Characterization of the importance of polysaccharide intercellular adhesin/hemagglutinin of Staphylococcus epidermidis in the pathogenesis of biomaterial-based infection in a mouse foreign body infection model

AraC-Type Regulator Rsp Adapts Staphylococcus aureus Gene Expression to Acute Infection

Staphylococcus aureus is an important human pathogen that can cause two categories of severe infections. Acute infections are characterized by pronounced toxin production, while chronic infections often involve biofilm formation. However, it is poorly understood how S. aureus controls the expression of genes associated with acute versus biofilm-associated virulence. We here identified an AraC-type transcriptional regulator, Rsp, that promotes the production of key toxins while repressing major biofilm-associated genes and biofilm formation. Genome-wide transcriptional analysis and modeling of regulatory networks indicated that upregulation of the accessory gene regulator (Agr) and downregulation of the ica operon coding for the biofilm exopolysaccharide polysaccharide intercellular adhesin (PIA) were central to the regulatory impact of Rsp on virulence. Notably, the Rsp protein directly bound to the agrP2 and icaADBC promoters, resulting in strongly increased levels of the Agr-controlled toxins phenol-soluble modulins (PSMs) and alpha-toxin and reduced production of PIA. Accordingly, Rsp was essential for the development of bacteremia and skin infection, representing major types of acute S. aureus infection. Our findings give important insight into how S. aureus adapts the expression of its broad arsenal of virulence genes to promote different types of disease manifestations and identify the Rsp regulator as a potential target for strategies to control acute S. aureus infection.

Emerging interactions between matrix components during biofilm development

Bacterial cells are most often found in the form of multicellular aggregates commonly referred to as biofilms. Biofilms offer their member cells several benefits, such as resistance to killing by antimicrobials and predation. During biofilm formation there is a production of extracellular substances that, upon assembly, constitute an extracellular matrix. The ability to generate a matrix encasing the microbial cells is a common feature of biofilms, but there is diversity in matrix composition and in interaction between matrix components. The different components of bacterial biofilm extracellular matrixes, known as matrix interactions, and resulting implications are discussed in this review.

Effects of polysaccharide intercellular adhesin (PIA) in an ex vivo model of whole blood killing and in prosthetic joint infection (PJI): A role for C5a

BACKGROUND

A major complication of using medical devices is the development of biofilm-associated infection caused by Staphylococcus epidermidis where polysaccharide intercellular adhesin (PIA) is a major mechanism of biofilm accumulation. PIA affects innate and humoral immunity in isolated cells and animal models. Few studies have examined these effects in prosthetic joint infection (PJI).

METHODS

This study used ex vivo whole blood modelling in controls together with matched-serum and staphylococcal isolates from patients with PJI.

RESULTS

Whole blood killing of PIA positive S. epidermidis and its isogenic negative mutant was identical. Differences were unmasked in immunosuppressed whole blood pre-treated with dexamethasone where PIA positive bacteria showed a more resistant phenotype. PIA expression was identified in three unique patterns associated with bacteria and leukocytes, implicating a soluble form of PIA. Purified PIA reduced whole blood killing while increasing C5a levels. In clinically relevant staphylococcal isolates and serum samples from PJI patients; firstly complement C5a was increased 3-fold compared to controls; secondly, the C5a levels were significantly higher in serum from PJI patients whose isolates preferentially formed PIA-associated biofilms.

CONCLUSIONS

These data demonstrate for the first time that the biological effects of PIA are mediated through C5a in patients with PJI.

Atmospheric pressure nonthermal plasmas for bacterial biofilm prevention and eradication

Biofilms are three-dimensional structures formed by surface-attached microorganisms and their extracellular products. Biofilms formed by pathogenic microorganisms play an important role in human diseases. Higher resistance to antimicrobial agents and changes in microbial physiology make treating biofilm infections very complex. Atmospheric pressure nonthermal plasmas (NTPs) are a novel and powerful tool for antimicrobial treatment. The microbicidal activity of NTPs has an unspecific character due to the synergetic actions of bioactive components of the plasma torch, including charged particles, reactive species, and UV radiation. This review focuses on specific traits of biofilms, their role in human diseases, and those effects of NTP that are helpful for treating biofilm infections. The authors discuss NTP-based strategies for biofilm control, such as surface modifications to prevent bacterial adhesion, killing bacteria in biofilms, and biofilm destruction with NTPs. The unspecific character of microbicidal activity, proven polymer modification and destruction abilities, low toxicity for human tissues and absence of long-living toxic compounds make NTPs a very promising tool for biofilm prevention and control.

Photoactive TiO₂ antibacterial coating on surgical external fixation pins for clinical application

External fixation is a method of osteosynthesis currently used in traumatology and orthopedic surgery. Pin tract infection is a common problem in clinical practice. Infection occurs after bacterial colonization of the pin due to its contact with skin and the local environment. One way to prevent such local contamination is to create a specific coating that could be applied in the medical field. In this work, we developed a surface coating for external fixator pins based on the photocatalytic properties of titanium dioxide, producing a bactericidal effect with sufficient mechanical strength to be compatible with surgical use. The morphology and structure of the sol-gel coating layers were characterized using, respectively, scanning electron microscopy and X-ray diffraction. The resistance properties of the coating were investigated by mechanical testing. Photodegradation of acid orange 7 in aqueous solution was used as a probe to assess the photocatalytic activity of the titanium dioxide layers under ultraviolet irradiation. The bactericidal effect induced by the process was evaluated against two strains, ie, Staphylococcus aureus and multiresistant Staphylococcus epidermidis. The coated pins showed good mechanical strength and an efficient antibacterial effect after 1 hour of ultraviolet irradiation.

A Commensal Strain of Staphylococcus epidermidis Overexpresses Membrane Proteins Associated with Pathogenesis When Grown in Biofilms

Staphylococcus epidermidis has emerged as one of the major nosocomial pathogens associated with infections of implanted medical devices. The most important factor in the pathogenesis of these infections is the formation of bacterial biofilms. Bacteria grown in biofilms are more resistant to antibiotics and to the immune defence system than planktonic bacteria. In these infections, the antimicrobial therapy usually fails and the removal of the biofilm-coated implanted device is the only effective solution. In this study, three proteomic approaches were performed to investigate membrane proteins associated to biofilm formation: (i) sample fractionation by gel electrophoresis, followed by isotopic labelling and LC-MS/MS analysis, (ii) in-solution sample preparation, followed by isotopic labelling and LC-MS/MS analysis and (iii) in-solution sample preparation and label-free LC-MS/MS analysis. We found that the commensal strain S. epidermidis CECT 231 grown in biofilms expressed higher levels of five membrane and membrane-associated proteins involved in pathogenesis: accumulation-associated protein, staphylococcal secretory antigen, signal transduction protein TRAP, ribonuclease Y and phenol soluble modulin beta 1 when compared with bacteria grown under planktonic conditions. These results indicate that a commensal strain can acquire a pathogenic phenotype depending on the mode of growth.

The role of microbial biofilms in prosthetic joint infections

Prosthetic joint infection (PJI) still remains a significant problem. In line with the forecasted rise in joint replacement procedures, the number of cases of PJI is also anticipated to rise. The formation of biofilm by causative pathogens is central to the occurrence and the recalcitrance of PJI. The subject of microbial biofilms is receiving increasing attention, probably as a result of the wide acknowledgement of the ubiquity of biofilms in the natural, industrial, and clinical contexts, as well as the notorious difficulty in eradicating them. In this review, we discuss the pertinent issues surrounding PJI and the challenges posed by biofilms regarding diagnosis and treatment. In addition, we discuss novel strategies of prevention and treatment of biofilm-related PJI.

An 18 kDa scaffold protein is critical for Staphylococcus epidermidis biofilm formation

Virulence of the nosocomial pathogen Staphylococcus epidermidis is crucially linked to formation of adherent biofilms on artificial surfaces. Biofilm assembly is significantly fostered by production of a bacteria derived extracellular matrix. However, the matrix composition, spatial organization, and relevance of specific molecular interactions for integration of bacterial cells into the multilayered biofilm community are not fully understood. Here we report on the function of novel 18 kDa Small basic protein (Sbp) that was isolated from S. epidermidis biofilm matrix preparations by an affinity chromatographic approach. Sbp accumulates within the biofilm matrix, being preferentially deposited at the biofilm–substratum interface. Analysis of Sbp-negative S. epidermidis mutants demonstrated the importance of Sbp for sustained colonization of abiotic surfaces, but also epithelial cells. In addition, Sbp promotes assembly of S. epidermidis cell aggregates and establishment of multilayered biofilms by influencing polysaccharide intercellular-adhesin (PIA) and accumulation associated protein (Aap) mediated intercellular aggregation. While inactivation of Sbp indirectly resulted in reduced PIA-synthesis and biofilm formation, Sbp serves as an essential ligand during Aap domain-B mediated biofilm accumulation. Our data support the conclusion that Sbp serves as an S. epidermidis biofilm scaffold protein that significantly contributes to key steps of surface colonization. Sbp-negative S. epidermidis mutants showed no attenuated virulence in a mouse catheter infection model. Nevertheless, the high prevalence of sbp in commensal and invasive S. epidermidis populations suggests that Sbp plays a significant role as a co-factor during both multi-factorial commensal colonization and infection of artificial surfaces.

Biofilm formation is a key phenotype allowing the otherwise harmless skin commensal S. epidermidis to establish chronic implant-associated infections, affecting millions of patients worldwide. S. epidermidis biofilm assembly relies on the production of an extracellular matrix that serves as glue to stabilize the multilayered bacterial architecture. Here we identified novel 18 kDa Small basic protein (Sbp) as a key component of the extracellular matrix that promotes pivotal steps of bacterial biofilm formation in vitro. Importantly, Sbp is deposited specifically at the interface between biofilm and substrate, as well as in larger humps interspersed within the bacterial cell architecture, thereby forming a proteinaceous biofilm scaffold. This localization enables Sbp to foster stable S. epidermidis interactions with an artificial surface and also contributes to S. epidermidis cell aggregation mechanisms, i.e., polysaccharide intercellular adhesin (PIA) and accumulation associated protein (Aap). In fact, by demonstrating direct Sbp-Aap interactions we provide the first evidence supporting the idea that specific molecular interactions between S. epidermidis and matrix components are involved in S. epidermidis biofilm accumulation. In conclusion, we here show that Sbp promotes key phenotypic features important for S. epidermidis to evolve as an opportunistic pathogen.

Role of exopolysaccharide in Aggregatibacter actinomycetemcomitans-induced bone resorption in a rat model for periodontal disease

Aggregatibacter actinomycetemcomitans a causative agent of periodontal disease in humans, forms biofilm on biotic and abiotic surfaces. A. actinomycetemcomitans biofilm is heterogeneous in nature and is composed of proteins, extracellular DNA and exopolysaccharide. To explore the role played by the exopolysaccharide in the colonization and disease progression, we employed genetic reduction approach using our rat model of A. actinomycetemcomitans-induced periodontitis. To this end, a genetically modified strain of A. actinomycetemcomitans lacking the pga operon was compared with the wild-type strain in the rat infection model. The parent and mutant strains were primarily evaluated for bone resorption and disease. Our study showed that colonization, bone resorption/disease and antibody response were all elevated in the wild-type fed rats. The bone resorption/disease caused by the pga mutant strain, lacking the exopolysaccharide, was significantly less (P < 0.05) than the bone resorption/disease caused by the wild-type strain. Further analysis of the expression levels of selected virulence genes through RT-PCR showed that the decrease in colonization, bone resorption and antibody titer in the absence of the exopolysaccharide might be due to attenuated levels of colonization genes, flp-1, apiA and aae in the mutant strain. This study demonstrates that the effect exerted by the exopolysaccharide in A. actinomycetemcomitans-induced bone resorption has hitherto not been recognized and underscores the role played by the exopolysaccharide in A. actinomycetemcomitans-induced disease.

Mechanisms of resistance to antimicrobial peptides in staphylococci

Staphylococci are commensal bacteria living on the epithelial surfaces of humans and other mammals. Many staphylococci, including the dangerous pathogen Staphylococcus aureus, can cause severe disease when they breach the epithelial barrier. Both during their commensal life and during infection, staphylococci need to evade mechanisms of innate host defense, of which antimicrobial peptides (AMPs) play a key role in particular on the skin. Mechanisms that staphylococci have developed to evade the bactericidal activity of AMPs are manifold, comprising repulsion of AMPs via alteration of cell wall and membrane surface charges, proteolytic inactivation, sequestration, and secretion. Furthermore, many staphylococci form biofilms, which represents an additional way of protection from antimicrobial agents, including AMPs. Finally, staphylococci can sense the presence of AMPs by sensor/regulator systems that control many of those resistance mechanisms. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

Structural basis of Staphylococcus epidermidis biofilm formation: mechanisms and molecular interactions

Staphylococcus epidermidis is a usually harmless commensal bacterium highly abundant on the human skin. Under defined predisposing conditions, most importantly implantation of a medical device, S. epidermidis, however, can switch from a colonizing to an invasive life style. The emergence of S. epidermidis as an opportunistic pathogen is closely linked to the biofilm forming capability of the species. During the past decades, tremendous advance regarding our understanding of molecular mechanisms contributing to surface colonization has been made, and detailed information is available for several factors active during the primary attachment, accumulative or dispersal phase of biofilm formation. A picture evolved in which distinct factors, though appearing to be redundantly organized, take over specific and exclusive functions during biofilm development. In this review, these mechanisms are described in molecular detail, with a highlight on recent insights into multi-functional S. epidermidis cell surface proteins contributing to surface adherence and intercellular adhesion. The integration of distinct biofilm-promoting factors into regulatory networks is summarized, with an emphasis on mechanism that could allow S. epidermidis to flexibly adapt to changing environmental conditions present during colonizing or invasive life-styles.

Effect of incubation atmosphere on the production and composition of staphylococcal biofilms

Staphylococcus aureus and Staphylococcus epidermidis are pathogenic bacteria that often cause invasive infections in humans. In this study, we characterized the composition and growth characteristics of staphylococcal biofilms under various incubation atmospheres. We assessed the effect of incubation atmosphere (aerobic, 5% CO2, anaerobic, and microaerobic) on the biofilm production capabilities of S. aureus strains isolated from healthy volunteers and from patients with catheter-related bloodstream infection. In addition, the composition of S. aureus and S. epidermidis biofilms was determined by assessment of biofilm degradation after treatment with DNase I, proteinase K, and dispersin B. The strains obtained from healthy volunteers and patients showed similar biofilm formation capabilities. Biofilms of S. aureus were rich in proteins when developed under ambient atmospheric conditions, 5% CO2, and microaerobic condition, whereas S. epidermidis biofilms contained large amounts of poly-β (1, 6)-N-acetyl-D-glucosamine when developed under ambient atmospheric conditions and microaerobic condition. The biofilm-producing capability of S. epidermidis was considerably higher than that of S. aureus under aerobic condition. Staphylococcal isolates obtained from healthy individuals and patients with catheter-related infections have similar biofilm-forming capabilities. Under microaerobic conditions, S. aureus and S. epidermidis form protein-rich and poly-β (1, 6)-N-acetyl-D-glucosamine-rich biofilms, respectively. These components may play an important role in the development of biofilms inside the body and may be the target molecules to prevent catheter-related infections caused by these organisms.

Accumulation-associated protein enhances Staphylococcus epidermidis biofilm formation under dynamic conditions and is required for infection in a rat catheter model

Biofilm formation is the primary virulence factor of Staphylococcus epidermidis. S. epidermidis biofilms preferentially form on abiotic surfaces and may contain multiple matrix components, including proteins such as accumulation-associated protein (Aap). Following proteolytic cleavage of the A domain, which has been shown to enhance binding to host cells, B domain homotypic interactions support cell accumulation and biofilm formation. To further define the contribution of Aap to biofilm formation and infection, we constructed an aap allelic replacement mutant and an icaADBC aap double mutant. When subjected to fluid shear, strains deficient in Aap production produced significantly less biofilm than Aap-positive strains. To examine the in vivo relevance of our findings, we modified our previously described rat jugular catheter model and validated the importance of immunosuppression and the presence of a foreign body to the establishment of infection. The use of our allelic replacement mutants in the model revealed a significant decrease in bacterial recovery from the catheter and the blood in the absence of Aap, regardless of the production of polysaccharide intercellular adhesin (PIA), a well-characterized, robust matrix molecule. Complementation of the aap mutant with full-length Aap (containing the A domain), but not the B domain alone, increased initial attachment to microtiter plates, as did in trans expression of the A domain in adhesion-deficient Staphylococcus carnosus. These results demonstrate Aap contributes to S. epidermidis infection, which may in part be due to A domain-mediated attachment to abiotic surfaces.

Antimicrobial Peptide Resistance Mechanisms of Gram-Positive Bacteria

Antimicrobial peptides, or AMPs, play a significant role in many environments as a tool to remove competing organisms. In response, many bacteria have evolved mechanisms to resist these peptides and prevent AMP-mediated killing. The development of AMP resistance mechanisms is driven by direct competition between bacterial species, as well as host and pathogen interactions. Akin to the number of different AMPs found in nature, resistance mechanisms that have evolved are just as varied and may confer broad-range resistance or specific resistance to AMPs. Specific mechanisms of AMP resistance prevent AMP-mediated killing against a single type of AMP, while broad resistance mechanisms often lead to a global change in the bacterial cell surface and protect the bacterium from a large group of AMPs that have similar characteristics. AMP resistance mechanisms can be found in many species of bacteria and can provide a competitive edge against other bacterial species or a host immune response. Gram-positive bacteria are one of the largest AMP producing groups, but characterization of Gram-positive AMP resistance mechanisms lags behind that of Gram-negative species. In this review we present a summary of the AMP resistance mechanisms that have been identified and characterized in Gram-positive bacteria. Understanding the mechanisms of AMP resistance in Gram-positive species can provide guidelines in developing and applying AMPs as therapeutics, and offer insight into the role of resistance in bacterial pathogenesis.

Coagulase-negative staphylococci

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

Biofilm Formation by Staphylococcus epidermidis on Foldable and Rigid Intraocular Lenses

Background:

Biofilm formation of Staphylococcus epidermidis is a major etiological factor of inducing device-related infections.

Objectives:

The ability of biofilm formation by the S. epidermidis was assessed in vitro on two brands of foldable (hydrophilic) and two brands of rigid (hydrophobic) intraocular lens materials in order to investigate the role of lens material in postoperative endophthalmitis.

Materials and Methods:

To ensure reproducibility of biofilm formation on intraocular lenses, two strains of S. epidermidis and three quantification methods were performed. The S. epidermidis strains, DSMZ3270 (biofilm-producer) and ATCC12228 (non-biofilm-producer) were applied. Organisms were cultivated on disks of different brands of foldable hydrophilic Intra Ocular Lens (IOL) made of acrylic (Didar, Iran; (A) and Omni, India; (B)), and rigid hydrophobic IOL made of polymethyl methacrylate (PMMA; Didar, Iran; (C) and Hexavision, France; (D)). Biofilms were stained with crystal violet (CV) dye, which is an index of biofilm formation. The bacterial population was counted after biofilm homogenization. Scanning electron microscopy (SEM) was performed to examine the extent of biofilm formation.

Results:

Adherence of DSMZ3270 strain on both types of foldable and rigid IOLs, was significantly more than ATCC12228 (P < 0.001-0.05 and, P < 0.01-0.05, respectively). The bacterial populations between the lenses were significantly different (P < 0.05). Subsequent studies demonstrated significant differences between brands of foldable and PMMA IOLs. According to statistical analyses the incubation time influenced the biofilm formation on both types of IOLs which meant that by increasing incubation time, the biofilm formation increased. According to the SEM pictures, biofilm seems to be lysed at 72 hours.

Conclusions:

These data demonstrated that the attachment of bacteria to hydrophilic acrylic IOLs was more than hydrophobic PMMA ones independent of the brand. According to these results the bacterial strain might have more hydrophilic properties. Augmenting the biomass of biofilm by passing of time demonstrated the key role of time in biofilm formation on the IOL surfaces. The differences between IOL brands in the biofilm formation indicated the influence of design parameters for IOLs.

Foreign Body Infection Models to Study Host-Pathogen Response and Antimicrobial Tolerance of Bacterial Biofilm

The number of implanted medical devices is steadily increasing and has become an effective intervention improving life quality, but still carries the risk of infection. These infections are mainly caused by biofilm-forming staphylococci that are difficult to treat due to the decreased susceptibility to both antibiotics and host defense mechanisms. To understand the particular pathogenesis and treatment tolerance of implant-associated infection (IAI) animal models that closely resemble human disease are needed. Applications of the tissue cage and catheter abscess foreign body infection models in the mouse will be discussed herein. Both models allow the investigation of biofilm and virulence of various bacterial species and a comprehensive insight into the host response at the same time. They have also been proven to serve as very suitable tools to study the anti-adhesive and anti-infective efficacy of different biomaterial coatings. The tissue cage model can additionally be used to determine pharmacokinetics, efficacy and cytotoxicity of antimicrobial compounds as the tissue cage fluid can be aspirated repeatedly without the need to sacrifice the animal. Moreover, with the advance in innovative imaging systems in rodents, these models may offer new diagnostic measures of infection. In summary, animal foreign body infection models are important tools in the development of new antimicrobials against IAI and can help to elucidate the complex interactions between bacteria, the host immune system, and prosthetic materials.

Efficacy of the novel oxazolidinone compound FYL-67 for preventing biofilm formation by Staphylococcus aureus

OBJECTIVES

Infections of hospitalized patients caused by biofilms formed by Staphylococcus aureus represent a major problem. Using in vitro and in vivo biofilm models, we evaluated the efficacy of the novel oxazolidinone FYL-67, by using linezolid (the only clinically approved oxazolidinone antibiotic) as a control, for inhibiting S. aureus biofilm formation.

METHODS

Antibiofilm activity was determined using strains of methicillin-susceptible S. aureus and methicillin-resistant S. aureus. We studied the mechanism(s) and pharmacodynamics of antibiofilm activity as follows: (i) effects of pre- and post-exposure to FYL-67 or linezolid on biofilm formation; (ii) the effect of FYL-67 on biofilm structure; (iii) the role of FYL-67 in biofilm composition; (iv) effects on cell morphology; and (v) efficacy of FYL-67 and linezolid using an in vivo murine model of catheter infection.

RESULTS

FYL-67 effectively inhibited biofilm formation using in vitro and in vivo assays.

CONCLUSIONS

Our data suggest that oxazolidinone compounds, such as FYL-67, may serve as antibiofilm agents.

A mouse model of Staphylococcus catheter-associated biofilm infection

Biofilms are adherent communities of bacteria contained within a complex matrix. Staphylococcal species are frequent etiological agents of device-associated biofilm infections in humans that are highly recalcitrant to antimicrobial therapy and alter host immune responses to facilitate bacterial persistence. Here we describe a mouse model of catheter-associated biofilm infection, which can be utilized to investigate the importance of various staphylococcal determinants on disease progression as well as the host immune response to staphylococcal biofilms.

Staphylococcus epidermidis pathogenesis

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

Molecular basis of in vivo biofilm formation by bacterial pathogens

Bacterial biofilms are involved in a multitude of serious chronic infections. In recent years, modeling of biofilm infection in vitro has led to the identification of microbial determinants that govern biofilm development. However, we lack information as to whether the biofilm formation mechanisms identified in vitro have relevance for biofilm-associated infection. Here, we discuss the molecular basis of biofilm formation. Staphylococci and Pseudomonas aeruginosa are used to illustrate key points because their biofilm development process has been well studied. We focus on in vivo findings, such as obtained in animal infection models, and critically evaluate the in vivo relevance of in vitro findings. Although conflicting results about the role of quorum sensing in biofilm formation have been obtained, we argue that integration of in vitro and in vivo studies allows a differentiated view of this mechanism as it relates to biofilm infection.

A zebrafish high throughput screening system used for Staphylococcus epidermidis infection marker discovery

BACKGROUND

Staphylococcus epidermidis bacteria are a major cause of biomaterial-associated infections in modern medicine. Yet there is little known about the host responses against this normally innocent bacterium in the context of infection of biomaterials. In order to better understand the factors involved in this process, a whole animal model with high throughput screening possibilities and markers for studying the host response to S. epidermidis infection are required.

RESULTS

We have used a zebrafish yolk injection system to study bacterial proliferation and the host response in a time course experiment of S. epidermidis infection. By combining an automated microinjection system with complex object parametric analysis and sorting (COPAS) technology we have quantified bacterial proliferation. This system was used together with transcriptome analysis at several time points during the infection period. We show that bacterial colony forming unit (CFU) counting can be replaced by high throughput flow-based fluorescence analysis of embryos enabling high throughput readout. Comparison of the host transcriptome response to S. epidermidis and Mycobacterium marinum infection in the same system showed that M. marinum has a far stronger effect on host gene regulation than S. epidermidis. However, multiple genes responded differently to S. epidermidis infection than to M. marinum, including a cell adhesion gene linked to specific infection by staphylococci in mammals.

CONCLUSIONS

Our zebrafish embryo infection model allowed (i) quantitative assessment of bacterial proliferation, (ii) identification of zebrafish genes serving as markers for infection with the opportunistic pathogen S. epidermidis, and (iii) comparison of the transcriptome response of infection with S. epidermidis and with the pathogen M. marinum. As a result we have identified markers that can be used to distinguish common and specific responses to S. epidermidis. These markers enable the future integration of our high throughput screening technology with functional analyses of immune response genes and immune modulating factors.

PelA deacetylase activity is required for Pel polysaccharide synthesis in Pseudomonas aeruginosa

The Pel polysaccharide serves as an intercellular adhesin for the formation and maintenance of biofilms in the opportunistic pathogen Pseudomonas aeruginosa. Pel biosynthesis requires the products of a seven-gene operon, pelA-pelG, all of which are necessary for Pel-dependent biofilm formation and Pel-related phenotypes. One of the genes, pelA, encodes a protein with a predicted polysaccharide deacetylase domain. In this work, the role of the putative deacetylase domain in Pel production was examined. We first established that purified recombinant PelA hydrolyzed the pseudosubstrate p-nitrophenyl acetate in vitro, and site-specific mutations of predicted deacetylase active-site residues reduced activity greater than 10-fold. Additionally, these mutants were deficient in Pel-dependent biofilm formation and wrinkly colony morphology in vivo. Subcellular fractionation experiments demonstrate that PelA localizes to both the membrane and periplasmic fractions. Finally, antiserum against the Pel polysaccharide was generated, and PelA deacetylase mutants do not produce Pel-reactive material. Taken together, these results suggest that the deacetylase activity of PelA is important for the production of the Pel polysaccharide.

Staphylococcus epidermidis biofilms induce lower complement activation in neonates as compared with adults

BACKGROUND

Staphylococcus epidermidis (SE) is an important cause of late-onset sepsis in neonates. SE frequently produces a polysaccharide intercellular adhesin (PIA) biofilm, important in the pathogenesis of these infections. Little is known about how the neonatal innate immune system reacts to SE biofilm-associated infections. Our hypothesis was that SE biofilms induce a lower complement activation in neonates as compared with adults.

METHODS

Cord blood from term infants (n = 15) and blood from adults (n = 6) were studied in an ex vivo whole-blood sepsis model. A PIA biofilm-producing strain (SE1457) and its isogenic mutant (M10), producing a non-PIA biofilm, were used.

RESULTS

Both SE biofilms induced stronger complement activation in adult than in cord blood (P ≤ 0.033). We found lower levels of antibodies toward both PIA (P = 0.002) and the whole bacterium (P = 0.001) in cord vs. adult blood. By contrast, the interleukin-8 (IL-8) and IL-6 secretion were higher in cord than in adult blood (P ≤ 0.002). The PIA biofilm induced stronger complement activation than the non-PIA biofilm.

CONCLUSION

We conclude that the neonatal complement system exhibits a maturational deficiency. This may reduce the ability of neonates to combat biofilm-associated SE infections.

Targeting macrophage activation for the prevention and treatment of Staphylococcus aureus biofilm infections

Biofilm infections often lead to significant morbidity due to their chronicity and recalcitrance to antibiotics. We have demonstrated that methicillin-resistant Staphylococcus aureus (MRSA) biofilms can evade macrophage (MΦ) antibacterial effector mechanisms by skewing MΦs toward an alternatively activated M2 phenotype. To overcome this immune evasion, we have used two complementary approaches. In the first, a proinflammatory milieu was elicited by local administration of classically activated M1 MΦs and in the second by treatment with the C5a receptor (CD88) agonist EP67, which invokes MΦ proinflammatory activity. Early administration of M1-activated MΦs or EP67 significantly attenuated biofilm formation in a mouse model of MRSA catheter-associated infection. Several proinflammatory mediators were significantly elevated in biofilm-infected tissues from MΦ- and EP67-treated animals, revealing effective reprogramming of the biofilm environment to a proinflammatory milieu. A requirement for MΦ proinflammatory activity was demonstrated by the fact that transfer of MyD88-deficient MΦs had minimal impact on biofilm growth. Likewise, neutrophil administration had no effect on biofilm formation. Treatment of established biofilm infections with M1-activated MΦs also significantly reduced catheter-associated biofilm burdens compared with antibiotic treatment. Collectively, these results demonstrate that targeting MΦ proinflammatory activity can overcome the local immune inhibitory environment created during biofilm infections and represents a novel therapeutic strategy.

Staphylococcal infections: mechanisms of biofilm maturation and detachment as critical determinants of pathogenicity

Biofilm-associated infections are a significant cause of morbidity and death. Staphylococci, above all Staphylococcus aureus and S. epidermidis, are the most frequent causes of biofilm-associated infections on indwelling medical devices. Although the mechanistic basis for the agglomeration of staphylococcal cells in biofilms has been investigated in great detail, we lack understanding of the forces and molecular determinants behind the structuring of biofilms and the detachment of cellular clusters from biofilms. These processes are of key importance for the formation of vital biofilms in vivo with the capacity of bacterial dissemination to secondary sites of infection. Recent studies showed that the phenol-soluble modulins, surfactant peptides secreted by staphylococci in a quorum-sensing controlled fashion, structure biofilms in vitro and in vivo and lead to biofilm detachment with the in vivo consequence of bacterial dissemination. These findings substantiate that quorum sensing and surfactants have widespread importance for biofilm maturation processes in bacteria and establish a novel theory of the molecular determinants driving dissemination of biofilm-associated infection.

MyD88-dependent signaling influences fibrosis and alternative macrophage activation during Staphylococcus aureus biofilm infection

Bacterial biofilms represent a significant therapeutic challenge based on their ability to evade host immune and antibiotic-mediated clearance. Recent studies have implicated IL-1β in biofilm containment, whereas Toll-like receptors (TLRs) had no effect. This is intriguing, since both the IL-1 receptor (IL-1R) and most TLRs impinge on MyD88-dependent signaling pathways, yet the role of this key adaptor in modulating the host response to biofilm growth is unknown. Therefore, we examined the course of S. aureus catheter-associated biofilm infection in MyD88 knockout (KO) mice. MyD88 KO animals displayed significantly increased bacterial burdens on catheters and surrounding tissues during early infection, which coincided with enhanced dissemination to the heart and kidney compared to wild type (WT) mice. The expression of several proinflammatory mediators, including IL-6, IFN-γ, and CXCL1 was significantly reduced in MyD88 KO mice, primarily at the later stages of infection. Interestingly, immunofluorescence staining of biofilm-infected tissues revealed increased fibrosis in MyD88 KO mice concomitant with enhanced recruitment of alternatively activated M2 macrophages. Taken in the context of previous studies with IL-1β, TLR2, and TLR9 KO mice, the current report reveals that MyD88 signaling is a major effector pathway regulating fibrosis and macrophage polarization during biofilm formation. Together these findings represent a novel example of the divergence between TLR and MyD88 action in the context of S. aureus biofilm infection.

Fluid flow induces biofilm formation in Staphylococcus epidermidis polysaccharide intracellular adhesin-positive clinical isolates

Staphylococcus epidermidis is a common cause of catheter-related bloodstream infections, resulting in significant morbidity and mortality and increased hospital costs. The ability to form biofilms plays a crucial role in pathogenesis; however, not all clinical isolates form biofilms under normal in vitro conditions. Strains containing the ica operon can display significant phenotypic variation with respect to polysaccharide intracellular adhesin (PIA)-based biofilm formation, including the induction of biofilms upon environmental stress. Using a parallel microfluidic approach to investigate flow as an environmental signal for S. epidermidis biofilm formation, we demonstrate that fluid shear alone induces PIA-positive biofilms of certain clinical isolates and influences biofilm structure. These findings suggest an important role of the catheter microenvironment, particularly fluid flow, in the establishment of S. epidermidis infections by PIA-dependent biofilm formation.

Deciphering mechanisms of staphylococcal biofilm evasion of host immunity

Biofilms are adherent communities of bacteria contained within a complex matrix. Although host immune responses to planktonic staphylococcal species have been relatively well-characterized, less is known regarding immunity to staphylococcal biofilms and how they modulate anti-bacterial effector mechanisms when organized in this protective milieu. Previously, staphylococcal biofilms were thought to escape immune recognition on the basis of their chronic and indolent nature. Instead, we have proposed that staphylococcal biofilms skew the host immune response away from a proinflammatory bactericidal phenotype toward an anti-inflammatory, pro-fibrotic response that favors bacterial persistence. This possibility is supported by recent studies from our laboratory using a mouse model of catheter-associated biofilm infection, where S. aureus biofilms led to the accumulation of alternatively activated M2 macrophages that exhibit anti-inflammatory and pro-fibrotic properties. In addition, relatively few neutrophils were recruited into S. aureus biofilms, representing another mechanism that deviates from planktonic infections. However, it is important to recognize the diversity of biofilm infections, in that studies by others have demonstrated the induction of distinct immune responses during staphylococcal biofilm growth in other models, suggesting influences from the local tissue microenvironment. This review will discuss the immune defenses that staphylococcal biofilms evade as well as conceptual issues that remain to be resolved. An improved understanding of why the host immune response is unable to clear biofilm infections could lead to targeted therapies to reverse these defects and expedite biofilm clearance.

Catheter colonization and abscess formation due to Staphylococcus epidermidis with normal and small-colony-variant phenotype is mouse strain dependent

Coagulase-negative staphylococci (CoNS) form a thick, multilayered biofilm on foreign bodies and are a major cause of nosocomial implant-associated infections. Although foreign body infection models are well-established, limited in vivo data are available for CoNS with small-colony-variant (SCV) phenotype described as causative agents in implant-associated infections. Therefore, we investigated the impact of the Staphylococcus epidermidis phenotype on colonization of implanted PVC catheters and abscess formation in three different mouse strains. Following introduction of a catheter subcutaneously in each flank of 8- to 12-week-old inbred C57BL/6JCrl (B6J), outbred Crl:CD1(ICR) (CD-1), and inbred BALB/cAnNCrl (BALB/c) male mice, doses of S. epidermidis O-47 wild type, its hemB mutant with stable SCV phenotype, or its complemented mutant at concentrations of 10(6) to 10(9) colony forming units (CFUs) were gently spread onto each catheter. On day 7, mice were sacrificed and the size of the abscesses as well as bacterial colonization was determined. A total of 11,500 CFUs of the complemented mutant adhered to the catheter in BALB/c followed by 9,960 CFUs and 9,900 CFUs from S. epidermidis wild type in BALB/c and CD-1, respectively. SCV colonization was highest in CD-1 with 9,500 CFUs, whereas SCVs were not detected in B6J. The minimum dose that led to colonization or abscess formation in all mouse strains was 10(7) or 10(8) CFUs of the normal phenotype, respectively. A minimum dose of 10(8) or 10(9) CFU of the hemB mutant with stable SCV phenotype led to colonization only or abscess formation, respectively. The largest abscesses were detected in BALB/c inoculated with wild type bacteria or SCV (64 mm(2) vs. 28 mm(2)). Our results indicate that colonization and abscess formation by different phenotypes of S. epidermidis in a foreign body infection model is most effective in inbred BALB/c followed by outbred CD-1 and inbred B6J mice.

Database screening and in vivo efficacy of antimicrobial peptides against methicillin-resistant Staphylococcus aureus USA300

Natural antimicrobial peptides (AMPs) are promising candidates for developing a generation of new antimicrobials to meet the challenge of antibiotic-resistant pathogens such as methicillin-resistant Staphylococcus aureus (MRSA). To facilitate the search for new candidates, we have utilised the Antimicrobial Peptide Database (APD), which contains natural AMPs from bacteria, fungi, plants and animals. This study demonstrates the identification of novel templates against MRSA by screening 30 peptides selected from the APD. These peptides are short (<25 residues), cysteine-free, cationic and represent candidates from different biological sources such as bacteria, insects, arachnids, tunicates, amphibians, fish and mammals. Six peptides, including ascaphin-8, database-screened antimicrobial peptide 1 (DASamP1), DASamP2, lycotoxin I, maculatin 1.3 and piscidin 1, were found to exert potent antimicrobial activity against an MRSA USA300 isolate. Although five of the six peptides showed broad-spectrum antibacterial activity, DASamP1 displayed killing of MRSA in vitro but not of Escherichia coli, Bacillus subtilis or Pseudomonas aeruginosa. In addition, DASamP1 suppressed early biofilm formation in a mouse model of catheter-associated MRSA infection. DASamP1 is a novel, short and potent peptide that will be a useful starting template for further developing novel anti-MRSA peptides.

Methicillin resistance alters the biofilm phenotype and attenuates virulence in Staphylococcus aureus device-associated infections

Clinical isolates of Staphylococcus aureus can express biofilm phenotypes promoted by the major cell wall autolysin and the fibronectin-binding proteins or the icaADBC-encoded polysaccharide intercellular adhesin/poly-N-acetylglucosamine (PIA/PNAG). Biofilm production in methicillin-susceptible S. aureus (MSSA) strains is typically dependent on PIA/PNAG whereas methicillin-resistant isolates express an Atl/FnBP-mediated biofilm phenotype suggesting a relationship between susceptibility to β-lactam antibiotics and biofilm. By introducing the methicillin resistance gene mecA into the PNAG-producing laboratory strain 8325-4 we generated a heterogeneously resistant (HeR) strain, from which a homogeneous, high-level resistant (HoR) derivative was isolated following exposure to oxacillin. The HoR phenotype was associated with a R₆₀₂H substitution in the DHHA1 domain of GdpP, a recently identified c-di-AMP phosphodiesterase with roles in resistance/tolerance to β-lactam antibiotics and cell envelope stress. Transcription of icaADBC and PNAG production were impaired in the 8325-4 HoR derivative, which instead produced a proteinaceous biofilm that was significantly inhibited by antibodies against the mecA-encoded penicillin binding protein 2a (PBP2a). Conversely excision of the SCCmec element in the MRSA strain BH1CC resulted in oxacillin susceptibility and reduced biofilm production, both of which were complemented by mecA alone. Transcriptional activity of the accessory gene regulator locus was also repressed in the 8325-4 HoR strain, which in turn was accompanied by reduced protease production and significantly reduced virulence in a mouse model of device infection. Thus, homogeneous methicillin resistance has the potential to affect agr- and icaADBC-mediated phenotypes, including altered biofilm expression and virulence, which together are consistent with the adaptation of healthcare-associated MRSA strains to the antibiotic-rich hospital environment in which they are frequently responsible for device-related infections in immuno-compromised patients.

The acquisition of mecA, which encodes penicillin binding protein 2a (PBP2a) and methicillin resistance, by Staphylococcus aureus has added to an already impressive array of virulence mechanisms including enzyme and toxin production, biofilm forming capacity and immune evasion. And yet clinical data does not indicate that healthcare-associated methicillin resistant S. aureus (MRSA) strains are more virulent than their methicillin-susceptible counterparts. Here our findings suggest that MRSA sacrifices virulence potential for antibiotic resistance and that expression of methicillin resistance alters the biofilm phenotype but does not interfere with the colonization of implanted medical devices in vivo. High level expression of PBP2a, which was associated with a mutation in the c-di-AMP phosphodiesterase gene gdpP, resulted in these pleiotrophic effects by blocking icaADBC-dependent polysaccharide type biofilm development and promoting an alternative PBP2a-mediated biofilm, repressing the accessory gene regulator and extracellular protease production, and attenuating virulence in a mouse device-infection model. Thus the adaptation of MRSA to the hospital environment has apparently focused on the acquisition of antibiotic resistance and retention of biofilm forming capacity, which are likely to be more advantageous than metabolically-expensive enzyme and toxin production in immunocompromised patients with implanted medical devices offering a route to infection.

Static biofilm cultures of Gram-positive pathogens grown in a microtiter format used for anti-biofilm drug discovery

An in vitro assay is presented for culturing staphylococcal biofilms and biofilms of nonmotile Gram-positive bacteria under static conditions in microtiter assay plates, and for the quantification of biofilm growth, using a simple staining procedure that measures amounts of bacterial cells and extracellular matrix. This basic assay can be adapted readily to study several aspects of biofilm formation, for high-throughput screening to identify small molecule inhibitors of biofilm formation or biofilm-defective mutants, and for quantifying the anti-biofilm activity of biofilm inhibitors.

Staphylococcus epidermidis polysaccharide intercellular adhesin activates complement

Staphylococcus epidermidis is a frequent cause of nosocomial infections. The central virulence factor of S. epidermidis is biofilm formation. Polysaccharide intercellular adhesin (PIA) constitutes the major biofilm matrix-component. PIA and biofilm have been implicated in S. epidermidis evasion of host immune defence. We examined the effects of S. epidermidis PIA on the inflammatory response with focus on complement activation. We used a human whole-blood ex vivo model of infection and compared the effects of a PIA-positive S. epidermidis strain (SE1457) and its PIA-negative isogenic mutant (M10). The independent effect of purified PIA on complement activation was investigated. In glucose-rich media, the mutant formed a proteinacious DNA-rich biofilm, whereas SE1457 formed a thick PIA-biofilm. In biofilm growth, SE1457 induced a stronger activation of the complement system compared with M10. We verified that purified PIA was independently responsible for a strong activation of the complement system. In contrast, M10 induced higher granulocyte activation by expression of CD11b and higher secretion of cytokines. We conclude that PIA has potent pro-inflammatory properties by activating the complement system. However, in a complex balance of the immune response, the decreased activation of granulocytes and cytokines by a PIA biofilm may limit host eradication of S. epidermidis.

Characterization and functional analysis of atl, a novel gene encoding autolysin in Streptococcus suis

Streptococcus suis serotype 2 (S. suis 2) is an important swine and human pathogen responsible for septicemia and meningitis. A novel gene, designated atl and encoding a major autolysin of S. suis 2 virulent strain HA9801, was identified and characterized in this study. The Atl protein contains 1,025 amino acids with a predicted molecular mass of 113 kDa and has a conserved N-acetylmuramoyl-l-alanine amidase domain. Recombinant Atl was expressed in Escherichia coli, and its bacteriolytic and fibronectin-binding activities were confirmed by zymography and Western affinity blotting. Two bacteriolytic bands were shown in the sodium dodecyl sulfate extracts of HA9801, while both were absent from the atl inactivated mutant. Cell chains of the mutant strain became longer than that of the parental strain. In the autolysis assay, HA9801 decreased to 20% of the initial optical density (OD) value, while the mutant strain had almost no autolytic activity. The biofilm capacity of the atl mutant was reduced ∼30% compared to the parental strain. In the zebrafish infection model, the 50% lethal dose of the mutant strain was increased up to 5-fold. Furthermore, the adherence to HEp-2 cells of the atl mutant was 50% less than that of the parental strain. Based on the functional analysis of the recombinant Atl and observed effects of atl inactivation on HA9801, we conclude that Atl is a major autolysin of HA9801. It takes part in cell autolysis, separation of daughter cells, biofilm formation, fibronectin-binding activity, cell adhesion, and pathogenesis of HA9801.

Molecular basis of Staphylococcus epidermidis infections

Staphylococcus epidermidis is the most important member of the coagulase-negative staphylococci and one of the most abundant colonizers of human skin. While for a long time regarded as innocuous, it has been identified as the most frequent cause of device-related infections occurring in the hospital setting and is therefore now recognized as an important opportunistic pathogen. S. epidermidis produces a series of molecules that provide protection from host defenses. Specifically, many proteins and exopolymers, such as the exopolysaccharide PIA, contribute to biofilm formation and inhibit phagocytosis and the activity of human antimicrobial peptides. Furthermore, recent research has identified a family of pro-inflammatory peptides in S. epidermidis, the phenol-soluble modulins (PSMs), which have multiple functions in immune evasion and biofilm development, and may be cytolytic. However, in accordance with the relatively benign relationship that S. epidermidis has with its host, production of aggressive members of the PSM family is kept at a low level. Interestingly, in contrast to S. aureus with its large arsenal of toxins developed for causing infection in the human host, most if not all "virulence factors" of S. epidermidis appear to have original functions in the commensal lifestyle of this bacterium.

CcpA coordinates central metabolism and biofilm formation in Staphylococcus epidermidis

Staphylococcus epidermidis is an opportunistic bacterium whose infections often involve the formation of a biofilm on implanted biomaterials. In S. epidermidis, the exopolysaccharide facilitating bacterial adherence in a biofilm is polysaccharide intercellular adhesin (PIA), whose synthesis requires the enzymes encoded within the intercellular adhesin operon (icaADBC). In vitro, the formation of S. epidermidis biofilms is enhanced by conditions that repress tricarboxylic acid (TCA) cycle activity, such as growth in a medium containing glucose. In many Gram-positive bacteria, repression of TCA cycle genes in response to glucose is accomplished by catabolite control protein A (CcpA). CcpA is a member of the GalR-LacI repressor family that mediates carbon catabolite repression, leading us to hypothesize that catabolite control of S. epidermidis biofilm formation is indirectly regulated by CcpA-dependent repression of the TCA cycle. To test this hypothesis, ccpA deletion mutants were constructed in strain 1457 and 1457-acnA and the effects on TCA cycle activity, biofilm formation and virulence were assessed. As anticipated, deletion of ccpA derepressed TCA cycle activity and inhibited biofilm formation; however, ccpA deletion had only a modest effect on icaADBC transcription. Surprisingly, deletion of ccpA in strain 1457-acnA, a strain whose TCA cycle is inactive and where icaADBC transcription is derepressed, strongly inhibited icaADBC transcription. These observations demonstrate that CcpA is a positive effector of biofilm formation and icaADBC transcription and a repressor of TCA cycle activity.

Staphylococcal biofilm disassembly

Staphylococcus aureus and Staphylococcus epidermidis are a frequent cause of biofilm-associated infections that are a tremendous burden on our healthcare system. Staphylococcal biofilms exhibit extraordinary resistance to antimicrobial killing, limiting the efficacy of antibiotic therapy, and surgical intervention is often required to remove infected tissues or implanted devices. Recent work has provided new insight into the molecular basis of biofilm development in these opportunistic pathogens. Extracellular bacterial products, environmental conditions, and polymicrobial interactions have all been shown to influence profoundly the ability of these bacteria to colonize and disperse from clinically relevant surfaces. We review new developments in staphylococcal biofilm disassembly and set them in the context of potential strategies to control biofilm infections.

NMR analysis of a stress response metabolic signaling network

We previously hypothesized that Staphylococcus epidermidis senses a diverse set of environmental and nutritional factors associated with biofilm formation through a modulation in the activity of the tricarboxylic acid (TCA) cycle. Herein, we report our further investigation of the impact of additional environmental stress factors on TCA cycle activity and provide a detailed description of our NMR methodology. S. epidermidis wild-type strain 1457 was treated with stressors that are associated with biofilm formation, a sublethal dose of tetracycline, 5% NaCl, 2% glucose, and autoinducer-2 (AI-2). As controls and to integrate our current data with our previous study, 4% ethanol stress and iron-limitation were also used. Consistent with our prior observations, the effect of many environmental stress factors on the S. epidermidis metabolome was essentially identical to the effect of TCA cycle inactivation in the aconitase mutant strain 1457-acnA::tetM. A detailed quantitative analysis of metabolite concentration changes using 2D (1)H-(13)C HSQC and (1)H-(1)H TOCSY spectra identified a network of 37 metabolites uniformly affected by the stressors and TCA cycle inactivation. We postulate that the TCA cycle acts as the central pathway in a metabolic signaling network.

Bacterial resistance mechanisms against host defense peptides

Host defense peptides and proteins are important components of the innate host defense against pathogenic microorganisms. They target negatively charged bacterial surfaces and disrupt microbial cytoplasmic membranes, which ultimately leads to bacterial destruction. Throughout evolution, pathogens devised several mechanisms to protect themselves from deleterious damage of host defense peptides. These strategies include (a) inactivation and cleavage of host defense peptides by production of host defense binding proteins and proteases, (b) repulsion of the peptides by alteration of pathogen's surface charge employing modifications by amino acids or amino sugars of anionic molecules (e.g., teichoic acids, lipid A and phospholipids), (c) alteration of bacterial membrane fluidity, and (d) expulsion of the peptides using multi drug pumps. Together with bacterial regulatory network(s) that regulate expression and activity of these mechanisms, they represent attractive targets for development of novel antibacterials.

Animal models of external traumatic wound infections

BACKGROUND

Despite advances in traumatic wound care and management, infections remain a leading cause of mortality,morbidity and economic disruption in millions of wound patients around the world. Animal models have become standard tools for studying a wide array of external traumatic wound infections and testing new antimicrobial strategies.

RESULTS

Animal models of external traumatic wound infections reported by different investigators vary in animal species used, microorganism strains, the number of microorganisms applied, the size of the wounds and for burn infections, the length of time the heated object or liquid is in contact with the skin.

METHODS

This review covers experimental infections in animal models of surgical wounds, skin abrasions, burns, lacerations,excisional wounds and open fractures.

CONCLUSIONS

As antibiotic resistance continues to increase,more new antimicrobial approaches are urgently needed.These should be tested using standard protocols for infections in external traumatic wounds in animal models.

Staphylococcus aureus biofilms prevent macrophage phagocytosis and attenuate inflammation in vivo

Biofilms are complex communities of bacteria encased in a matrix composed primarily of polysaccharides, extracellular DNA, and protein. Staphylococcus aureus can form biofilm infections, which are often debilitating due to their chronicity and recalcitrance to antibiotic therapy. Currently, the immune mechanisms elicited during biofilm growth and their impact on bacterial clearance remain to be defined. We used a mouse model of catheter-associated biofilm infection to assess the functional importance of TLR2 and TLR9 in the host immune response during biofilm formation, because ligands for both receptors are present within the biofilm. Interestingly, neither TLR2 nor TLR9 impacted bacterial density or inflammatory mediator secretion during biofilm growth in vivo, suggesting that S. aureus biofilms circumvent these traditional bacterial recognition pathways. Several potential mechanisms were identified to account for biofilm evasion of innate immunity, including significant reductions in IL-1β, TNF-α, CXCL2, and CCL2 expression during biofilm infection compared with the wound healing response elicited by sterile catheters, limited macrophage invasion into biofilms in vivo, and a skewing of the immune response away from a microbicidal phenotype as evidenced by decreases in inducible NO synthase expression concomitant with robust arginase-1 induction. Coculture studies of macrophages with S. aureus biofilms in vitro revealed that macrophages successful at biofilm invasion displayed limited phagocytosis and gene expression patterns reminiscent of alternatively activated M2 macrophages. Collectively, these findings demonstrate that S. aureus biofilms are capable of attenuating traditional host proinflammatory responses, which may explain why biofilm infections persist in an immunocompetent host.