrotonate tautomerase (4-OT) family, was pro-immunogenic in mice when fused to a range of pathogen antigens from both S. aureus and P. falciparum, and delivered by either DNA vaccination, viral vector vaccines or as protein-in-adjuvant formulations. The adjuvant effect did not depend on enzymatic activity, but was abrogated by mutations disrupting the hexameric structure of the protein. We therefore propose that SAR1376, and perhaps other members of the 4-OT protein family, represent very small domains which can be fused to a wide range of antigens, enhancing immune responses against them.

Staphylococcus aureus biofilm: a complex developmental organism

Chronic biofilm-associated infections caused by Staphylococcus aureus often lead to significant increases in morbidity and mortality, particularly when associated with indwelling medical devices. This has triggered a great deal of research attempting to understand the molecular mechanisms that control S. aureus biofilm formation and the basis for the recalcitrance of these multicellular structures to antibiotic therapy. The purpose of this review is to summarize our current understanding of S. aureus biofilm development, focusing on the description of a newly-defined, five-stage model of biofilm development and the mechanisms required for each stage. Importantly, this model includes an alternate view of the processes involved in microcolony formation in S. aureus and suggests that these structures originate as a result of stochastically regulated metabolic heterogeneity and proliferation within a maturing biofilm population, rather than a subtractive process involving the release of cell clusters from a thick, unstructured biofilm. Importantly, it is proposed that this new model of biofilm development involves the genetically programmed generation of metabolically distinct subpopulations of cells, resulting in an overall population that is better able to adapt to rapidly changing environmental conditions.

Genomic and transcriptomic comparison between Staphylococcus aureus strains associated with high and low within herd prevalence of intra-mammary infection

Background

Staphylococcus aureus (Staph. aureus) is one of the major pathogens causing mastitis in dairy ruminants worldwide. The chronic nature of Staph. aureus infection enhances the contagiousness risk and diffusion in herds. In order to identify the factors involved in intra-mammary infection (IMI) and diffusion in dairy cows, we investigated the molecular characteristics of two groups of Staph. aureus strains belonging to ST8 and ST398, differing in clinical properties, through comparison of whole genome and whole transcriptome sequencing.

Results

The two groups of strains, one originated from high IMI prevalence herds and the other from low IMI prevalence herds, present a peculiar set of genes and polymorphisms related to phenotypic features, such as bacterial invasion of mammary epithelial cells and host adaptation. Transcriptomic analysis supports the high propensity of ST8 strain to chronicity of infection and to a higher potential cytotoxicity.

Conclusions

Our data are consistent with the invasiveness and host adaptation feature for the strains GTB/ST8 associated to high within-herd prevalence of mastitis. Variation in genes coding for surface exposed proteins and those associated to virulence and defence could constitute good targets for further research.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-0931-8) contains supplementary material, which is available to authorized users.

Inhibitory effects of antibiofilm compound 1 against Staphylococcus aureus biofilms

A novel benzimidazole molecule that was identified in a small-molecule screen and is known as antibiofilm compound 1 (ABC-1) has been found to prevent bacterial biofilm formation by multiple bacterial pathogens, including Staphylococcus aureus, without affecting bacterial growth. Here, the biofilm inhibiting ability of 156 μM ABC-1 was tested in various biofilm-forming strains of S. aureus. It was demonstrated that ABC-1 inhibits biofilm formation by these strains at micromolar concentrations regardless of the strains' dependence on Polysaccharide Intercellular Adhesin (PIA), cell wall-associated protein dependent or cell wall- associated extracellular DNA (eDNA). Of note, ABC-1 treatment primarily inhibited Protein A (SpA) expression in all strains tested. spa gene disruption showed decreased biofilm formation; however, the mutants still produced more biofilm than ABC-1 treated strains, implying that ABC-1 affects not only SpA but also other factors. Indeed, ABC-1 also attenuated the accumulation of PIA and eDNA on cell surface. Our results suggest that ABC-1 has pleotropic effects on several biofilm components and thus inhibits biofilm formation by S. aureus.

Inhibition of major integrin αV β3 reduces Staphylococcus aureus attachment to sheared human endothelial cells

Essentials Staphylococcus aureus (S. aureus) binds and impairs function of vascular endothelial cells (EC). We investigated the molecular signals triggered by S. aureus adhesion to EC. Inhibition of the EC integrin αVβ3 reduces S. aureus binding and rescues EC function. αVβ3 blockade represents an attractive target to treat S. aureus bloodborne infections.

SUMMARY

Background Vascular endothelial dysfunction with associated edema and organ failure is one of the hallmarks of sepsis. Although a large number of microorganisms can cause sepsis, Staphylococcus aureus (S. aureus) is one of the primary etiologic agents. Currently, there are no approved specific treatments for sepsis, and the initial management bundle is therefore focused on cardiorespiratory resuscitation and mitigation of the immediate threat of uncontrolled infection. The continuous emergence of antibiotic-resistant strains of bacteria necessitates the development of new therapeutic approaches for this disease. Objective To identify the molecular mechanisms leading to endothelial dysfunction as a result of S. aureus binding.

METHODS

Binding of wild type and Clumping factor A (ClfA) deficient S. aureus Newman to the endothelium was measured in vitro and in the mesenteric circulation of C57Bl/6 mice. The effects of the αV β3 blocker-cilengitide-on bacterial binding, endothelial VE-cadherin expression, apoptosis, proliferation and permeability were assessed. Results The major S. aureus cell wall protein ClfA bound to endothelial cell αV β3 in the presence of fibrinogen. This interaction resulted in disturbances in barrier function mediated by VE-cadherin in endothelial cell monolayers, and ultimately cell death by apoptosis. With a low concentration of cilengitide, ClfA binding to αV β3 was significantly inhibited both in vitro and in vivo. Moreover, preventing S. aureus from attaching to αV β3 resulted in a significant reduction in endothelial dysfunction following infection. Conclusion Inhibition of S. aureus ClfA binding to endothelial cell αV β3 by cilengitide prevents endothelial dysfunction.

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.

Fibrinogen Is at the Interface of Host Defense and Pathogen Virulence in Staphylococcus aureus Infection

Fibrinogen not only plays a pivotal role in hemostasis but also serves key roles in antimicrobial host defense. As a rapidly assembled provisional matrix protein, fibrin(ogen) can function as an early line of host protection by limiting bacterial growth, suppressing dissemination of microbes to distant sites, and mediating host bacterial killing. Fibrinogen-mediated host antimicrobial activity occurs predominantly through two general mechanisms, namely, fibrin matrices functioning as a protective barrier and fibrin(ogen) directly or indirectly driving host protective immune function. The potential of fibrin to limit bacterial infection and disease has been countered by numerous bacterial species evolving and maintaining virulence factors that engage hemostatic system components within vertebrate hosts. Bacterial factors have been isolated that simply bind fibrinogen or fibrin, promote fibrin polymer formation, or promote fibrin dissolution. Staphylococcus aureus is an opportunistic gram-positive bacterium, the causative agent of a wide range of human infectious diseases, and a prime example of a pathogen exquisitely sensitive to host fibrinogen. Indeed, current data suggest fibrinogen serves as a context-dependent determinant of host defense or pathogen virulence in Staphylococcus infection whose ultimate contribution is dictated by the expression of S. aureus virulence factors, the path of infection, and the tissue microenvironment.

A combination of positive dielectrophoresis driven on-line enrichment and aptamer-fluorescent silica nanoparticle label for rapid and sensitive detection of Staphylococcus aureus

Staphylococcus aureus (S. aureus) is an important human pathogen that causes several diseases ranging from superficial skin infections to life-threatening diseases. Here, a method combining positive dielectrophoresis (pDEP) driven on-line enrichment and aptamer-fluorescent silica nanoparticle label has been developed for the rapid and sensitive detection of S. aureus in microfluidic channels. An aptamer, having high affinity to S. aureus, is used as the molecular recognition tool and immobilized onto chloropropyl functionalized fluorescent silica nanoparticles through a click chemistry approach to obtain S. aureus aptamer-nanoparticle bioconjugates (Apt(S.aureus)/FNPs). The pDEP driven on-line enrichment technology was used for accumulating the Apt(S.aureus)/FNP labeled S. aureus. After incubating with S. aureus, the mixture of Apt(S.aureus)/FNP labeled S. aureus and Apt(S.aureus)/FNPs was directly introduced into the pDEP-based microfluidic system. By applying an AC voltage in a pDEP frequency region, the Apt(S.aureus)/FNP labelled S. aureus moved to the electrodes and accumulated in the electrode gap, while the free Apt(S.aureus)/FNPs flowed away. The signal that came from the Apt(S.aureus)/FNP labelled S. aureus in the focused detection areas was then detected. Profiting from the specificity of aptamer, signal amplification of FNP label and pDEP on-line enrichment, this assay can detect as low as 93 and 270 cfu mL(-1)S. aureus in deionized water and spiked water samples, respectively, with higher sensitivities than our previously reported Apt(S.aureus)/FNP based flow cytometry. Moreover, without the need for separation and washing steps usually required for FNP label involved bioassays, the total assay time including sample pretreatment was within 2 h.

Evaluation of double locus (clfB and spa) sequence typing for studying molecular epidemiology of methicillin-resistant Staphylococcus aureus in Taiwan

BACKGROUND

Pulsed-field gel electrophoresis (PFGE) is the "gold standard" for epidemiological investigation of methicillin-resistant Staphylococcus aureus (MRSA), but several DNA sequence-based methods have been developed in MRSA typing because of the unambiguous results.

METHODS

Ninety-one MRSA isolates were collected from the blood cultures of different patients from July 2008 to December 2008 in central Taiwan. The molecular characteristics of each isolate, including double locus sequence typing (DLST; spa and clfB typing), Staphylococcus cassette chromosome mec (SCCmec), and PFGE were determined for comparison.

RESULTS

Five major clfB types (types A-E), 18 spa types, 33 DLST genotypes, five SCCmec types, 17 pulsotypes have been observed. Three major DLST genotypes (A1-t002, C0-t037, and B1-t437) and two major pulsotypes (6 and 8) were identified. Most clfB type A isolates (97.1%) were SCCmec type II and all clfB type C isolates (100%) were SCCmec type III. Most clfB type B isolates (88.9%) were SCCmec type IV (59.3%) and V_T (29.6%). All (100%) clfB subtypes A1, A2, and C isolates and 70.4% of clfB type B isolates belonged to healthcare-associated-MRSA. The average congruence was 57.7% between DLST and PFGE, and 96.6% between clfB and SCCmec type. The index of discrimination of SCCmec, clfB, spa, PFGE, and DLST was 0.72, 0.79, 0.80, 0.81, and 0.87, respectively.

CONCLUSION

ClfB type has high congruence with SCCmec type. The DLST method in this study yielded a higher discriminatory power than PFGE in local investigation of molecular epidemiology of MRSA and a promising alternative to PFGE.

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.

Platelets and infections - complex interactions with bacteria

Platelets can be considered sentinels of vascular system due to their high number in the circulation and to the range of functional immunoreceptors they express. Platelets express a wide range of potential bacterial receptors, including complement receptors, FcγRII, Toll-like receptors but also integrins conventionally described in the hemostatic response, such as GPIIb–IIIa or GPIb. Bacteria bind these receptors either directly, or indirectly via fibrinogen, fibronectin, the first complement C1q, the von Willebrand Factor, etc. The fate of platelet-bound bacteria is questioned. Several studies reported the ability of activated platelets to internalize bacteria such as Staphylococcus aureus or Porphyromonas gingivalis, though there is no clue on what happens thereafter. Are they sheltered from the immune system in the cytoplasm of platelets or are they lysed? Indeed, while the presence of phagolysosome has not been demonstrated in platelets, they contain antimicrobial peptides that were shown to be efficient on S. aureus. Besides, the fact that bacteria can bind to platelets via receptors involved in hemostasis suggests that they may induce aggregation; this has indeed been described for Streptococcus sanguinis, S. epidermidis, or C. pneumoniae. On the other hand, platelets are able to display an inflammatory response to an infectious triggering. We, and others, have shown that platelet release soluble immunomodulatory factors upon stimulation by bacterial components. Moreover, interactions between bacteria and platelets are not limited to only these two partners. Indeed, platelets are also essential for the formation of neutrophil extracellular traps by neutrophils, resulting in bacterial clearance by trapping bacteria and concentrating antibacterial factors but in enhancing thrombosis. In conclusion, the platelet–bacteria interplay is a complex game; its fine analysis is complicated by the fact that the inflammatory component adds to the aggregation response.

Calcium binding proteins and calcium signaling in prokaryotes

With the continued increase of genomic information and computational analyses during the recent years, the number of newly discovered calcium binding proteins (CaBPs) in prokaryotic organisms has increased dramatically. These proteins contain sequences that closely resemble a variety of eukaryotic calcium (Ca(2+)) binding motifs including the canonical and pseudo EF-hand motifs, Ca(2+)-binding β-roll, Greek key motif and a novel putative Ca(2+)-binding domain, called the Big domain. Prokaryotic CaBPs have been implicated in diverse cellular activities such as division, development, motility, homeostasis, stress response, secretion, transport, signaling and host-pathogen interactions. However, the majority of these proteins are hypothetical, and only few of them have been studied functionally. The finding of many diverse CaBPs in prokaryotic genomes opens an exciting area of research to explore and define the role of Ca(2+) in organisms other than eukaryotes. This review presents the most recent developments in the field of CaBPs and novel advancements in the role of Ca(2+) in prokaryotes.

Temporal and stochastic control of Staphylococcus aureus biofilm development

Biofilm communities contain distinct microniches that result in metabolic heterogeneity and variability in gene expression. Previously, these niches were visualized within Staphylococcus aureus biofilms by observing differential expression of the cid and lrg operons during tower formation. In the present study, we examined early biofilm development and identified two new stages (designated “multiplication” and “exodus”) that were associated with changes in matrix composition and a distinct reorganization of the cells as the biofilm matured. The initial attachment and multiplication stages were shown to be protease sensitive but independent of most cell surface-associated proteins. Interestingly, after 6 h of growth, an exodus of the biofilm population that followed the transition of the biofilm to DNase I sensitivity was demonstrated. Furthermore, disruption of the gene encoding staphylococcal nuclease (nuc) abrogated this exodus event, causing hyperproliferation of the biofilm and disrupting normal tower development. Immediately prior to the exodus event, S. aureus cells carrying a nuc::gfp promoter fusion demonstrated Sae-dependent expression but only in an apparently random subpopulation of cells. In contrast to the existing model for tower development in S. aureus, the results of this study suggest the presence of a Sae-controlled nuclease-mediated exodus of biofilm cells that is required for the development of tower structures. Furthermore, these studies indicate that the differential expression of nuc during biofilm development is subject to stochastic regulatory mechanisms that are independent of the formation of metabolic microniches.

In this study, we provide a novel view of four early stages of biofilm formation by the human pathogen Staphylococcus aureus. We identified an initial nucleoprotein matrix during biofilm development that is DNase I insensitive until a critical point when a nuclease-mediated exodus of the population is induced prior to tower formation. Unlike the previously described dispersal of cells that occurs after tower development, we found that the mechanism controlling this exodus event is dependent on the Sae regulatory system and independent of Agr. In addition, we revealed that the gene encoding the secreted staphylococcal nuclease was expressed in only a subpopulation of cells, consistent with a model in which biofilms exhibit multicellular characteristics, including the presence of specialized cells and a division of labor that imparts functional consequences to the remainder of the population.

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.

Surface adhesins and exopolymers of selected foodborne pathogens

The ability of bacteria to bind different compounds and to adhere to biotic and abiotic surfaces provides them with a range of advantages, such as colonization of various tissues, internalization, avoidance of an immune response, and survival and persistence in the environment. A variety of bacterial surface structures are involved in this process and these promote bacterial adhesion in a more or less specific manner. In this review, we will focus on those surface adhesins and exopolymers in selected foodborne pathogens that are involved mainly in primary adhesion. Their role in biofilm development will also be considered when appropriate. Both the clinical impact and the implications for food safety of such adhesion will be discussed.

SdrC induces staphylococcal biofilm formation through a homophilic interaction

The molecular pathogenesis of many Staphylococcus aureus infections involves growth of bacteria as biofilm. In addition to polysaccharide intercellular adhesin (PIA) and extracellular DNA, surface proteins appear to mediate the transition of bacteria from planktonic growth to sessile lifestyle as well as biofilm growth, and can enable these processes even in the absence of PIA expression. However, the molecular mechanisms by which surface proteins contribute to biofilm formation are incompletely understood. Here we demonstrate that self-association of the serine-aspartate repeat protein SdrC promotes both bacterial adherence to surfaces and biofilm formation. However, this homophilic interaction is not required for the attachment of bacteria to abiotic surfaces. We identified the subdomain that mediates SdrC dimerization and subsequent cell-cell interactions. In addition, we determined that two adjacently located amino acid sequences within this subdomain are required for the SdrC homophilic interaction. Comparative amino acid sequence analysis indicated that these binding sites are conserved. In summary, our study identifies SdrC as a novel molecular determinant in staphylococcal biofilm formation and describes the mechanism responsible for intercellular interactions. Furthermore, these findings contribute to a growing body of evidence suggesting that homophilic interactions between surface proteins present on neighbouring bacteria induce biofilm growth.

Clostridium difficile has a single sortase, SrtB, that can be inhibited by small-molecule inhibitors

Background

Bacterial sortases are transpeptidases that covalently anchor surface proteins to the peptidoglycan of the Gram-positive cell wall. Sortase protein anchoring is mediated by a conserved cell wall sorting signal on the anchored protein, comprising of a C-terminal recognition sequence containing an “LPXTG-like” motif, followed by a hydrophobic domain and a positively charged tail.

Results

We report that Clostridium difficile strain 630 encodes a single sortase (SrtB). A FRET-based assay was used to confirm that recombinant SrtB catalyzes the cleavage of fluorescently labelled peptides containing (S/P)PXTG motifs. Strain 630 encodes seven predicted cell wall proteins with the (S/P)PXTG sorting motif, four of which are conserved across all five C. difficile lineages and include potential adhesins and cell wall hydrolases. Replacement of the predicted catalytic cysteine residue at position 209 with alanine abolishes SrtB activity, as does addition of the cysteine protease inhibitor MTSET to the reaction. Mass spectrometry reveals the cleavage site to be between the threonine and glycine residues of the (S/P)PXTG peptide. Small-molecule inhibitors identified through an in silico screen inhibit SrtB enzymatic activity to a greater degree than MTSET.

Conclusions

These results demonstrate for the first time that C. difficile encodes a single sortase enzyme, which cleaves motifs containing (S/P)PXTG in-vitro. The activity of the sortase can be inhibited by mutation of a cysteine residue in the predicted active site and by small-molecule inhibitors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0219-1) contains supplementary material, which is available to authorized users.

Staphylococci: colonizers and pathogens of human skin

 Staphylococci are abundant bacteria of the human skin microbiome. Several species, particularly Staphylococcus aureus and Staphylococcus epidermidis, are opportunistic pathogens and cause significant disease. The human skin serves many functions and here we review its role as an antimicrobial barrier and the staphylococcal mechanisms to colonize and counteract the various stresses present in this niche. Successful colonization is achieved using a diversity of adhesins, surface proteins and secreted enzymes to counteract the antimicrobial peptides, enzymes and lipid matrix components present in the acid mantle. Further mechanisms enable these bacteria to overcome osmotic and acid stresses and desiccation in order to survive the exacting demands of an ever-changing landscape.

Specific capture and detection of Staphylococcus aureus with high-affinity modified aptamers to cell surface components

Slow off-rate modified aptamer (SOMAmer) reagents were generated to several Staphylococcus aureus cell surface-associated proteins via SELEX with multiple modified DNA libraries using purified recombinant or native proteins. High-affinity binding agents with sub-nanomolar Kd 's were obtained for staphylococcal protein A (SpA), clumping factors (ClfA, ClfB), fibronectin-binding proteins (FnbA, FnbB) and iron-regulated surface determinants (Isd). Further screening revealed several SOMAmers that specifically bound to Staph. aureus cells from all strains that were tested, but not to other staphylococci or other bacteria. SpA and ClfA SOMAmers proved useful for the selective capture and enrichment of Staph. aureus cells, as shown by culture and PCR, leading to improved limits of detection and efficient removal of PCR inhibitors. Detection of Staph. aureus cells was enhanced by several orders of magnitude when the bacterial cell surface was coated with SOMAmers followed by qPCR of the SOMAmers. Furthermore, fluorescence-labelled SpA SOMAmers demonstrated their utility as direct detection agents in flow cytometry. Significance and impact of the study: Monitoring for microbial contamination of food, water, nonsterile products or the environment is typically based on culture, PCR or antibodies. Aptamers that bind with high specificity and affinity to well-conserved cell surface epitopes represent a promising novel type of reagents to detect bacterial cells without the need for culture or cell lysis, including for the capture and enrichment of bacteria present at low cell densities and for the direct detection via qPCR or fluorescent staining.

Staphylococcus aureus surface proteins involved in adaptation to oxacillin identified using a novel cell shaving approach

Staphylococcus aureus is a Gram-positive pathogen responsible for a variety of infections, and some strains are resistant to virtually all classes of antibiotics. Cell shaving proteomics using a novel probability scoring algorithm to compare the surfaceomes of the methicillin-resistant, laboratory-adapted S. aureus COL strain with a COL strain in vitro adapted to high levels of oxacillin (APT). APT displayed altered cell morphology compared with COL and increased aggregation in biofilm assays. Increased resistance to β-lactam antibiotics was observed, but adaptation to oxacillin did not confer multidrug resistance. Analysis of the S. aureus COL and APT surfaceomes identified 150 proteins at a threshold determined by the scoring algorithm. Proteins unique to APT included the LytR-CpsA-Psr (LCP) domain-containing MsrR and SACOL2302. Quantitative RT-PCR showed increased expression of sacol2302 in APT grown with oxacillin (>6-fold compared with COL). Overexpression of sacol2302 in COL to levels consistent with APT (+ oxacillin) did not influence biofilm formation or β-lactam resistance. Proteomics using iTRAQ and LC-MS/MS identified 1323 proteins (∼50% of the theoretical S. aureus proteome), and cluster analysis demonstrated elevated APT abundances of LCP proteins, capsule and peptidoglycan biosynthesis proteins, and proteins involved in wall remodelling. Adaptation to oxacillin also induced urease proteins, which maintained culture pH compared to COL. These results show that S. aureus modifies surface architecture in response to antibiotic adaptation.

Irradiated wild-type and Spa mutant Staphylococcus aureus induce anti-S. aureus immune responses in mice which do not protect against subsequent intravenous challenge

Staphylococcus aureus remains an important human and animal pathogen. Its pathogenicity is determined in part by expression of the Spa-immune subversion protein, neutralising the activity of which provides partial protection in murine models, as does experimental infection with live S. aureus with Spa gene deletions followed by antibiotic-mediated cure in mice. Together, these data raise the question of whether Spa mutant S. aureus might represent a viable vaccine. Here, we find that gamma-irradiated S. aureus strains, both wild-type and null mutant of spa, are immunogenic in mice when administered intramuscularly, eliciting large amounts of anti-S. aureus antibodies, as judged by whole-cell immunoassay on fixed microorganisms. We used an intravenous challenge system to assess vaccine efficacy, the sensitivity of which was increased by studying renal bacterial concentrations in both kidneys. Despite this, protection from intravenous challenge was not observed (mean difference between vaccinated and unvaccinated mice 0.27 log(10) with 95% confidence interval -0.922 to 1.467). Surprisingly, antibody responses elicited against a panel of protective cell surface proteins were very low, indicating that most antibody induced is not protective. Additionally, these data suggest a limited role for irradiated wild-type or spa mutant S. aureus as vaccines.

Covering all the Bases: Preclinical Development of an Effective Staphylococcus aureus Vaccine

A key aspect of the pathogenesis of the Gram positive bacterium Staphylococcus aureus is its ability to rapidly adapt to the host environment during the course of an infection. To successfully establish infection, the organism deploys a variety of survival and immune evasion strategies, ranging from the acquisition of essential nutrients and expression of adhesins, which promote colonization and survival, to the elaboration of virulence factors such as capsule, which aids host immune evasion. The ability of S. aureus to deploy different virulence factors must be taken into account for S. aureus vaccine design. Here, we present a strategy for designing an effective vaccine against S. aureus disease by evaluating vaccine candidate performance in multiple in vivo models targeted to mimic aspects of human disease, and by co-development of functional in vitro immunoassays that measure the neutralization of relevant S. aureus virulence factors.

N-acetylglucosaminylation of serine-aspartate repeat proteins promotes Staphylococcus aureus bloodstream infection

Staphylococcus aureus secretes products that convert host fibrinogen to fibrin and promote its agglutination with fibrin fibrils, thereby shielding bacteria from immune defenses. The agglutination reaction involves ClfA (clumping factor A), a surface protein with serine-aspartate (SD) repeats that captures fibrin fibrils and fibrinogen. Pathogenic staphylococci express several different SD proteins that are modified by two glycosyltransferases, SdgA and SdgB. Here, we characterized three genes of S. aureus, aggA, aggB (sdgA), and aggC (sdgB), and show that aggA and aggC contribute to staphylococcal agglutination with fibrin fibrils in human plasma. We demonstrate that aggB (sdgA) and aggC (sdgB) are involved in GlcNAc modification of the ClfA SD repeats. However, only sdgB is essential for GlcNAc modification, and an sdgB mutant is defective in the pathogenesis of sepsis in mice. Thus, GlcNAc modification of proteins promotes S. aureus replication in the bloodstream of mammalian hosts.