Transcriptional profiling of a Staphylococcus aureus clinical isolate and its isogenic agr and sarA mutants reveals global differences in comparison to the laboratory strain RN6390

Increased production of aureolysin and staphopain A is a primary determinant of the reduced virulence of Staphylococcus aureus sarA mutants in osteomyelitis

We previously demonstrated that mutation of sarA in Staphylococcus aureus limits biofilm formation, cytotoxicity for osteoblasts and osteoclasts, and virulence in osteomyelitis, and that all of these phenotypes can be attributed to the increased production of extracellular proteases. Here we extend these studies to assess the individual importance of these proteases alone and in combination with each other using the methicillin-resistant USA300 strain LAC, the methicillin-susceptible USA200 strain UAMS-1, and isogenic sarA mutants that were also unable to produce aureolysin (Aur), staphopain A (ScpA), staphylococcal serine protease A (subsp.), staphopain B (SspB), and the staphylococcal serine protease-like proteins A-F (SplA-F). Biofilm formation was restored in LAC and UAMS-1 sarA mutants by subsequent mutation of aur and scpA, while mutation of aur had the greatest impact on cytotoxicity to mammalian cells, particularly with conditioned medium (CM) from the more cytotoxic strain LAC. However, SDS-PAGE and western blot analysis of CM confirmed that mutation of sspAB was also required to mimic the phenotype of sarA mutants unable to produce any extracellular proteases. Nevertheless, in a murine model of post-traumatic osteomyelitis, mutation of aur and scpA had the greatest impact on restoring the virulence of LAC and UAMS-1 sarA mutants, with concurrent mutation of sspAB and the spl operon having relatively little effect. These results demonstrate that the increased production of Aur and ScpA in combination with each other is a primary determinant of the reduced virulence of S. aureus sarA mutants in diverse clinical isolates including both methicillin-resistant and methicillin-susceptible strains.IMPORTANCEPrevious work established that SarA plays a primary role in limiting the production of extracellular proteases to prevent them from limiting the abundance of S. aureus virulence factors. Eliminating the production of all 10 extracellular proteases in the methicillin-resistant strain LAC has also been shown to enhance virulence in a murine sepsis model, and this has been attributed to the specific proteases Aur and ScpA. The importance of this work lies in our demonstration that the increased production of these same proteases largely accounts for the decreased virulence of sarA mutants in a murine model of post-traumatic osteomyelitis not only in LAC but also in the methicillin-susceptible human osteomyelitis isolate UAMS-1. This confirms that sarA-mediated repression of Aur and ScpA production plays a critical role in the posttranslational regulation of S. aureus virulence factors in diverse clinical isolates and diverse forms of S. aureus infection.

Insights into the Effects and Mechanism of Andrographolide-Mediated Recovery of Susceptibility of Methicillin-Resistant Staphylococcus aureus to β-Lactam Antibiotics

The frequent resistance associated with β-lactam antibiotics and the high frequency of mutations in β-lactamases constitute a major clinical challenge that can no longer be ignored. Andrographolide (AP), a natural active compound, has been shown to restore susceptibility to β-lactam antibiotics. Fluorescence quenching and molecular simulation showed that AP quenched the intrinsic fluorescence of β-lactamase BlaZ and stably bound to the residues in the catalytic cavity of BlaZ. Of note, AP was found to reduce the stability of the cell wall (CW) in methicillin-resistant Staphylococcus aureus (MRSA), and in combination with penicillin G (PEN), it significantly induced CW roughness and dispersion and even caused its disintegration, while the same concentration of PEN did not. In addition, transcriptome sequencing revealed that AP induced a significant stress response and increased peptidoglycan (PG) synthesis but disrupted its cross-linking, and it repressed the expression of critical genes such as mecA, blaZ, and sarA. We also validated these findings by quantitative reverse transcription-PCR (qRT-PCR). Association analysis using the GEO database showed that the alterations caused by AP were similar to those caused by mutations in the sarA gene. In summary, AP was able to restore the susceptibility of MRSA to β-lactam antibiotics, mainly by inhibiting the β-lactamase BlaZ, by downregulating the expression of critical resistance genes such as mecA and blaZ, and by disrupting CW homeostasis. In addition, restoration of susceptibility to antibiotics could be achieved by inhibiting the global regulator SarA, providing an effective solution to alleviate the problem of bacterial resistance. IMPORTANCE Increasingly, alternatives to antibiotics are being used to mitigate the rapid onset and development of bacterial resistance, and the combination of natural compounds with traditional antibiotics has become an effective therapeutic strategy. Therefore, we attempted to discover more mechanisms to restore susceptibility and effective dosing strategies. Andrographolide (AP), as a natural active ingredient, can mediate recovery of susceptibility of MRSA to β-lactam antibiotics. AP bound stably to the β-lactamase BlaZ and impaired its hydrolytic activity. Notably, AP was able to downregulate the expression of critical resistance genes such as mecA, blaZ, and sarA. Meanwhile, it disrupted the CW cross-linking and homeostasis, while the same concentration of penicillin could not. The multiple inhibitory effect of AP resensitizes intrinsically resistant bacteria to β-lactam antibiotics, effectively prolonging the use cycle of these antibiotics and providing an effective solution to reduce the dosage of antibiotics and providing a theoretical reference for the prevention and control of MRSA.

Staphylococcus aureus binding to Seraph® 100 Microbind® Affinity Filter: Effects of surface protein expression and treatment duration

INTRODUCTION

Extracorporeal blood purification systems represent a promising alternative for treatment of blood stream infections with multiresistant bacteria.

OBJECTIVES

The aim of this study was to analyse the binding activity of S. aureus to Seraph affinity filters based on heparin coated beads and to identify effectors influencing this binding activity.

RESULTS

To test the binding activity, we used gfp-expressing S. aureus Newman strains inoculated either in 0.9% NaCl or in blood plasma and determined the number of unbound bacteria by FACS analyses after passing through Seraph affinity filters. The binding activity of S. aureus was clearly impaired in human plasma: while a percent removal of 42% was observed in 0.9% NaCl (p-value 0.0472) using Seraph mini columns, a percent removal of only 10% was achieved in human plasma (p-value 0.0934). The different composition of surface proteins in S. aureus caused by the loss of SarA, SigB, Lgt, and SaeS had no significant influence on its binding activity. In a clinically relevant approach using the Seraph® 100 Microbind® Affinity Filter and 1000 ml of human blood plasma from four different donors, the duration of treatment was shown to have a critical effect on the rate of bacterial reduction. Within the first four hours, the number of bacteria decreased continuously and the reduction in bacteria reached statistical significance after two hours of treatment (percentage reduction 64%, p-value 0.01165). The final reduction after four hours of treatment was close to 90% and is dependent on donor. The capacity of Seraph® 100 for S. aureus in human plasma was approximately 5 x 108 cells.

CONCLUSIONS

The Seraph affinity filter, based on heparin-coated beads, is a highly efficient method for reducing S. aureus in human blood plasma, with efficiency dependent on blood plasma composition and treatment duration.

Global Downregulation of Penicillin Resistance and Biofilm Formation by MRSA Is Associated with the Interaction between Kaempferol Rhamnosides and Quercetin

The rapid development of methicillin-resistant Staphylococcus aureus (MRSA) drug resistance and the formation of biofilms seriously challenge the clinical application of classic antibiotics. Extracts of the traditional herb Chenopodium ambrosioides L. were found to have strong antibiofilm activity against MRSA, but their mechanism of action remains poorly understood. This study was designed to investigate the antibacterial and antibiofilm activities against MRSA of flavonoids identified from C. ambrosioides L. in combination with classic antibiotics, including ceftazidime, erythromycin, levofloxacin, penicillin G, and vancomycin. Liquid chromatography-mass spectrometry (LC-MS) was used to analyze the nonvolatile chemical compositions. Reverse transcription (RT)-PCR was used to investigate potential multitargets of flavonoids based on global transcriptional responses of virulence and antibiotic resistance. A synergistic antibacterial and biofilm-inhibiting activity of the alcoholic extract of the ear of C. ambrosioides L. in combination with penicillin G was observed against MRSA, which proved to be closely related to the interaction of the main components of kaempferol rhamnosides with quercetin. In regard to the mechanism, the increased sensitivity of MRSA to penicillin G was shown to be related to the downregulation of penicillinase with SarA as a potential drug target, while the antibiofilm activity was mainly related to downregulation of various virulence factors involved in the initial and mature stages of biofilm development, with SarA and/or σB as drug targets. This study provides a theoretical basis for further exploration of the medicinal activity of kaempferol rhamnosides and quercetin and their application in combination with penicillin G against MRSA biofilm infection. IMPORTANCE In this study, the synergistic antibacterial and antibiofilm effects of the traditional herb C. ambrosioides L. and the classic antibiotic penicillin G on MRSA provide a potential strategy to deal with the rapid development of MRSA antibiotic resistance. This study also provides a theoretical basis for further optimizing the combined effect of kaempferol rhamnosides, quercetin, and penicillin G and exploring anti-MRSA biofilm infection research with SarA and σB as drug targets.

Limiting protease production plays a key role in the pathogenesis of the divergent clinical isolates of Staphylococcus aureus LAC and UAMS-1

Using the USA300, methicillin-resistant Staphylococcus aureus strain LAC, we previously examined the impact of regulatory mutations implicated in biofilm formation on protease production and virulence in a murine sepsis model. Here we examined the impact of these mutations in the USA200, methicillin-sensitive strain UAMS-1. Mutation of agr, mgrA, rot, sarA and sigB attenuated the virulence of UAMS-1. A common characteristic of codY, rot, sigB, and sarA mutants was increased protease production, with mutation of rot having the least impact followed by mutation of codY, sigB and sarA, respectively. Protein A was undetectable in conditioned medium from all four mutants, while extracellular nuclease was only present in the proteolytically cleaved NucA form. The abundance of high molecular weight proteins was reduced in all four mutants. Biofilm formation was reduced in codY, sarA and sigB mutants, but not in the rot mutant. Eliminating protease production partially reversed these phenotypes and enhanced biofilm formation. This was also true in LAC codY, rot, sarA and sigB mutants. Eliminating protease production enhanced the virulence of LAC and UAMS-1 sarA, sigB and rot mutants in a murine sepsis model but did not significantly impact the virulence of the codY mutant in either strain. Nevertheless, these results demonstrate that repressing protease production plays an important role in defining critical phenotypes in diverse clinical isolates of S. aureus and that Rot, SigB and SarA play critical roles in this regard.

Expanding the Staphylococcus aureus SarA Regulon to Small RNAs

SarA, a transcriptional regulator of Staphylococcus aureus, is a major global regulatory system that coordinates the expression of target genes involved in its pathogenicity. Various studies have identified a large number of SarA target genes, but an in-depth characterization of the sarA regulon, including small regulatory RNAs (sRNAs), has not yet been done. In this study, we utilized transcriptome sequencing (RNA-Seq) and chromatin immunoprecipitation sequencing (ChIP-Seq) to determine a comprehensive list of SarA-regulated targets, including both mRNAs and sRNAs. RNA-Seq analysis indicated 390 mRNAs and 51 sRNAs differentially expressed in a ΔsarA mutant, while ChIP-Seq revealed 354 mRNAs and 55 sRNA targets in the S. aureus genome. We confirmed the authenticity of several novel SarA targets by Northern blotting and electrophoretic mobility shift assays. Among them, we characterized repression of sprG2, a gene that encodes the toxin of a type I toxin-antitoxin system, indicating a multilayer lockdown of toxin expression by both SarA and its cognate antitoxin, SprF2. Finally, a novel SarA consensus DNA binding sequence was generated using the upstream promoter sequences of 15 novel SarA-regulated sRNA targets. A genome-wide scan with a deduced SarA motif enabled the discovery of new potential SarA target genes which were not identified in our RNA-Seq and ChIP-Seq analyses. The strength of this new consensus was confirmed with one predicted sRNA target. The RNA-Seq and ChIP-Seq combinatory analysis gives a snapshot of the regulation, whereas bioinformatic analysis reveals a permanent view of targets based on sequence. Altogether these experimental and in silico methodologies are effective to characterize transcriptional factor (TF) regulons and functions.

IMPORTANCEStaphylococcus aureus, a commensal and opportunist pathogen, is responsible for a large number of human and animal infections, from benign to severe. Gene expression adaptation during infection requires a complex network of regulators, including transcriptional factors (TF) and sRNAs. TF SarA influences virulence, metabolism, biofilm formation, and resistance to some antibiotics. SarA directly regulates expression of around 20 mRNAs and a few sRNAs. Here, we combined high-throughput expression screening methods combined with binding assays and bioinformatics for an in-depth investigation of the SarA regulon. This combinatory approach allowed the identification of 85 unprecedented mRNAs and sRNAs targets, with at least 14 being primary. Among novel SarA direct targets, we characterized repression of sprG2, a gene that encodes the toxin of a toxin-antitoxin system, indicating a multilayer lockdown of toxin expression by both SarA and its cognate antitoxin, SprF2.

Evaluation of a bone filler scaffold for local antibiotic delivery to prevent Staphylococcus aureus infection in a contaminated bone defect

We previously reported the development of an osteogenic bone filler scaffold consisting of degradable polyurethane, hydroxyapatite, and decellularized bovine bone particles. The current study was aimed at evaluating the use of this scaffold as a means of local antibiotic delivery to prevent infection in a bone defect contaminated with Staphylococcus aureus. We evaluated two scaffold formulations with the same component ratios but differing overall porosity and surface area. Studies with vancomycin, daptomycin, and gentamicin confirmed that antibiotic uptake was concentration dependent and that increased porosity correlated with increased uptake and prolonged antibiotic release. We also demonstrate that vancomycin can be passively loaded into either formulation in sufficient concentration to prevent infection in a rabbit model of a contaminated segmental bone defect. Moreover, even in those few cases in which complete eradication was not achieved, the number of viable bacteria in the bone was significantly reduced by treatment and there was no radiographic evidence of osteomyelitis. Radiographs and microcomputed tomography (µCT) analysis from the in vivo studies also suggested that the addition of vancomycin did not have any significant effect on the scaffold itself. These results demonstrate the potential utility of our bone regeneration scaffold for local antibiotic delivery to prevent infection in contaminated bone defects.

Mild Lactic Acid Stress Causes Strain-Dependent Reduction in SEC Protein Levels

Staphylococcal enterotoxin C (SEC) is a major cause of staphylococcal food poisoning in humans and plays a role in bovine mastitis. Staphylococcus aureus (S. aureus) benefits from a competitive growth advantage under stress conditions encountered in foods such as a low pH. Therefore, understanding the role of stressors such as lactic acid on SEC production is of pivotal relevance to food safety. However, stress-dependent cues and their effects on enterotoxin expression are still poorly understood. In this study, we used human and animal strains harboring different SEC variants in order to evaluate the influence of mild lactic acid stress (pH 6.0) on SEC expression both on transcriptional and translational level. Although only a modest decrease in sec mRNA levels was observed under lactic acid stress, protein levels showed a significant decrease in SEC levels for some strains. These findings indicate that post-transcriptional modifications can act in SEC expression under lactic acid stress.

Protein Biomarker Discovery for Methicillin-Sensitive, Heterogeneous Vancomycin-Intermediate and Vancomycin-Intermediate Staphylococcus aureus Strains Using Label-Free Data-Independent Acquisition Proteomics

Rapid identification of methicillin-sensitive Staphylococcus aureus (MSSA), heterogeneous vancomycin-intermediate S. aureus (hVISA), and vancomycin-intermediate S. aureus (VISA) is important for accurate treatment, timely intervention, and prevention of outbreaks. Here, 90 S. aureus isolates were analyzed for protein biomarker discovery, including MSSA, vancomycin-susceptible S. aureus (VSSA), hVISA, and VISA strains. Label-free data-independent acquisition proteomics was used to identify protein biomarkers that allow for discrimination among MSSA, hVISA, and VISA strains. There were 8786 nonredundant peptides identified, corresponding to 418 different annotated nonredundant proteins. Two VISA protein biomarkers, two hVISA protein biomarkers, and one MSSA protein biomarker with high sensitivities and specificities were discovered and verified. Data are available via MassIVE with identifier MSV000085776.

The type VII secretion system protects Staphylococcus aureus against antimicrobial host fatty acids

The Staphylococcus aureus type VII secretion system (T7SS) exports several proteins that are pivotal for bacterial virulence. The mechanisms underlying T7SS-mediated staphylococcal survival during infection nevertheless remain unclear. Here we report that S. aureus lacking T7SS components are more susceptible to host-derived antimicrobial fatty acids. Unsaturated fatty acids such as linoleic acid (LA) elicited an increased inhibition of S. aureus mutants lacking T7SS effectors EsxC, EsxA and EsxB, or the membrane-bound ATPase EssC, compared to the wild-type (WT). T7SS mutants generated in different S. aureus strain backgrounds also displayed an increased sensitivity to LA. Analysis of bacterial membrane lipid profiles revealed that the esxC mutant was less able to incorporate LA into its membrane phospholipids. Although the ability to bind labelled LA did not differ between the WT and mutant strains, LA induced more cell membrane damage in the T7SS mutants compared to the WT. Furthermore, proteomic analyses of WT and mutant cell fractions revealed that, in addition to compromising membranes, T7SS defects induce oxidative stress and hamper their response to LA challenge. Thus, our findings indicate that T7SS contribute to maintaining S. aureus membrane integrity and homeostasis when bacteria encounter antimicrobial fatty acids.

Part II: Minimum Quality Threshold in Preclinical Sepsis Studies (MQTiPSS) for Types of Infections and Organ Dysfunction Endpoints

Although the clinical definitions of sepsis and recommended treatments are regularly updated, a systematic review has not been done for preclinical models. To address this deficit, a Wiggers-Bernard Conference on preclinical sepsis modeling reviewed the 260 most highly cited papers between 2003 and 2012 using sepsis models to create a series of recommendations. This Part II report provides recommendations for the types of infections and documentation of organ injury in preclinical sepsis models. Concerning the types of infections, the review showed that the cecal ligation and puncture model was used for 44% of the studies while 40% injected endotoxin. Recommendation #8 (numbered sequentially from Part I): endotoxin injection should not be considered as a model of sepsis; live bacteria or fungal strains derived from clinical isolates are more appropriate. Recommendation #9: microorganisms should replicate those typically found in human sepsis. Sepsis-3 states that sepsis is life-threatening organ dysfunction caused by a dysregulated host response to infection, but the review of the papers showed limited attempts to document organ dysfunction. Recommendation #10: organ dysfunction definitions should be used in preclinical models. Recommendation #11: not all activities in an organ/system need to be abnormal to verify organ dysfunction. Recommendation #12: organ dysfunction should be measured in an objective manner using reproducible scoring systems. Recommendation #13: not all experiments must measure all parameters of organ dysfunction, but investigators should attempt to fully capture as much information as possible. These recommendations are proposed as "best practices" for animal models of sepsis.

Typing methods based on whole genome sequencing data

Whole genome sequencing (WGS) of foodborne pathogens has become an effective method for investigating the information contained in the genome sequence of bacterial pathogens. In addition, its highly discriminative power enables the comparison of genetic relatedness between bacteria even on a sub-species level. For this reason, WGS is being implemented worldwide and across sectors (human, veterinary, food, and environment) for the investigation of disease outbreaks, source attribution, and improved risk characterization models. In order to extract relevant information from the large quantity and complex data produced by WGS, a host of bioinformatics tools has been developed, allowing users to analyze and interpret sequencing data, starting from simple gene-searches to complex phylogenetic studies. Depending on the research question, the complexity of the dataset and their bioinformatics skill set, users can choose between a great variety of tools for the analysis of WGS data. In this review, we describe the relevant approaches for phylogenomic studies for outbreak studies and give an overview of selected tools for the characterization of foodborne pathogens based on WGS data. Despite the efforts of the last years, harmonization and standardization of typing tools are still urgently needed to allow for an easy comparison of data between laboratories, moving towards a one health worldwide surveillance system for foodborne pathogens.

Heterogeneous ceramide distributions alter spatially resolved growth of Staphylococcus aureus on human stratum corneum

Contemporary studies have revealed dramatic changes in the diversity of bacterial microbiota between healthy and diseased skin. However, the prevailing use of swabs to extract the microorganisms has meant that only population 'snapshots' are obtained, and all spatially resolved information of bacterial growth is lost. Here we report on the temporospatial growth of Staphylococcus aureus on the surface of the human stratum corneum (SC); the outermost layer of skin. This bacterial species dominates bacterial populations on skin with atopic dermatitis (AD). We first establish that the distribution of ceramides naturally present in the SC is heterogeneous, and correlates with the tissue's structural topography. This distribution subsequently impacts the growth of bacterial biofilms. In the SC retaining healthy ceramide concentrations, biofilms exhibit no spatial preference for growth. By contrast, a depletion of ceramides consistent with reductions known to occur with AD enables S. aureus to use the patterned network of topographical canyons as a conduit for growth. The ability of ceramides to govern bacterial growth is confirmed using a topographical skin canyon analogue coated with the ceramide subcomponent d-sphingosine. Our work appears to explain the causal link between ceramide depletion and increased S. aureus populations that is observed in AD. It may also provide insight into disease transmission as well as improving pre-operative skin cleansing techniques.

The Role of Regulatory Mechanisms and Environmental Parameters in Staphylococcal Food Poisoning and Resulting Challenges to Risk Assessment

Prevention, prediction, control, and handling of bacterial foodborne diseases - an ongoing, serious, and costly concern worldwide - are continually facing a wide array of difficulties. Not the least due to that food matrices, highly variable and complex, can impact virulence expression in diverse and unpredictable ways. This review aims to present a comprehensive overview of challenges related to the presence of enterotoxigenic Staphylococcus aureus in the food production chain. It focuses on characteristics, expression, and regulation of the highly stable staphylococcal enterotoxins and in particular staphylococcal enterotoxin A (SEA). Together with the robustness of the pathogen under diverse environmental conditions and the range of possible entry routes into the food chain, this poses some of the biggest challenges in the control of SFP. Furthermore, the emergence of new enterotoxins, found to be connected with SFP, brings new questions around their regulatory mechanisms and expression in different food environments. The appearance of increasing amounts of antibiotic resistant strains found in food is also highlighted. Finally, potentials and limitations of implementing existing risk assessment models are discussed. Various quantitative microbial risk assessment approaches have attempted to quantify the growth of the bacterium and production of disease causing levels of toxin under various food chain and domestic food handling scenarios. This requires employment of predictive modeling tools, quantifying the spatiotemporal population dynamics of S. aureus in response to intrinsic and extrinsic food properties. In this context, the armory of predictive modeling employs both kinetic and probabilistic models to estimate the levels that potentiate toxin production, the time needed to reach that levels, and overall, the likelihood of toxin production. Following risk assessment, the main challenge to mitigate the risk of S. aureus intoxication is first to prevent growth of the organism and then to hamper the production of enterotoxins, or at least prevent the accumulation of high levels (e.g., >10-20 ng) in food. The necessity for continued studies indeed becomes apparent based on the challenges to understand, control, and predict enterotoxin production in relation to the food environment. Different types of food, preservatives, processing, and packaging conditions; regulatory networks; and different staphylococcal enterotoxin-producing S. aureus strains need to be further explored to obtain more complete knowledge about the virulence of this intriguing pathogen.

Protease-Mediated Growth of Staphylococcus aureus on Host Proteins Is opp3 Dependent

Staphylococcus aureus has the ability to cause infections in a variety of niches, suggesting a robust metabolic capacity facilitating proliferation under various nutrient conditions. The mature skin abscess is glucose depleted, indicating that peptides and free amino acids are important sources of nutrients for S. aureus. Our studies have found that mutations in both pyruvate carboxykinase and glutamate dehydrogenase, enzymes that function in essential gluconeogenesis reactions when amino acids serve as the major carbon source, reduce bacterial burden in a murine skin abscess model. Moreover, peptides liberated from collagen by host protease MMP-9 as well as the staphylococcal protease aureolysin support S. aureus growth in an Opp3-dependent manner under nutrient-limited conditions. Additionally, the presence of peptides induces aureolysin expression. Overall, these studies define one pathway by which S. aureus senses a nutrient-limiting environment and induces factors that function to acquire and utilize carbon from host-derived sources.

Staphylococcus aureus has the ability to cause infections in multiple organ systems, suggesting an ability to rapidly adapt to changing carbon and nitrogen sources. Although there is little information about the nutrients available at specific sites of infection, a mature skin abscess has been characterized as glucose depleted, indicating that peptides and free amino acids are an important source of nutrients for the bacteria. Our studies have found that mutations in enzymes necessary for growth on amino acids, including pyruvate carboxykinase (ΔpckA) and glutamate dehydrogenase (ΔgudB), reduced the ability of the bacteria to proliferate within a skin abscess, suggesting that peptides and free amino acids are important for S. aureus growth. Furthermore, we found that collagen, an abundant host protein that is present throughout a skin abscess, serves as a reservoir of peptides. To liberate peptides from the collagen, we identified that the host protease, MMP-9, as well as the staphylococcal proteases aureolysin and staphopain B function to cleave collagen into peptide fragments that can support S. aureus growth under nutrient-limited conditions. Moreover, the oligopeptide transporter Opp3 is the primary staphylococcal transporter responsible for peptide acquisition. Lastly, we observed that the presence of peptides (3-mer to 7-mer) induces the expression of aureolysin, suggesting that S. aureus has the ability to detect peptides in the environment.

Anti-Biofilm Activity of Graphene Quantum Dots via Self-Assembly with Bacterial Amyloid Proteins

Bacterial biofilms represent an essential part of Earth's ecosystem that can cause multiple ecological, technological, and health problems. The environmental resilience and sophisticated organization of biofilms are enabled by the extracellular matrix that creates a protective network of biomolecules around the bacterial community. Current anti-biofilm agents can interfere with extracellular matrix production but, being based on small molecules, are degraded by bacteria and rapidly diffuse away from biofilms. Both factors severely reduce their efficacy, while their toxicity to higher organisms creates additional barriers to their practicality. In this paper, we report on the ability of graphene quantum dots to effectively disperse mature amyloid-rich Staphylococcus aureus biofilms, interfering with the self-assembly of amyloid fibers, a key structural component of the extracellular matrix. Mimicking peptide-binding biomolecules, graphene quantum dots form supramolecular complexes with phenol-soluble modulins, the peptide monomers of amyloid fibers. Experimental and computational results show that graphene quantum dots efficiently dock near the N-terminus of the peptide and change the secondary structure of phenol-soluble modulins, which disrupts their fibrillation and represents a strategy for mitigation of bacterial communities.

A Fluorescence-based Method to Study Bacterial Gene Regulation in Infected Tissues

Bacterial virulence genes are often regulated at the transcriptional level by multiple factors that respond to different environmental signals. Some factors act directly on virulence genes; others control pathogenesis by adjusting the expression of downstream regulators or the accumulation of signals that affect regulator activity. While regulation has been studied extensively during in vitro growth, relatively little is known about how gene expression is adjusted during infection. Such information is important when a particular gene product is a candidate for therapeutic intervention. Transcriptional approaches like quantitative, real-time RT-PCR and RNA-Seq are powerful ways to examine gene expression on a global level but suffer from many technical challenges including low abundance of bacterial RNA compared to host RNA, and sample degradation by RNases. Evaluating regulation using fluorescent reporters is relatively easy and can be multiplexed with fluorescent proteins with unique spectral properties. The method allows for single-cell, spatiotemporal analysis of gene expression in tissues that exhibit complex three-dimensional architecture and physiochemical gradients that affect bacterial regulatory networks. Such information is lost when data are averaged over the bulk population. Herein, we describe a method for quantifying gene expression in bacterial pathogens in situ. The method is based on simple tissue processing and direct observation of fluorescence from reporter proteins. We demonstrate the utility of this system by examining the expression of Staphylococcus aureus thermonuclease (nuc), whose gene product is required for immune evasion and full virulence ex vivo and in vivo. We show that nuc-gfp is strongly expressed in renal abscesses and reveal heterogeneous gene expression due in part to apparent spatial regulation of nuc promoter activity in abscesses fully engaged with the immune response. The method can be applied to any bacterium with a manipulatable genetic system and any infection model, providing valuable information for preclinical studies and drug development.

Small RNA-Based Regulation of Bacterial Quorum Sensing and Biofilm Formation

Quorum sensing is a vital property of bacteria that enables community-wide coordination of collective behaviors. A key example of such a behavior is biofilm formation, in which groups of bacteria invest in synthesizing a protective, joint extracellular matrix. Quorum sensing involves the production, release, and subsequent detection of extracellular signaling molecules called autoinducers. The architecture of quorum-sensing signal transduction pathways is highly variable among different species of bacteria, but frequently involves posttranscriptional regulation carried out by small regulatory RNA molecules. This review illustrates the diverse roles small trans-acting regulatory RNAs can play, from constituting a network's core to auxiliary roles in adjusting the rate of autoinducer synthesis, mediating cross talk among different parts of a network, or integrating different regulatory inputs to trigger appropriate changes in gene expression. The emphasis is on describing how the study of small RNA-based regulation in quorum sensing and biofilm formation has uncovered new general properties or expanded our understanding of bacterial riboregulation.

Severe infections emerge from commensal bacteria by adaptive evolution

Bacteria responsible for the greatest global mortality colonize the human microbiota far more frequently than they cause severe infections. Whether mutation and selection among commensal bacteria are associated with infection is unknown. We investigated de novo mutation in 1163 Staphylococcus aureus genomes from 105 infected patients with nose colonization. We report that 72% of infections emerged from the nose, with infecting and nose-colonizing bacteria showing parallel adaptive differences. We found 2.8-to-3.6-fold adaptive enrichments of protein-altering variants in genes responding to rsp, which regulates surface antigens and toxin production; agr, which regulates quorum-sensing, toxin production and abscess formation; and host-derived antimicrobial peptides. Adaptive mutations in pathogenesis-associated genes were 3.1-fold enriched in infecting but not nose-colonizing bacteria. None of these signatures were observed in healthy carriers nor at the species-level, suggesting infection-associated, short-term, within-host selection pressures. Our results show that signatures of spontaneous adaptive evolution are specifically associated with infection, raising new possibilities for diagnosis and treatment.

Expression of Staphylococcal Enterotoxins under Stress Encountered during Food Production and Preservation

Staphylococcal food poisoning (SFP) is the most prevalent cause of food-borne intoxications worldwide. Consumption of enterotoxins preformed in food causes violent vomiting and can be fatal in children and the elderly. While being repressed by competing bacteria in most matrices, Staphylococcus aureus benefits from crucial competitive advantages in foods with high osmolarity or low pH. During recent years, the long-standing belief in the feasibility of assessing SFP risk based on colony-forming units of S. aureus present in food products has been disproven. Instead, researchers and food business operators are acutely aware of the imminent threat arising from unforeseeable enterotoxin production under stress conditions. This paradigm shift led to a variety of new publications enabling an improved understanding of enterotoxin expression under stress conditions encountered in food. The wealth of data provided by these studies is extremely diverse, as it is based on different methodological approaches, staphylococcal strains, stressors, and enterotoxins. Therefore, in this review, we aggregated and critically evaluated the complex findings of these studies, to provide readers with a current overview of the state of research in the field.

Transcriptional Regulator TetR21 Controls the Expression of the Staphylococcus aureus LmrS Efflux Pump

TetR21 controls the expression of Tet38 and LmrS efflux pumps. A tetR21 mutant, QT21, exhibited a 4-fold increase in the transcription level of lmrSStaphylococcus aureuslmrS overexpressor showed increases of 4-fold and 2-fold, respectively, in the MICs of chloramphenicol and erythromycin, while the MICs of lmrS mutant QT18 and lmrS-tetR21 mutant QT1821 remained similar to those of parental strain RN6390. TetR21 does not bind to the promoter of lmrS, suggesting indirect regulation of lmrS.

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.

Active and Passive Immunization Against Staphylococcus aureus Periprosthetic Osteomyelitis in Rats

BACKGROUND/AIM

Staphylococcus aureus infection associated with orthopedic implants cannot always be controlled. We used a knee prosthesis model with implant-related osteomyelitis in rats to explore induction of an effective immune response with active and passive immunization.

MATERIALS AND METHODS

Fifty-two Sprague-Dawley rats were divided into active (N=28) and passive immunization groups (N=24). A bacterial inoculum of 10³ S. aureus MN8 was injected into the tibia and the femur marrow before insertion of a non-constrained knee prosthesis in each rat. The active-immunization group received a synthetic oligosaccharide of polysaccharide poly-N-acetylglucosamine (PNAG), 9G1cNH₂ and the passive-immunization group received immunization with immunoglobulin from rabbits infected with S. aureus.

RESULTS/CONCLUSION

Active immunization against PNAG significantly reduced the consequences of osteomyelitis infection from PNAG-producing intercellular adhesion (ica⁺) but not ica^- S. aureus. Passive immunization resulted in better clinical assessments in animals challenged with either ica⁺ or ica^- S. aureus, suggesting a lack of specificity in this antiserum.

Effect of food-related stress conditions and loss of agr and sigB on seb promoter activity in S. aureus

Staphylococcal enterotoxin B (SEB) causes staphylococcal food poisoning and is produced in up to ten times higher quantities than other major enterotoxins. While Staphylococcus aureus growth is often repressed by competing flora, the organism exhibits a decisive growth advantage under some stress conditions. So far, data on the influence of food-related stressors and regulatory mutations on seb expression is limited and largely based on laboratory strains, which were later reported to harbor mutations. Therefore, the aim of this study was to investigate the influence of stress and regulatory mutations on seb promoter activity. To this end, transcriptional fusions were created in two strains, USA300 and HG003, carrying different seb upstream sequences fused to a blaZ reporter. NaCl, nitrite, and glucose stress led to significantly decreased seb promoter activity, while lactic acid stress resulted in significantly increased seb promoter activity. Loss of agr decreased seb promoter activity and loss of sigB increased promoter activity, with the magnitude of change depending on the strain. These results demonstrate that mild stress conditions encountered during food production and preservation can induce significant changes in seb promoter activity.

The role of biofilms in persistent infections and factors involved in ica-independent biofilm development and gene regulation in Staphylococcus aureus

Staphylococcus aureus biofilms represent a unique micro-environment that directly contribute to the bacterial fitness within hospital settings. The accumulation of this structure on implanted medical devices has frequently caused the development of persistent and chronic S. aureus-associated infections, which represent an important social and economic burden worldwide. ica-independent biofilms are composed of an assortment of bacterial products and modulated by a multifaceted and overlapping regulatory network; therefore, biofilm composition can vary among S. aureus strains. In the microniches formed by biofilms-produced by a number of bacterial species and composed by different structural components-drug refractory cell subpopulations with distinct physiological characteristics can emerge and result in therapeutic failures in patients with recalcitrant bacterial infections. In this review, we highlight the importance of biofilms in the development of persistence and chronicity in some S. aureus diseases, the main molecules associated with ica-independent biofilm development and the regulatory mechanisms that modulate ica-independent biofilm production, accumulation, and dispersion.

The metalloprotease SepA governs processing of accumulation-associated protein and shapes intercellular adhesive surface properties in Staphylococcus epidermidis

The otherwise harmless skin inhabitant Staphylococcus epidermidis is a major cause of healthcare-associated medical device infections. The species' selective pathogenic potential depends on its production of surface adherent biofilms. The Cell wall-anchored protein Aap promotes biofilm formation in S. epidermidis, independently from the polysaccharide intercellular adhesin PIA. Aap requires proteolytic cleavage to act as an intercellular adhesin. Whether and which staphylococcal proteases account for Aap processing is yet unknown. Here, evidence is provided that in PIA-negative S. epidermidis 1457Δica, the metalloprotease SepA is required for Aap-dependent S. epidermidis biofilm formation in static and dynamic biofilm models. qRT-PCR and protease activity assays demonstrated that under standard growth conditions, sepA is repressed by the global regulator SarA. Inactivation of sarA increased SepA production, and in turn augmented biofilm formation. Genetic and biochemical analyses demonstrated that SepA-related induction of biofilm accumulation resulted from enhanced Aap processing. Studies using recombinant proteins demonstrated that SepA is able to cleave the A domain of Aap at residue 335 and between the A and B domains at residue 601. This study identifies the mechanism behind Aap-mediated biofilm maturation, and also demonstrates a novel role for a secreted staphylococcal protease as a requirement for the development of a biofilm.

Comparative virulence studies and transcriptome analysis of Staphylococcus aureus strains isolated from animals

Several studies have been conducted to check the prevalence of methicillin-resistant strains of Staphylococcus aureus (MRSA) in animals and animal-derived food products but limited data are available regarding their virulence and associated gene expression profile. In the present study, antibiotic resistance and virulence of MRSA and methicillin-sensitive S. aureus animal isolates were determined in vitro by agar dilution, biofilm formation, adhesion, invasion and intracellular survivability assays. In addition, the pathogenicity of these isolates was examined in a murine model of S. aureus sepsis. MRSA1679a, a strain isolated from chicken, was observed to be highly virulent, in cell culture and in mouse model, and exhibited extensive resistant profile. Comparative gene expression profile of MRSA1679a and the reference human MRSA strain (ATCC 29213) was performed using Illumina-based transcriptome and RT-qPCR analyses. Several virulence elements including 22 toxin genes were detected in MRSA animal-isolate. In addition, we observed enhanced expression of crucial virulence regulators, such as sarA and KdpDE in MRSA animal-isolate compared to the human isolate. Collectively, gene expression profile including several virulence and drug-resistance factors confirmed the unique and highly virulent determinants of the MRSA strain of poultry origin which warrants further attention due to significant threat to public health.

Impact of sarA and Phenol-Soluble Modulins on the Pathogenesis of Osteomyelitis in Diverse Clinical Isolates of Staphylococcus aureus

We used a murine model of acute, posttraumatic osteomyelitis to evaluate the virulence of two divergent Staphylococcus aureus clinical isolates (the USA300 strain LAC and the USA200 strain UAMS-1) and their isogenic sarA mutants. The results confirmed that both strains caused comparable degrees of osteolysis and reactive new bone formation in the acute phase of osteomyelitis. Conditioned medium (CM) from stationary-phase cultures of both strains was cytotoxic to cells of established cell lines (MC3TC-E1 and RAW 264.7 cells), primary murine calvarial osteoblasts, and bone marrow-derived osteoclasts. Both the cytotoxicity of CM and the reactive changes in bone were significantly reduced in the isogenic sarA mutants. These results confirm that sarA is required for the production and/or accumulation of extracellular virulence factors that limit osteoblast and osteoclast viability and that thereby promote bone destruction and reactive bone formation during the acute phase of S. aureus osteomyelitis. Proteomic analysis confirmed the reduced accumulation of multiple extracellular proteins in the LAC and UAMS-1 sarA mutants. Included among these were the alpha class of phenol-soluble modulins (PSMs), which were previously implicated as important determinants of osteoblast cytotoxicity and bone destruction and repair processes in osteomyelitis. Mutation of the corresponding operon reduced the cytotoxicity of CM from both UAMS-1 and LAC cultures for osteoblasts and osteoclasts. It also significantly reduced both reactive bone formation and cortical bone destruction by CM from LAC cultures. However, this was not true for CM from cultures of a UAMS-1 psmα mutant, thereby suggesting the involvement of additional virulence factors in such strains that remain to be identified.

Staphylococcus aureus Shifts toward Commensalism in Response to Corynebacterium Species

Staphylococcus aureus-human interactions result in a continuum of outcomes from commensalism to pathogenesis. S. aureus is a clinically important pathogen that asymptomatically colonizes ~25% of humans as a member of the nostril and skin microbiota, where it resides with other bacteria including commensal Corynebacterium species. Commensal Corynebacterium spp. are also positively correlated with S. aureus in chronic polymicrobial diabetic foot infections, distinct from acute monomicrobial S. aureus infections. Recent work by our lab and others indicates that microbe-microbe interactions between S. aureus and human skin/nasal commensals, including Corynebacterium species, affect S. aureus behavior and fitness. Thus, we hypothesized that S. aureus interactions with Corynebacterium spp. diminish S. aureus virulence. We tested this by assaying for changes in S. aureus gene expression during in vitro mono- versus coculture with Corynebacterium striatum, a common skin and nasal commensal. We observed a broad shift in S. aureus gene transcription during in vitro growth with C. striatum, including increased transcription of genes known to exhibit increased expression during human nasal colonization and decreased transcription of virulence genes. S. aureus uses several regulatory pathways to transition between commensal and pathogenic states. One of these, the quorum signal accessory gene regulator (agr) system, was strongly inhibited in response to Corynebacterium spp. Phenotypically, S. aureus exposed to C. striatum exhibited increased adhesion to epithelial cells, reflecting a commensal state, and decreased hemolysin activity, reflecting an attenuation of virulence. Consistent with this, S. aureus displayed diminished fitness in experimental in vivo coinfection with C. striatum when compared to monoinfection. These data support a model in which S. aureus shifts from virulence toward a commensal state when exposed to commensal Corynebacterium species.

Regulatory Requirements for Staphylococcus aureus Nitric Oxide Resistance

The ability of Staphylococcus aureus to resist host innate immunity augments the severity and pervasiveness of its pathogenesis. Nitric oxide (NO˙) is an innate immune radical that is critical for the efficient clearance of a wide range of microbial pathogens. Exposure of microbes to NO˙ typically results in growth inhibition and induction of stress regulons. S. aureus, however, induces a metabolic state in response to NO˙ that allows for continued replication and precludes stress regulon induction. The regulatory factors mediating this distinctive response remain largely undefined. Here, we employ a targeted transposon screen and transcriptomics to identify and characterize five regulons essential for NO˙ resistance in S. aureus: three virulence regulons not formerly associated with NO˙ resistance, SarA, CodY, and Rot, as well as two regulons with established roles, Fur and SrrAB. We provide new insights into the contributions of Fur and SrrAB during NO˙ stress and show that the S. aureus ΔsarA mutant, the most sensitive of the newly identified mutants, exhibits metabolic dysfunction and widespread transcriptional dysregulation following NO˙ exposure. Altogether, our results broadly characterize the regulatory requirements for NO˙ resistance in S. aureus and suggest an intriguing overlap between the regulation of NO˙ resistance and virulence in this well-adapted human pathogen.

IMPORTANCE

The prolific human pathogen Staphylococcus aureus is uniquely capable of resisting the antimicrobial radical nitric oxide (NO˙), a crucial component of the innate immune response. However, a complete understanding of how S. aureus regulates an effective response to NO˙ is lacking. Here, we implicate three central virulence regulators, SarA, CodY, and Rot, as major players in the S. aureus NO˙ response. Additionally, we elaborate on the contribution of two regulators, SrrAB and Fur, already known to play a crucial role in S. aureus NO˙ resistance. Our study sheds light on a unique facet of S. aureus pathogenicity and demonstrates that the transcriptional response of S. aureus to NO˙ is highly pleiotropic and intrinsically tied to metabolism and virulence regulation.

Cyanide Toxicity to Burkholderia cenocepacia Is Modulated by Polymicrobial Communities and Environmental Factors

Microbes within polymicrobial communities can establish positive and negative interactions that have the potential to influence the overall behavior of the community. Pseudomonas aeruginosa and species of the Burkholderia cepacia complex (Bcc) can co-exist in the lower airways, however several studies have shown that P. aeruginosa can effectively kill the Bcc in vitro, for which hydrogen cyanide (HCN) was recently proposed to play a critical role. Here we show that modification of the environment (i.e., culture medium), long-term genetic adaptation of P. aeruginosa to the cystic fibrosis (CF) lung, or the addition of another bacterial species to the community can alter the sensitivity of Burkholderia cenocepacia to P. aeruginosa toxins. We specifically demonstrate that undefined rich media leads to higher susceptibility of B. cenocepacia to P. aeruginosa toxins like cyanide as compared to a synthetic medium (SCFM), that mimics the CF lung nutritional content. Overall, our study shows that the polymicrobial environment can have profound effects on negative interactions mediated by P. aeruginosa against B. cenocepacia. In fact, evolved P. aeruginosa or the presence of other species such as Staphylococcus aureus can directly abolish the direct competition mediated by cyanide and consequently maintaining a higher level of species diversity within the community.

Identification of virulence factors and antibiotic resistance markers using bacterial genomics

In recent years, the number of multidrug-resistant bacteria has increased rapidly and several epidemics were signaled in different regions of the world. Faced with this situation that presents a major global public health concern, the development and the use of new and rapid technologies is more than urgent. The use of the next-generation sequencing platforms by microbiologists and infectious disease specialists has allowed great progress in the medical field. Here, we review the usefulness of whole-genome sequencing for the detection of virulence and antibiotic resistance associated genes.

Various checkpoints prevent the synthesis of Staphylococcus aureus peptidoglycan hydrolase LytM in the stationary growth phase

In Staphylococcus aureus, peptidoglycan metabolism plays a role in the host inflammatory response and pathogenesis. Transcription of the peptidoglycan hydrolases is activated by the essential 2-component system WalKR at low cell density. During stationary growth phase, WalKR is not active and transcription of the peptidoglycan hydrolase genes is repressed. In this work, we studied regulation of expression of the glycylglycine endopeptidase LytM. We show that, in addition to the transcriptional regulation mediated by WalKR, the synthesis of LytM is negatively controlled by a unique mechanism at the stationary growth phase. We have identified 2 different mRNAs encoding lytM, which vary in the length of their 5' untranslated (5'UTR) regions. LytM is predominantly produced from the WalKR-regulated mRNA transcript carrying a short 5'UTR. The lytM mRNA is also transcribed as part of a polycistronic operon with the upstream SA0264 gene and is constitutively expressed. Although SA0264 protein can be synthesized from the longer operon transcript, lytM cannot be translated because its ribosome-binding site is sequestered into a translationally inactive secondary structure. In addition, the effector of the agr system, RNAIII, can inhibit translation of lytM present on the operon without altering the transcript level but does not have an effect on the translation of the upstream gene. We propose that this dual regulation of lytM expression, at the transcriptional and post-transcriptional levels, contributes to prevent cell wall damage during the stationary phase of growth.

Effects of Subinhibitory Concentrations of Ceftaroline on Methicillin-Resistant Staphylococcus aureus (MRSA) Biofilms

Ceftaroline (CPT) is a novel cephalosporin with in vitro activity against Staphylococcus aureus. Ceftaroline exhibits a level of binding affinity for PBPs in S. aureus including PBP2a of methicillin-resistant S. aureus (MRSA). The aims of this study were to investigate the morphological, physiological and molecular responses of MRSA clinical strains and MRSA biofilms to sub-MICs (1/4 and 1/16 MIC) of ceftaroline by using transmission, scanning and confocal microscopy. We have also used quantitative Real-Time PCR to study the effect of sub-MICs of ceftaroline on the expression of the staphylococcal icaA, agrA, sarA and sasF genes in MRSA biofilms. In one set of experiments, ceftaroline was able to inhibit biofilm formation in all strains tested at MIC, however, a strain dependent behavior in presence of sub-MICs of ceftaroline was shown. In a second set of experiments, destruction of preformed biofilms by addition of ceftaroline was evaluated. Ceftaroline was able to inhibit biofilm formation at MIC in all strains tested but not at the sub-MICs. Destruction of preformed biofilms was strain dependent because the biofilm formed by a matrix-producing strain was resistant to a challenge with ceftaroline at MIC, whereas in other strains the biofilm was sensitive. At sub-MICs, the impact of ceftaroline on expression of virulence genes was strain-dependent at 1/4 MIC and no correlation between ceftaroline-enhanced biofilm formation and gene regulation was established at 1/16 MIC. Our findings suggest that sub-MICs of ceftaroline enhance bacterial attachment and biofilm formation by some, but not all, MRSA strains and, therefore, stress the importance of maintaining effective bactericidal concentrations of ceftaroline to fight biofilm-MRSA related infections.

Regulatory Mutations Impacting Antibiotic Susceptibility in an Established Staphylococcus aureus Biofilm

We previously determined the extent to which mutations of different Staphylococcus aureus regulatory loci impact biofilm formation as assessed under in vitro conditions. Here we extend these studies to determine the extent to which those regulatory loci that had the greatest effect on biofilm formation also impact antibiotic susceptibility. The experiments were done under in vitro and in vivo conditions using two clinical isolates of S. aureus (LAC and UAMS-1) and two functionally diverse antibiotics (daptomycin and ceftaroline). Mutation of the staphylococcal accessory regulator (sarA) or sigB was found to significantly increase susceptibilities to both antibiotics and in both strains in a manner that could not be explained by changes in the MICs. The impact of a mutation in sarA was comparable to that of a mutation in sigB and greater than the impact observed with any other mutant. These results suggest that therapeutic strategies targeting sarA and/or sigB have the greatest potential to facilitate the ability to overcome the intrinsic antibiotic resistance that defines S. aureus biofilm-associated infections.

Comparative impact of diverse regulatory loci on Staphylococcus aureus biofilm formation

The relative impact of 23 mutations on biofilm formation was evaluated in the USA300, methicillin-resistant strain LAC. Mutation of sarA, atl, codY, rsbU, and sigB limited biofilm formation in comparison to the parent strain, but the limitation imposed by mutation of sarA was greater than that imposed by mutation of any of these other genes. The reduced biofilm formation of all mutants other than the atl mutant was correlated with increased levels of extracellular proteases. Mutation of fur- and mgrA-enhanced biofilm formation but in LAC had no impact on protease activity, nuclease activity, or accumulation of the polysaccharide intercellular adhesin (PIA). The increased capacity of these mutants to form a biofilm was reversed by mutation of sarA, and this was correlated with increased protease production. Mutation of sarA, mgrA, and sigB had the same phenotypic effect in the methicillin-sensitive strain UAMS-1, but mutation of codY increased rather than decreased biofilm formation. As with the UAMS-1 mgrA mutant, this was correlated with increased production of PIA. Examination of four additional clinical isolates suggests that the differential impact of codY on biofilm formation may be a conserved characteristic of methicillin-resistant versus methicillin-sensitive strains.

Investigational therapies targeting quorum-sensing for the treatment of Staphylococcus aureus infections

INTRODUCTION

Antibiotic resistance is a serious global health concern for developed and developing nations. MRSA represents a particularly severe public health threat that is associated with high morbidity and mortality. The lack of novel antibiotics has led scientists to explore therapies targeting bacterial virulence mechanisms and virulence regulators, including those controlling cell-cell communication.

AREAS COVERED

The authors discuss the role of quorum-sensing in Staphylococcus aureus infections and components of the system that are being targeted using novel investigational drugs. In particular, the authors examine the role of the accessory gene regulator (Agr) system in virulence regulation of S. aureus pathogenesis. Finally, the authors present and compare natural and synthetic compounds that have been found to interfere with Agr functionality.

EXPERT OPINION

There is a great need to develop new therapeutic methods to combat S. aureus infections. These include anti-virulence therapies that target key global regulators involved with the establishment and propagation of infection. Several molecules have been found to interfere with S. aureus virulence regulation, especially those targeting the Agr quorum-sensing signaling molecule. These preliminary findings warrant further investigation and validation, with the goal of refining a compound that has broad-spectrum inhibitory effects on most S. aureus strains and Agr subtypes.

A defense-offense multi-layered regulatory switch in a pathogenic bacterium

Cells adapt to environmental changes by efficiently adjusting gene expression programs. Staphylococcus aureus, an opportunistic pathogenic bacterium, switches between defensive and offensive modes in response to quorum sensing signal. We identified and studied the structural characteristics and dynamic properties of the core regulatory circuit governing this switch by deterministic and stochastic computational methods, as well as experimentally. This module, termed here Double Selector Switch (DSS), comprises the RNA regulator RNAIII and the transcription factor Rot, defining a double-layered switch involving both transcriptional and post-transcriptional regulations. It coordinates the inverse expression of two sets of target genes, immuno-modulators and exotoxins, expressed during the defensive and offensive modes, respectively. Our computational and experimental analyses show that the DSS guarantees fine-tuned coordination of the inverse expression of its two gene sets, tight regulation, and filtering of noisy signals. We also identified variants of this circuit in other bacterial systems, suggesting it is used as a molecular switch in various cellular contexts and offering its use as a template for an effective switching device in synthetic biology studies.

Impact of individual extracellular proteases on Staphylococcus aureus biofilm formation in diverse clinical isolates and their isogenic sarA mutants

We demonstrate that the purified Staphylococcus aureus extracellular proteases aureolysin, ScpA, SspA, and SspB limit biofilm formation, with aureolysin having the greatest impact. Using protease-deficient derivatives of LAC, we confirmed that this is due to the individual proteases themselves. Purified aureolysin, and to a lesser extent ScpA and SspB, also promoted dispersal of an established biofilm. Mutation of the genes encoding these proteases also only partially restored biofilm formation in an FPR3757 sarA mutant and had little impact on restoring virulence in a murine bacteremia model. In contrast, eliminating the production of all of these proteases fully restored both biofilm formation and virulence in a sarA mutant generated in the closely related USA300 strain LAC. These results confirm an important role for multiple extracellular proteases in S. aureus pathogenesis and the importance of sarA in repressing their production. Moreover, purified aureolysin limited biofilm formation in 14 of 15 methicillin-resistant isolates and 11 of 15 methicillin-susceptible isolates, while dispersin B had little impact in UAMS-1, LAC, or 29 of 30 contemporary isolates of S. aureus. This suggests that the role of sarA and its impact on protease production is important in diverse strains of S. aureus irrespective of their methicillin resistance status.

A semi-quantitative model of Quorum-Sensing in Staphylococcus aureus, approved by microarray meta-analyses and tested by mutation studies

Staphylococcus aureus (SA) causes infections including severe sepsis by antibiotic-resistant strains. It forms biofilms to protect itself from the host and antibiotics. Biofilm and planktonic lifestyle are regulated by a complex quorum sensing system (QS) with the central regulator agr. To study biofilm formation and QS we set up a Boolean node interaction network (94 nodes, 184 edges) that included different two component systems such as agr, sae and arl. Proteins such as sar, rot and sigB were included. Each gene node represents the resulting activity of its gene products (mRNA and protein). Network consistency was tested according to previous knowledge and the literature. Regulator mutation combinations (agr-, sae-, sae-/agr-, sigB+, sigB+/sae-) were tested in silico in the model and compared regarding system changes and responses to experimental gene expression data. High connectivity served as a guide to identify master regulators, and their detailed behaviour was studied both in vitro and in the model. System analysis showed two stable states, biofilm forming versus planktonic, with clearly different sub-networks turned on. Predicted node activity changes from the in silico model were in line with microarray gene expression data of different knockout strains. Additional in silico predictions about node activity and biofilm formation were compared to new in vitro experiments (northern blots and biofilm adherence assays) which confirmed these. Further experiments in silico as well as in vitro showed the sae locus as the central modulator of biofilm production. Sae knockout strains showed stronger biofilms. Wild type phenotype was rescued by sae complementation. The in silico network provides a theoretical model that agrees well with the presented experimental data on how integration of different inputs is achieved in the QS of SA. It faithfully reproduces the behaviour of QS mutants and their biofilm forming ability and allows predictions about mutations and mutation combinations for any node in the network. The model and simulations allow us to study QS and biofilm formation in SA including behaviour of MRSA strains and mutants. The in vitro and in silico evidence stresses the role of sae and agr in fine-tuning biofilm repression and/or SA dissemination.

Agr function is upregulated by photodynamic therapy for Staphylococcus aureus and is related to resistance to photodynamic therapy

Photodynamic therapy (PDT) has been considered a feasible alternative for antimicrobial therapy of multidrug-resistant pathogens. However, bacterial response mechanisms against PDT-generated photo-oxidative stress remain largely unknown. Herein, it is shown that the accessory gene regulator Agr is involved in Staphylococcus aureus response to photo-oxidative stress generated by laser-induced PDT with the photosensitizer chlorin e6 . Transcriptional profiling revealed that sublethal PDT induces a general stress response and also activates Agr-dependent gene regulation. Moreover, mutant S. aureus lacking Agr function showed hypersusceptibility to two independent PDT conditions with higher energy densities, demonstrating Agr-dependent S. aureus resistance against PDT.

Phosphatidylinositol-specific phospholipase C contributes to survival of Staphylococcus aureus USA300 in human blood and neutrophils

Staphylococcus aureus is an important human pathogen that employs a large repertoire of secreted virulence factors to promote disease pathogenesis. Many strains of S. aureus possess a plc gene that encodes a phosphatidylinositol (PI)-specific phospholipase C (PI-PLC) capable of hydrolyzing PI and cleaving glycosyl-PI (GPI)-linked proteins from cell surfaces. Despite being secreted by virulent staphylococci, the contribution of PI-PLC to the capacity of S. aureus to cause disease remains undefined. Our goal in these studies was to understand PI-PLC in the context of S. aureus biology. Among a collection of genetically diverse clinical isolates of S. aureus, community-associated methicillin-resistant S. aureus (CA-MRSA) USA300 secreted the most PI-PLC. Screening a collection of two-component system (TCS) mutants of S. aureus, we identified both the agr quorum-sensing system and the SrrAB TCS to be positive regulators of plc gene expression. Real-time PCR and PI-PLC enzyme assays of the TCS mutants, coupled with SrrA promoter binding studies, demonstrated that SrrAB was the predominant transcriptional activator of plc. Furthermore, plc regulation was linked to oxidative stress both in vitro and in vivo in a SrrAB-dependent manner. A Δplc mutant in a CA-MRSA USA300 background exhibited a survival defect in human whole blood and in isolated neutrophils. However, the same mutant strain displayed no survival defect in murine models of infection or murine whole blood. Overall, these data identify potential links between bacterial responses to the host innate immune system and to oxidative stress and suggest how PI-PLC could contribute to the pathogenesis of S. aureus infections.

Model organisms retain an "ecological memory" of complex ecologically relevant environmental variation

Although tractable model organisms are essential to characterize the molecular mechanisms of evolution and adaptation, the ecological relevance of their behavior is not always clear because certain traits are easily lost during long-term laboratory culturing. Here, we demonstrate that despite their long tenure in the laboratory, model organisms retain "ecological memory" of complex environmental changes. We have discovered that Halobacterium salinarum NRC-1, a halophilic archaeon that dominates microbial communities in a dynamically changing hypersaline environment, simultaneously optimizes fitness to total salinity, NaCl concentration, and the [K]/[Mg] ratio. Despite being maintained under controlled conditions over the last 50 years, peaks in the three-dimensional fitness landscape occur in salinity and ionic compositions that are not replicated in laboratory culturing but are routinely observed in the natural hypersaline environment of this organism. Intriguingly, adaptation to variations in ion composition was associated with differential regulation of anaerobic metabolism genes, suggesting an intertwined relationship between responses to oxygen and salinity. Our results suggest that the ecological memory of complex environmental variations is imprinted in the networks for coordinating multiple cellular processes. These coordination networks are also essential for dealing with changes in other physicochemically linked factors present during routine laboratory culturing and, hence, retained in model organisms.

Impact of vancomycin on sarA-mediated biofilm formation: role in persistent endovascular infections due to methicillin-resistant Staphylococcus aureus

BACKGROUND

Staphylococcus aureus is the most common cause of endovascular infections. The staphylococcal accessory regulator A locus (sarA) is a major virulence determinant that may potentially impact methicillin-resistant S. aureus (MRSA) persistence in such infections via its influence on biofilm formation.

METHODS

Two healthcare-associated MRSA isolates from patients with persistent bacteremia and 2 prototypical community-acquired MRSA strains, as well as their respective isogenic sarA mutants, were studied for in vitro biofilm formation, fibronectin-binding capacity, autolysis, and protease and nuclease activities. These assays were done in the presence or absence of sub-minimum inhibitory concentrations (MICs) of vancomycin. In addition, these strain pairs were compared for intrinsic virulence and responses to vancomycin therapy in experimental infective endocarditis, a prototypical biofilm model.

RESULTS

All sarA mutants displayed significantly reduced biofilm formation and binding to fibronectin but increased protease production in vitro, compared with their respective parental strains. Interestingly, exposure to sub-MICs of vancomycin significantly promoted biofilm formation and fibronectin-binding in parental strains but not in sarA mutants. In addition, all sarA mutants became exquisitely susceptible to vancomycin therapy, compared with their respective parental strains, in the infective endocarditis model.

CONCLUSIONS

These observations suggest that sarA activation is important in persistent MRSA endovascular infection, potentially in the setting of biofilm formation.

Staphylococcus aureus sarA regulates inflammation and colonization during central nervous system biofilm formation

Infection is a frequent and serious complication following the treatment of hydrocephalus with CSF shunts, with limited therapeutic options because of biofilm formation along the catheter surface. Here we evaluated the possibility that the sarA regulatory locus engenders S. aureus more resistant to immune recognition in the central nervous system (CNS) based on its reported ability to regulate biofilm formation. We utilized our established model of CNS catheter-associated infection, similar to CSF shunt infections seen in humans, to compare the kinetics of bacterial titers, cytokine production and inflammatory cell influx elicited by wild type S. aureus versus an isogenic sarA mutant. The sarA mutant was more rapidly cleared from infected catheters compared to its isogenic wild type strain. Consistent with this finding, several pro-inflammatory cytokines and chemokines, including IL-17, CXCL1, and IL-1β were significantly increased in the brain following infection with the sarA mutant versus wild type S. aureus, in agreement with the fact that the sarA mutant displayed impaired biofilm growth and favored a planktonic state. Neutrophil influx into the infected hemisphere was also increased in the animals infected with the sarA mutant compared to wild type bacteria. These changes were not attributable to extracellular protease activity, which is increased in the context of SarA mutation, since similar responses were observed between sarA and a sarA/protease mutant. Overall, these results demonstrate that sarA plays an important role in attenuating the inflammatory response during staphylococcal biofilm infection in the CNS via a mechanism that remains to be determined.