Role of ArlRS in autolysis in methicillin-sensitive and methicillin-resistant Staphylococcus aureus strains

'Computational studies on coumestrol-ArlR interaction to target ArlRS signaling cascade involved in MRSA virulence'

Two component signaling system ArlRS (Autolysis-related locus) regulates adhesion, biofilm formation and virulence in methicillin resistant Staphylococcus aureus. It consists of a histidine kinase ArlS and response regulator ArlR. ArlR is composed of a N-terminal receiver domain and DNA-binding effector domain at C-terminal. ArlR receiver domain dimerizes upon signal recognition and activates DNA binding by effector domain and subsequent virulence expression. In silico simulation and structural data suggest that coumestrol, a phytochemical found in Pueraria montana, forges a strong intermolecular interaction with residues involved in dimer formation and destabilizes ArlR dimerization, an essential conformational switch required for downstream effector domain to bind to virulent loci. Structural and energy profiles of simulated ArlR-coumestrol complexes suggest lower affinity between ArlR monomers due to structural rigidity at the dimer interface hindering the conformational rearrangements relevant for dimer formation. These analyses could be an attractive strategy to develop therapeutics and potent leads molecules response regulators of two component systems in which are involved in MRSA virulence as well as other drug-resistant pathogens.Communicated by Ramaswamy H. Sarma.

Staphylococcus aureus senses human neutrophils via PerR to coordinate the expression of the toxin LukAB

Staphylococcus aureus is a gram-positive pathogen that poses a major health concern, in part due to its large array of virulence factors that allow infection and evasion of the immune system. One of these virulence factors is the bicomponent pore-forming leukocidin LukAB. The regulation of lukAB expression is not completely understood, especially in the presence of immune cells such as human polymorphonuclear neutrophils (hPMNs). Here, we screened for transcriptional regulators of lukAB during the infection of primary hPMNs. We uncovered that PerR, a peroxide sensor, is vital for hPMN-mediated induction of lukAB and that PerR upregulates cytotoxicity during the infection of hPMNs. Exposure of S. aureus to hydrogen peroxide (H2O2) alone also results in increased lukAB promoter activity, a phenotype dependent on PerR. Collectively, our data suggest that S. aureus uses PerR to sense the H2O2 produced by hPMNs to stimulate the expression of lukAB, allowing the bacteria to withstand these critical innate immune cells.IMPORTANCEStaphylococcus aureus utilizes a diverse set of virulence factors, such as leukocidins, to subvert human neutrophils, but how these toxins are regulated is incompletely defined. Here, we identified the peroxide-sensitive repressor, PerR, as a required protein involved in the induction of lukAB in the presence of primary human neutrophils, a phenotype directly linked to the ability of PerR to sense H2O2. Thus, we show that S. aureus coordinates sensing and resistance to oxidative stress with toxin production to promote pathogen survival.

The Staphylococcus aureus ArlS Kinase Inhibitor Tilmicosin Has Potent Anti-Biofilm Activity in Both Static and Flow Conditions

Staphylococcus aureus can form biofilms on biotic surfaces or implanted materials, leading to biofilm-associated diseases in humans and animals that are refractory to conventional antibiotic treatment. Recent studies indicate that the unique ArlRS regulatory system in S. aureus is a promising target for screening inhibitors that may eradicate formed biofilms, retard virulence and break antimicrobial resistance. In this study, by screening in the library of FDA-approved drugs, tilmicosin was found to inhibit ArlS histidine kinase activity (IC50 = 1.09 μM). By constructing a promoter-fluorescence reporter system, we found that tilmicosin at a concentration of 0.75 μM or 1.5 μM displayed strong inhibition on the expression of the ArlRS regulon genes spx and mgrA in the S. aureus USA300 strain. Microplate assay and confocal laser scanning microscopy showed that tilmicosin at a sub-minimal inhibitory concentration (MIC) had a potent inhibitory effect on biofilms formed by multiple S. aureus strains and a strong biofilm-forming strain of S. epidermidis. In addition, tilmicosin at three-fold of MIC disrupted USA300 mature biofilms and had a strong bactericidal effect on embedded bacteria. Furthermore, in a BioFlux flow biofilm assay, tilmicosin showed potent anti-biofilm activity and synergized with oxacillin against USA300.

Staphylococcus aureus susceptibility to complestatin and corbomycin depends on the VraSR two-component system

The overuse of antibiotics in humans and livestock has driven the emergence and spread of antimicrobial resistance and has therefore prompted research on the discovery of novel antibiotics. Complestatin (Cm) and corbomycin (Cb) are glycopeptide antibiotics with an unprecedented mechanism of action that is active even against methicillin-resistant and daptomycin-resistant Staphylococcus aureus. They bind to peptidoglycan and block the activity of peptidoglycan hydrolases required for remodeling the cell wall during growth. Bacterial signaling through two-component transduction systems (TCSs) has been associated with the development of S. aureus antimicrobial resistance. However, the role of TCSs in S. aureus susceptibility to Cm and Cb has not been previously addressed. In this study, we determined that, among all 16 S. aureus TCSs, VraSR is the only one controlling the susceptibility to Cm and Cb. Deletion of vraSR increased bacterial susceptibility to both antibiotics. Epistasis analysis with members of the vraSR regulon revealed that deletion of spdC, which encodes a membrane protein that scaffolds SagB for cleavage of peptidoglycan strands to achieve physiological length, in the vraSR mutant restored Cm and Cb susceptibility to wild-type levels. Moreover, deletion of either spdC or sagB in the wild-type strain increased resistance to both antibiotics. Further analyses revealed a significant rise in the relative amount of peptidoglycan and its total degree of cross-linkage in ΔspdC and ΔsagB mutants compared to the wild-type strain, suggesting that these changes in the cell wall provide resistance to the damaging effect of Cm and Cb. IMPORTANCE Although Staphylococcus aureus is a common colonizer of the skin and digestive tract of humans and many animals, it is also a versatile pathogen responsible for causing a wide variety and number of infections. Treatment of these infections requires the bacteria to be constantly exposed to antibiotic treatment, which facilitates the selection of antibiotic-resistant strains. The development of new antibiotics is, therefore, urgently needed. In this paper, we investigated the role of the sensory system of S. aureus in susceptibility to two new antibiotics: corbomycin and complestatin. The results shed light on the cell-wall synthesis processes that are affected by the presence of the antibiotic and the sensory system responsible for coordinating their activity.

Staphylococcus aureus cell wall maintenance - the multifaceted roles of peptidoglycan hydrolases in bacterial growth, fitness, and virulence

Staphylococcus aureus is an important human and livestock pathogen that is well-protected against environmental insults by a thick cell wall. Accordingly, the wall is a major target of present-day antimicrobial therapy. Unfortunately, S. aureus has mastered the art of antimicrobial resistance, as underscored by the global spread of methicillin-resistant S. aureus (MRSA). The major cell wall component is peptidoglycan. Importantly, the peptidoglycan network is not only vital for cell wall function, but it also represents a bacterial Achilles' heel. In particular, this network is continuously opened by no less than 18 different peptidoglycan hydrolases (PGHs) encoded by the S. aureus core genome, which facilitate bacterial growth and division. This focuses attention on the specific functions executed by these enzymes, their subcellular localization, their control at the transcriptional and post-transcriptional levels, their contributions to staphylococcal virulence and their overall importance in bacterial homeostasis. As highlighted in the present review, our understanding of the different aspects of PGH function in S. aureus has been substantially increased over recent years. This is important because it opens up new possibilities to exploit PGHs as innovative targets for next-generation antimicrobials, passive or active immunization strategies, or even to engineer them into effective antimicrobial agents.

Exploiting transposons in the study of Staphylococcus aureus pathogenesis and virulence

The opportunistic pathogen Staphylococcus aureus has an extremely complex relationship with humans. While the bacteria can exist as a commensal in many, it can cause a wide range of diseases and infections when turned pathogenic. Its presence is a determinant of chronicity and poor prognosis in numerous diseases, and its genomic plasticity causes S. aureus antimicrobial resistance to be one of the most dire contemporary medical problems to solve. Genetic manipulation of S. aureus has led to numerous findings that are vital in the fight against its pathogenesis. The utilisation of transposon mutant libraries for the systematic inspection of the S. aureus genome led to many landmark discoveries pertaining to the bacteria's pathogenicity, antimicrobial resistance acquisition, and virulence regulation. In this review, we describe mutant libraries, and their significant contributions, from various S. aureus strains created with commonly used transposons. The general workflow for the construction of libraries will be presented, along with a discussion of the challenges of undertaking the task of large-scale library construction. As the accessibility of transposon mutant library construction, screening, and analysis increases, this genetic tool could be further exploited in the study of the S. aureus genome.

The Major Autolysin Atl Regulates the Virulence of Staphylococcus aureus by Controlling the Sorting of LukAB

Infections caused by the Gram-positive bacterium Staphylococcus aureus remain a significant health threat globally. The production of bicomponent pore-forming leukocidins plays an important role in S. aureus pathogenesis. Transcriptionally, these toxins are primarily regulated by the Sae and Agr regulatory systems. However, the posttranslational regulation of these toxins is largely unexplored. In particular, one of the leukocidins, LukAB, has been shown to be both secreted into the extracellular milieu and associated with the bacterial cell envelope. Here, we report that a major cell wall hydrolase, autolysin (Atl), controls the sorting of LukAB from the cell envelope to the extracellular milieu, an effect independent of transcriptional regulation. By influencing the sorting of LukAB, Atl modulates S. aureus cytotoxicity toward primary human neutrophils. Mechanistically, we found that the reduction in peptidoglycan cleavage and increased LukAB secretion in the atl mutant can be reversed through the supplementation of exogenous mutanolysin. Altogether, our study revealed that the cell wall hydrolase activity of Atl and the cleavage of peptidoglycan play an important role in controlling the sorting of S. aureus toxins during secretion.

Baicalin inhibits biofilm formation by influencing primary adhesion and aggregation phases in Staphylococcus saprophyticus

The ability to form biofilms on surfaces makes Staphylococcus saprophyticus (S. saprophyticus) becomes the main pathogenic factor in nosocomial infections. Previously, we demonstrated that baicalin (Bac) inhibited azithromycin-resistant S. saprophyticus (ARSS) biofilm formation. This investigation aims to explore the influence of baicalin on primary adhesion and aggregation phases of biofilm formation, and the treatment effect of baicalin and azithromycin on ARSS biofilm-associated infection. Crystal violet (CV) staining and scanning electron microscope (SEM) observations clearly showed that sub-inhibitory concentration baicalin inhibited ARSS biofilm formation when baicalin was added before the adhesion and aggregation phases. Baicalin significantly increased the relative adhesion inhibition rate and decreased the rate of bacteria aggregation in a dose-dependent manner. Moreover, CLSM and cell lysis assays revealed that baicalin inhibited the production of surface proteins and cell autolysis in bacteria adhesion and aggregation phases of biofilm formation. Meanwhile, the relative expressions of adhesion-related and autolysis-related genes were down-regulated by baicalin. In vivo, the combination of baicalin and azithromycin succeeded in eradicating ARSS from the mouse cutaneous infection model and decreasing the pathological injuries, the expressions of cytokines in infected tissue, and the number of inflammatory cells in the blood. Simultaneously, baicalin decreased the bacterial burdens in tubes, the level of TNF-α, and the number of monocytes and neutrophils compared with that of the SS and azithromycin groups. Based on these results, baicalin inhibited the adhesion and aggregation phases of biofilm formation by influenced the production of surface proteins and cell autolysis. Baicalin and azithromycin synergetically treated ARSS biofilm-associated infection.

Staphylococcus aureus Synergized with Candida albicans to Increase the Pathogenesis and Drug Resistance in Cutaneous Abscess and Peritonitis Murine Models

The mixed species of Staphylococcus aureus and Candida albicans can cause infections on skin, mucosa or bloodstream; however, mechanisms of their cross-kingdom interactions related to pathogenesis and drug resistance are still not clear. Here an increase of S. aureus proliferation and biofilm formation was observed in S. aureus and C. albicans dual-species culture, and the synergistic pathogenic effect was then confirmed in both local (cutaneous abscess) and systemic infection (peritonitis) murine models. According to the transcriptome analysis of the dual-species culture, virulence factors of S. aureus were significantly upregulated. Surprisingly, the beta-lactams and vancomycin-resistant genes in S. aureus as well as azole-resistant genes in C. albicans were also significantly increased. The synergistic effects on drug resistance to both antibacterial and antifungal agents were further proved both in vitro and in cutaneous abscess and peritonitis murine models treated by methicillin, vancomycin and fluconazole. The synergistic interactions between S. aureus and C. albicans on pathogenesis and drug resistance highlight the importance of targeting the microbial interactions in polyspecies-associated infections.

Apt (Adenine Phosphoribosyltransferase) Mutation in Laboratory-Selected Vancomycin-Intermediate Staphylococcus aureus

Comparative genomic sequencing of laboratory-derived vancomycin-intermediate Staphylococcusaureus (VISA) (MM66-3 and MM66-4) revealed unique mutations in both MM66-3 (in apt and ssaA6), and MM66-4 (in apt and walK), compared to hetero-VISA parent strain MM66. Transcriptional profiling revealed that both MM66 VISA shared 79 upregulated genes and eight downregulated genes. Of these, 30.4% of the upregulated genes were associated with the cell envelope, whereas 75% of the downregulated genes were associated with virulence. In concordance with mutations and transcriptome alterations, both VISA strains demonstrated reduced autolysis, reduced growth in the presence of salt and reduced virulence factor activity. In addition to mutations in genes linked to cell wall metabolism (ssaA6 and walK), the same mutation in apt which encodes adenine phosphoribosyltransferase, was confirmed in both MM66 VISA. Apt plays a role in both adenine metabolism and accumulation and both MM66 VISA grew better than MM66 in the presence of adenine or 2-fluoroadenine indicating a reduction in the accumulation of these growth inhibiting compounds in the VISA strains. MM66 apt mutants isolated via 2-fluoroadenine selection also demonstrated reduced susceptibility to the cell wall lytic dye Congo red and vancomycin. Finding that apt mutations contribute to reduced vancomycin susceptibility once again suggests a role for altered purine metabolism in a VISA mechanism.

Complete genome analysis of a virulent Vibrio scophthalmi strain VSc190401 isolated from diseased marine fish half-smooth tongue sole, Cynoglossus semilaevis

Background

Vibrio scophthalmi is an opportunistic bacterial pathogen, which is widely distributed in the marine environment. Earlier studies have suggested that it is a normal microorganism in the turbot gut. However, recent studies have confirmed that this bacterial strain can cause diseases in many different marine animals. Therefore, it is necessary to investigate its whole genome for better understanding its physiological and pathogenic mechanisms.

Results

In the present study, we obtained a pathogenic strain of V. scophthalmi from diseased half-smooth tongue sole (Cynoglossus semilaevis) and sequenced its whole genome. Its genome contained two circular chromosomes and two plasmids with a total size of 3,541,838 bp, which harbored 3185 coding genes. Among these genes, 2648, 2298, and 1915 genes could be found through annotation information in COG, Blast2GO, and KEGG databases, respectively. Moreover, 10 genomic islands were predicted to exist in the chromosome I through IslandViewer online system. Comparison analysis in VFDB and PHI databases showed that this strain had 334 potential virulence-related genes and 518 pathogen-host interaction-related genes. Although it contained genes related to four secretion systems of T1SS, T2SS, T4SS, and T6SS, there was only one complete T2SS secretion system. Based on CARD database blast results, 180 drug resistance genes belonging to 27 antibiotic resistance categories were found in the whole genome of such strain. However, there were many differences between the phenotype and genotype of drug resistance.

Conclusions

Based on the whole genome analysis, the pathogenic V. scophthalmi strain contained many types of genes related to pathogenicity and drug resistance. Moreover, it showed inconsistency between phenotype and genotype on drug resistance. These results suggested that the physiological mechanism seemed to be complex.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-020-02028-7.

Deciphering the activation and recognition mechanisms of Staphylococcus aureus response regulator ArlR

Staphylococcus aureus ArlRS is a key two-component regulatory system necessary for adhesion, biofilm formation, and virulence. The response regulator ArlR consists of a C-terminal DNA-binding effector domain and an N-terminal receiver domain that is phosphorylated by ArlS, the cognate transmembrane sensor histidine kinase. We demonstrate that the receiver domain of ArlR adopts the canonical α5β5 response regulator assembly, which dimerizes upon activation, using beryllium trifluoride as an aspartate phosphorylation mimic. Activated ArlR recognizes a 20-bp imperfect inverted repeat sequence in the ica operon, which is involved in intercellular adhesion polysaccharide production. Crystal structures of the inactive and activated forms reveal that activation induces a significant conformational change in the β4-α4 and β5-α5-connecting loops, in which the α4 and α5 helices constitute the homodimerization interface. Crystal structures of the DNA-binding ArlR effector domain indicate that it is able to dimerize via a non-canonical β1–β2 hairpin domain swapping, raising the possibility of a new mechanism for signal transduction from the receiver domain to effector domain. Taken together, the current study provides structural insights into the activation of ArlR and its recognition, adding to the diversity of response regulation mechanisms that may inspire novel antimicrobial strategies specifically targeting Staphylococcus.

The Staphylococcus aureus ArlRS two-component system regulates virulence factor expression through MgrA

The Gram-positive bacterium, Staphylococcus aureus, is a versatile pathogen that can sense and adapt to a wide variety of environments within the human host, in part through its 16 two-component regulatory systems. The ArlRS two-component system has been shown to affect many cellular processes in S. aureus, including autolysis, biofilm formation, capsule synthesis and virulence. Yet the molecular details of this regulation remained largely unknown. We used RNA sequencing to identify the ArlRS regulon, and found 70% overlap with that of the global regulator MgrA. These genes included cell wall-anchored adhesins (ebh, sdrD), polysaccharide and capsule synthesis genes, cell wall remodeling genes (lytN, ddh), the urease operon, genes involved in metal transport (feoA, mntH, sirA), anaerobic metabolism genes (adhE, pflA, nrdDG) and a large number of virulence factors (lukSF, lukAB, nuc, gehB, norB, chs, scn and esxA). We show that ArlR directly activates expression of mgrA and identify a probable ArlR-binding site (TTTTCTCAT-N4 -TTTTAATAA). A highly similar sequence is also found in the spx P2 promoter, which was recently shown to be regulated by ArlRS. We also demonstrate that ArlS has kinase activity toward ArlR in vitro, although it has slower kinetics than other similar histidine kinases.

The DNA-binding mechanism of the TCS response regulator ArlR from Staphylococcus aureus

ArlRS is an essential two-component system in Staphylococcus aureus that regulates the transcription of virulence factors and participate in numerous pathogenic and symbiotic processes. In this work, we identified different DNA binding properties and oligomerization states among the DNA-binding domain of ArlR (ArlR^DBD) and the phosphorylated and unphosphorylated full-length ArlR. Based on a 2.5-Å resolution crystal structure of ArlR^DBD and subsequent mutagenesis experiments, we confirmed the DNA-binding site of ArlR and the preferred binding sequences in the agr promoter that enables the DNA recognition process. Finally, we propose a putative transcription regulation mechanism for ArlR. This work will facilitate our understanding of the DNA binding affinity regulatory mechanism between the phosphorylated and unphosphorylated response regulator in the two-component system.

The role of ArlRS in regulating oxacillin susceptibility in methicillin-resistant Staphylococcus aureus indicates it is a potential target for antimicrobial resistance breakers

Methicillin-resistant Staphylococcus aureus (MRSA), also known as oxacillin-resistant S. aureus, is a leading cause of community and hospital associated infections globally. In this work, we found that deletion of the arlRS two-component system genes in the USA300 and USA500 strains resulted in increased susceptibilities to oxacillin (8–16-fold decrease in minimal inhibitory concentrations). In USA300ΔarlRS, transcriptional levels of mecA or blaZ showed no obvious change, while mRNA levels of spx showed a 4-fold decrease at 4 h and a 6.3-fold decrease at 10 h. Overexpression of spx in ΔarlRS restored oxacillin resistance to a similar level in USA300. In addition, gel shift assay showed that the recombinant ArlR bound to spx promoter region. Furthermore, silencing of spx led to a significant increase of oxacillin susceptibility in multiple MRSA isolates. Our results indicate that ArlRS plays a strong role in regulating oxacillin resistance in MRSA strains, which involves direct modulation of spx expression. Moreover, oritavancin showed inhibition to ATPase activity of the recombinant histidine kinase ArlS (IC₅₀ = 5.47 μM). Oritavancin had synergy effect on oxacillin activity against the MRSA strains in both planktonic and biofilm state. Our data suggest that ArlRS is an attractive target for breaking antimicrobial resistance of MRSA.

The ClpCP Complex Modulates Respiratory Metabolism in Staphylococcus aureus and Is Regulated in a SrrAB-Dependent Manner

The staphylococcal respiratory regulator (SrrAB) modulates energy metabolism in Staphylococcus aureus Studies have suggested that regulated protein catabolism facilitates energy homeostasis. Regulated proteolysis in S. aureus is achieved through protein complexes composed of a peptidase (ClpQ or ClpP) in association with an AAA⁺ family ATPase (typically, ClpC or ClpX). In the present report, we tested the hypothesis that SrrAB regulates a Clp complex to facilitate energy homeostasis in S. aureus Strains deficient in one or more Clp complexes were attenuated for growth in the presence of puromycin, which causes enrichment of misfolded proteins. A ΔsrrAB strain had increased sensitivity to puromycin. Epistasis experiments suggested that the puromycin sensitivity phenotype of the ΔsrrAB strain was a result of decreased ClpC activity. Consistent with this, transcriptional activity of clpC was decreased in the ΔsrrAB mutant, and overexpression of clpC suppressed the puromycin sensitivity of the ΔsrrAB strain. We also found that ClpC positively influenced respiration and that it did so upon association with ClpP. In contrast, ClpC limited fermentative growth, while ClpP was required for optimal fermentative growth. Metabolomics studies demonstrated that intracellular metabolic profiles of the ΔclpC and ΔsrrAB mutants were distinct from those of the wild-type strain, supporting the notion that both ClpC and SrrAB affect central metabolism. We propose a model wherein SrrAB regulates energy homeostasis, in part, via modulation of regulated proteolysis.IMPORTANCE Oxygen is used as a substrate to derive energy by the bacterial pathogen Staphylococcus aureus during infection; however, S. aureus can also grow fermentatively in the absence of oxygen. To successfully cause infection, S. aureus must tailor its metabolism to take advantage of respiratory activity. Different proteins are required for growth in the presence or absence of oxygen; therefore, when cells transition between these conditions, several proteins would be expected to become unnecessary. In this report, we show that regulated proteolysis is used to modulate energy metabolism in S. aureus We report that the ClpCP protein complex is involved in specifically modulating aerobic respiratory growth but is dispensable for fermentative growth.

Regulation of Staphylococcus aureus Virulence

Staphylococcus aureus is a Gram-positive opportunistic pathogen that has evolved a complex regulatory network to control virulence. One of the main functions of this interconnected network is to sense various environmental cues and respond by altering the production of virulence factors necessary for survival in the host, including cell surface adhesins and extracellular enzymes and toxins. Of these S. aureus regulatory systems, one of the best studied is the accessory gene regulator (agr), which is a quorum-sensing system that senses the local concentration of a cyclic peptide signaling molecule. This system allows S. aureus to sense its own population density and translate this information into a specific gene expression pattern. Besides agr, this pathogen uses other two-component systems to sense specific cues and coordinates responses with cytoplasmic regulators of the SarA protein family and alternative sigma factors. These divergent regulatory systems integrate the various environmental and host-derived signals into a network that ensures optimal pathogen response to the changing conditions. This article gives an overview of the most important and best-studied S. aureus regulatory systems and summarizes the functions of these regulators during host interactions. The regulatory systems discussed include the agr quorum-sensing system; the SaeRS, SrrAB, and ArlRS two-component systems, the cytoplasmic SarA-family regulators (SarA, Rot, and MgrA); and the alternative sigma factors (SigB and SigH).

The ArlRS two-component system is a regulator of Staphylococcus aureus-induced endothelial cell damage

Staphylococcus aureus endovascular infections retain a high morbidity and mortality despite antibiotics and supportive care. The destruction of endothelial cells (ECs) is a critical step in the pathogenesis of S. aureus endovascular infections. In order to better understand S. aureus-induced EC damage, we systematically screened a collection of two-component regulatory system mutants of methicillin-resistant S. aureus (MRSA) USA300 strain JE2 for damage induction in human umbilical vein ECs (HUVECs). This screen revealed that the two-component regulatory system ArlRS is required for maximum damage: arlRS inactivation leads to a > 70% reduction in damage. In a different genetic S. aureus background (RN6390, MSSA strain) arlRS inactivation had a smaller but also significant effect on EC damage. In both strains, the reduction in EC damage was accompanied by a significant reduction in internalization. In conclusion, we determined a novel role of ArlRS in S. aureus-induced EC damage, which will help to better understand the pathogenesis of S. aureus endovascular infection.

Nucleotide Sequence Variations in Autolysis Genes of ST59 Methicillin-Resistant Staphylococcus aureus Isolates

Biofilm formation is a virulence factor of bacteria. The goal of this study was to understand the mechanisms of biofilm formation by methicillin-resistant Staphylococcus aureus (MRSA). Whole-genome sequencing of eight MRSA strains was performed to identify sequence variations in genes related to biofilm formation. Thirty-one genes involved in MRSA biofilm formation were analyzed and 11 amino acid sequence variations in four genes related to autolysis were found. These variations include E121D and H387 N in ArlS; Q117K, T424S, K428T, A509S, V752E, A754V, and T771A in Atl; T184K in CidC; and D251N in CidR. Among the 26 clinical MRSA isolates studied, 13 isolates were nonbiofilm producers and were found to harbor these mutations. Furthermore, all of these 13 isolates belonged to ST59. Ten of these 13 ST59 isolates became able to produce biofilms when they were incubated with extracellular DNA from MRSA N315. Results of this study suggest that sequence variations in arlS, atl, cidC, and cidR genes may render MRSA unable to produce biofilms. Further investigations are needed to correlate these sequence variations with the biofilm-forming ability of MRSA isolates.

Candida albicans Mycofilms Support Staphylococcus aureus Colonization and Enhances Miconazole Resistance in Dual-Species Interactions

Polymicrobial inter-kingdom biofilm infections represent a clinical management conundrum. The presence of co-isolation of bacteria and fungi complicates the ability to routinely administer single antimicrobial regimens, and synergy between the microorganisms influences infection severity. We therefore investigated the nosocomial pathogens Staphylococcus aureus and Candida albicans with respect to antimicrobial intervention. We characterized the interaction using biofilm assays and evaluated the effect of miconazole treatment using in vitro and in vivo assays. Finally, we assessed the impact of biofilm extracellular matrix (ECM) on these interactions. Data indicated that the C. albicans mycofilms supported adhesion and colonization by S. aureus through close interactions with hyphal elements, significantly increasing S. aureus biofilm formation throughout biofilm maturation. Miconazole sensitivity was shown to be reduced in both mono- and dual-species biofilms compared to planktonic cells. Within a three-dimensional biofilm model sensitivity was also hindered. Galleria mellonella survival analysis showed both enhanced pathogenicity of the dual-species infection, which was concomitantly desensitized to miconazole treatment. Analysis of the ECM revealed the importance of extracellular DNA, which supported the adhesion of S. aureus and the development of the dual-species biofilm structures. Collectively, these data highlight the clinical importance of dual-species inter-kingdom biofilm infections, though also provides translational opportunities to manage them more effectively.

Impaired respiration elicits SrrAB-dependent programmed cell lysis and biofilm formation in Staphylococcus aureus

Biofilms are communities of microorganisms attached to a surface or each other. Biofilm-associated cells are the etiologic agents of recurrent Staphylococcus aureus infections. Infected human tissues are hypoxic or anoxic. S. aureus increases biofilm formation in response to hypoxia, but how this occurs is unknown. In the current study we report that oxygen influences biofilm formation in its capacity as a terminal electron acceptor for cellular respiration. Genetic, physiological, or chemical inhibition of respiratory processes elicited increased biofilm formation. Impaired respiration led to increased cell lysis via divergent regulation of two processes: increased expression of the AtlA murein hydrolase and decreased expression of wall-teichoic acids. The AltA-dependent release of cytosolic DNA contributed to increased biofilm formation. Further, cell lysis and biofilm formation were governed by the SrrAB two-component regulatory system. Data presented support a model wherein SrrAB-dependent biofilm formation occurs in response to the accumulation of reduced menaquinone.

DOI:http://dx.doi.org/10.7554/eLife.23845.001

Millions of bacteria live on the human body. Generally these bacteria co-exist with us peacefully, but sometimes certain bacteria may enter the body and cause infections, such as gum disease or a bone infection called osteomyelitis. Many of these infections are thought to occur when the bacteria become able to form complex communities called biofilms. Bacteria living in a biofilm cooperate and make lifestyle choices as a community, so in this way, they behave like a single organism containing many cells.

A sticky glue-like material called the matrix holds the bacteria in a biofilm together. This matrix protects the bacteria in the biofilm from both the human immune system and antibiotics, allowing infections to develop and making them difficult to treat.

Previous research has shown that the supply and level of oxygen in infected tissues decreases as an infection gets worse. One bacterium that typically lives peacefully on our bodies, called Staphylococcus aureus, can sometimes cause serious biofilm-associated infections. S. aureus forms biofilms more readily when oxygen is in short supply, but it was not known how these biofilms form. Understanding how S. aureus forms biofilms could help scientists develop better treatments for bacterial infections.

Most bacterial cells have a cell wall to provide them with structural support. Mashruwala et al. found that, when oxygen levels are low, S. aureus decreases the production of a type of sugar that makes up the cell wall. At the same time, the bacteria produce more of an enzyme that breaks down cell walls. Together, these processes cause some of the bacteria cells to break open. The contents of these broken cells, including their DNA, help form the matrix that will hold together and protect the other bacterial cells in the biofilm. The experiments also identified a protein called SrrAB that switches on the process that ruptures the cells when oxygen is low.

The findings of Mashruwala et al. show how bacteria grown in the laboratory form biofilms when they are starved of oxygen. The next steps following on from this work are to find out whether the same thing happens when bacteria infect animals and whether drugs that block the rupturing of bacterial cells could be used to treat infections.

DOI:http://dx.doi.org/10.7554/eLife.23845.002

The Staphylococcus aureus SrrAB Regulatory System Modulates Hydrogen Peroxide Resistance Factors, Which Imparts Protection to Aconitase during Aerobic Growth

The SrrAB two-component regulatory system (TCRS) positively influences the transcription of genes involved in aerobic respiration in response to changes in respiratory flux. Hydrogen peroxide (H₂O₂) can arise as a byproduct of spontaneous interactions between dioxygen and components of respiratory pathways. H₂O₂ damages cellular factors including protein associated iron-sulfur cluster prosthetic groups. We found that a Staphylococcus aureus strain lacking the SrrAB two-component regulatory system (TCRS) is sensitive to H₂O₂ intoxication. We tested the hypothesis that SrrAB manages the mutually inclusive expression of genes required for aerobic respiration and H₂O₂ resistance. Consistent with our hypothesis, a ΔsrrAB strain had decreased transcription of genes encoding for H₂O₂ resistance factors (kat, ahpC, dps). SrrAB was not required for the inducing the transcription of these genes in cells challenged with H₂O₂. Purified SrrA bound to the promoter region for dps suggesting that SrrA directly influences dps transcription. The H₂O₂ sensitivity of the ΔsrrAB strain was alleviated by iron chelation or deletion of the gene encoding for the peroxide regulon repressor (PerR). The positive influence of SrrAB upon H₂O₂ metabolism bestowed protection upon the solvent accessible iron-sulfur (FeS) cluster of aconitase from H₂O₂ poisoning. SrrAB also positively influenced transcription of scdA (ytfE), which encodes for a FeS cluster repair protein. Finally, we found that SrrAB positively influences H₂O₂ resistance only during periods of high dioxygen-dependent respiratory activity. SrrAB did not influence H₂O₂ resistance when cellular respiration was diminished as a result of decreased dioxygen availability, and negatively influenced it in the absence of respiration (fermentative growth). We propose a model whereby SrrAB-dependent regulatory patterns facilitate the adaptation of cells to changes in dioxygen concentrations, and thereby aids in the prevention of H₂O₂ intoxication during respiratory growth upon dixoygen.

The Two-Component System ArlRS and Alterations in Metabolism Enable Staphylococcus aureus to Resist Calprotectin-Induced Manganese Starvation

During infection the host imposes manganese and zinc starvation on invading pathogens. Despite this, Staphylococcus aureus and other successful pathogens remain capable of causing devastating disease. However, how these invaders adapt to host-imposed metal starvation and overcome nutritional immunity remains unknown. We report that ArlRS, a global staphylococcal virulence regulator, enhances the ability of S. aureus to grow in the presence of the manganese-and zinc-binding innate immune effector calprotectin. Utilization of calprotectin variants with altered metal binding properties revealed that strains lacking ArlRS are specifically more sensitive to manganese starvation. Loss of ArlRS did not alter the expression of manganese importers or prevent S. aureus from acquiring metals. It did, however, alter staphylococcal metabolism and impair the ability of S. aureus to grow on amino acids. Further studies suggested that relative to consuming glucose, the preferred carbon source of S. aureus, utilizing amino acids reduced the cellular demand for manganese. When forced to use glucose as the sole carbon source S. aureus became more sensitive to calprotectin compared to when amino acids are provided. Infection experiments utilizing wild type and calprotectin-deficient mice, which have defects in manganese sequestration, revealed that ArlRS is important for disease when manganese availability is restricted but not when this essential nutrient is freely available. In total, these results indicate that altering cellular metabolism contributes to the ability of pathogens to resist manganese starvation and that ArlRS enables S. aureus to overcome nutritional immunity by facilitating this adaptation.

The ubiquitous pathogen Staphylococcus aureus is a serious threat to human health due to the continued spread of antibiotic resistance. This spread has made it challenging to treat staphylococcal infections and led to the call for new approaches to treat this devastating pathogen. One approach is to disrupt the ability of S. aureus to adapt to nutrient availability during infection. During infection, the host imposes manganese and zinc starvation on invading pathogens. However, the mechanisms utilized by Staphylococcus aureus to overcome this host defense are unknown. We report that ArlRS, a global staphylococcal virulence regulator, is important for resisting manganese starvation during infection. Loss of ArlRS does not prevent S. aureus from acquiring metals but instead renders the bacterium incapable of adapting to limited manganese availability. ArlRS mutants also have metabolic defects and a reduced ability to grow on amino acids. When using glucose as a carbon source S. aureus is more sensitive to manganese starvation and increases the expression of manganese transporters relative to when amino acids are provided suggesting a higher demand for manganese. These observations indicate that ArlRS contributes to resisting nutritional immunity by altering metabolism to reduce the staphylococcal demand for manganese.

SpoVG Regulates Cell Wall Metabolism and Oxacillin Resistance in Methicillin-Resistant Staphylococcus aureus Strain N315

Increasing cases of infections caused by methicillin-resistant Staphylococcus aureus (MRSA) strains in healthy individuals have raised concerns worldwide. MRSA strains are resistant to almost the entire family of β-lactam antibiotics due to the acquisition of an extra penicillin-binding protein, PBP2a. Studies have shown that spoVG is involved in oxacillin resistance, while the regulatory mechanism remains elusive. In this study, we have found that SpoVG plays a positive role in oxacillin resistance through promoting cell wall synthesis and inhibiting cell wall degradation in MRSA strain N315. Deletion of spoVG in strain N315 led to a significant decrease in oxacillin resistance and a dramatic increase in Triton X-100-induced autolytic activity simultaneously. Real-time quantitative reverse transcription-PCR revealed that the expression of 8 genes related to cell wall metabolism or oxacillin resistance was altered in the spoVG mutant. Electrophoretic mobility shift assay indicated that SpoVG can directly bind to the putative promoter regions of lytN (murein hydrolase), femA, and lytSR (the two-component system). These findings suggest a molecular mechanism in which SpoVG modulates oxacillin resistance by regulating cell wall metabolism in MRSA.

Bacterial Cytological Profiling (BCP) as a Rapid and Accurate Antimicrobial Susceptibility Testing Method for Staphylococcus aureus

Successful treatment of bacterial infections requires the timely administration of appropriate antimicrobial therapy. The failure to initiate the correct therapy in a timely fashion results in poor clinical outcomes, longer hospital stays, and higher medical costs. Current approaches to antibiotic susceptibility testing of cultured pathogens have key limitations ranging from long run times to dependence on prior knowledge of genetic mechanisms of resistance. We have developed a rapid antimicrobial susceptibility assay for Staphylococcus aureus based on bacterial cytological profiling (BCP), which uses quantitative fluorescence microscopy to measure antibiotic induced changes in cellular architecture. BCP discriminated between methicillin-susceptible (MSSA) and -resistant (MRSA) clinical isolates of S. aureus (n = 71) within 1–2 h with 100% accuracy. Similarly, BCP correctly distinguished daptomycin susceptible (DS) from daptomycin non-susceptible (DNS) S. aureus strains (n = 20) within 30 min. Among MRSA isolates, BCP further identified two classes of strains that differ in their susceptibility to specific combinations of beta-lactam antibiotics. BCP provides a rapid and flexible alternative to gene-based susceptibility testing methods for S. aureus, and should be readily adaptable to different antibiotics and bacterial species as new mechanisms of resistance or multidrug-resistant pathogens evolve and appear in mainstream clinical practice.

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Bacterial cytological profiling identifies antibiotic resistant S. aureus.

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BCP predicts best treatment options for multidrug resistant MRSA.

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Resistant strains are correctly identified within 1 h.

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BCP does not require prior knowledge of resistance mechanism.

There is a great need for rapid antimicrobial susceptibility testing (AST) as it can dramatically improve clinical outcome for bacterial infections. Most currently proposed ASTs are dependent on knowledge of known resistance genes or based solely on growth/lysis. We have developed a new diagnostic method for rapidly determining antibiotic susceptibility of Staphylococcus aureus using quantitative fluorescence microscopy to measure antibiotic induced changes in cellular architecture. Our test has the potential to change the way antibiotic susceptibility testing is done in the future and is readily adaptable to different antibiotics and bacterial species regardless of the mechanisms of resistance.

Exploration of Modulated Genetic Circuits Governing Virulence Determinants in Staphylococcus aureus

The expression of virulence genes in the human pathogen Staphylococcus aureus is strongly influenced by the multiple global regulators. The signal transduction cascade of these global regulators is accountable for recognizing and integrating the environmental cues to regulate the virulence regulon. While the production of virulent factors by individual global regulators are comparatively straightforward to define, auto-regulation of these global regulators and their impact on other regulators is more complex process. There are several reports on the production of virulent factors that are precisely regulated by switching processes of multiple global regulators including some prominent accessory regulators such as agr, sae and sar which allows S. aureus to coordinate the gene expression, and thus, provide organism an ability to act collectively. This review implicates the mechanisms involved in the global regulation of various virulence factors along with a comprehensive discussion on the differences between these signal transduction systems, their auto-induction and, coordination of classical and some comparatively new bacterial signal transduction systems.

SaeRS-dependent inhibition of biofilm formation in Staphylococcus aureus Newman

The SaeRS two-component regulatory system of Staphylococcus aureus is known to affect the expression of many genes. The SaeS protein is the histidine kinase responsible for phosphorylation of the response regulator SaeR. In S. aureus Newman, the sae system is constitutively expressed due to a point mutation in saeS, relative to other S. aureus strains, which results in substitution of proline for leucine at amino acid 18. Strain Newman is unable to form a robust biofilm and we report here that the biofilm-deficient phenotype is due to the saeS^P allele. Replacement of the Newman saeS^P with saeS^L, or deletion of saeRS, resulted in a biofilm-proficient phenotype. Newman culture supernatants were observed to inhibit biofilm formation by other S. aureus strains, but did not affect biofilm formation by S. epidermidis. Culture supernatants of Newman saeS^L or Newman ΔsaeRS had no significant effect on biofilm formation. The inhibitory factor was inactivated by incubation with proteinase K, but survived heating, indicating that the inhibitory protein is heat-stable. The inhibitory protein was found to affect the attachment step in biofilm formation, but had no effect on preformed biofilms. Replacement of saeS^L with saeS^P in the biofilm-proficient S. aureus USA300 FPR3757 resulted in the loss of biofilm formation. Culture supernatants of USA300 FPR3757 saeS^P, did not inhibit biofilm formation by other staphylococci, suggesting that the inhibitory factor is produced but not secreted in the mutant strain. A number of biochemical methods were utilized to isolate the inhibitory protein. Although a number of candidate proteins were identified, none were found to be the actual inhibitor. In an effort to reduce the number of potential inhibitory genes, RNA-Seq analyses were done with wild-type strain Newman and the saeS^L and ΔsaeRS mutants. RNA-Seq results indicated that sae regulates many genes that may affect biofilm formation by Newman.

The Role of Two-Component Signal Transduction Systems in Staphylococcus aureus Virulence Regulation

Staphylococcus aureus is a versatile, opportunistic human pathogen that can asymptomatically colonize a human host but can also cause a variety of cutaneous and systemic infections. The ability of S. aureus to adapt to such diverse environments is reflected in the presence of complex regulatory networks fine-tuning metabolic and virulence gene expression. One of the most widely distributed mechanisms is the two-component signal transduction system (TCS) which allows a pathogen to alter its gene expression profile in response to environmental stimuli. The simpler TCSs consist of only a transmembrane histidine kinase (HK) and a cytosolic response regulator. S. aureus encodes a total of 16 conserved pairs of TCSs that are involved in diverse signalling cascades ranging from global virulence gene regulation (e.g. quorum sensing by the Agr system), the bacterial response to antimicrobial agents, cell wall metabolism, respiration and nutrient sensing. These regulatory circuits are often interconnected and affect each other's expression, thus fine-tuning staphylococcal gene regulation. This manuscript gives an overview of the current knowledge of staphylococcal environmental sensing by TCS and its influence on virulence gene expression and virulence itself. Understanding bacterial gene regulation by TCS can give major insights into staphylococcal pathogenicity and has important implications for knowledge-based drug design and vaccine formulation.

Stress-triggered signaling affecting survival or suicide of Streptococcus pneumoniae

Streptococcus pneumoniae is a major human pathogen that can survive to stress conditions, such as the acidic environment of inflammatory foci, and tolerates lethal pH through a mechanism known as the acid tolerance response. We previously described that S. pneumoniae activates acidic-stress induced lysis in response to acidified environments, favoring the release of cell wall compounds, DNA and virulence factors. Here, we demonstrate that F(0)F(1)-ATPase is involved in the response to acidic stress. Chemical inhibitors (DCCD, optochin) of this proton pump repressed the ATR induction, but caused an increased ASIL. Confirming these findings, mutants of the subunit c of this enzyme showed the same phenotypes as inhibitors. Importantly, we demonstrated that F(0)F(1)-ATPase and ATR are necessary for the intracellular survival of the pneumococcus in macrophages. Alternatively, a screening of two-component system (TCS) mutants showed that ATR and survival in pneumocytes were controlled in contrasting ways by ComDE and CiaRH, which had been involved in the ASIL mechanism. Briefly, CiaRH was essential for ATR (ComE represses activation) whereas ComE was necessary for ASIL (CiaRH protects against induction). They did not regulate F0F1-ATPase expression, but control LytA expression on the pneumococcal surface. These results suggest that both TCSs and F(0)F(1)-ATPase control a stress response and decide between a survival or a suicide mechanism by independent pathways, either in vitro or in pneumocyte cultures. This biological model contributes to the current knowledge about bacterial response under stress conditions in host tissues, where pathogens need to survive in order to establish infections.

Role of the two-component regulatory system arlRS in ica operon and aap positive but non-biofilm-forming Staphylococcus epidermidis isolates from hospitalized patients

The ica operon and aap gene are important factors for Staphylococcus epidermidis biofilm formation. However, we found 15 out of 101 S. epidermidis strains isolated from patients had both the ica operon and the aap gene in the genome but could not form biofilms (ica(+)aap(+)/BF(-) isolates). Compared with standard strain RP62A, the 15 ica(+)aap(+)/BF(-) isolates had similar growth curves and initial attachment abilities, but had much lower apparent transcription levels of the icaA gene and significantly less production of polysaccharide intercellular adhesion (PIA). Furthermore, the expression of accumulation-associated protein in ica(+)aap(+)/BF(-) isolates was much weaker than in RP62A. The mRNA levels of icaADBC transcription-related regulatory genes, including icaR, sarA, rsbU, srrA, arlRS and luxS, were measured in the 15 ica(+)aap(+)/BF(-) clinical isolates. The mRNA levels of arlR and rsbU in all of the ica(+)aap(+)/BF(-) isolates were lower than in RP62A at 4 h. At 10 h, 14/15 of the isolates showed lower mRNA levels of arlR and rsbU than shown by RP62A. However, expression of sarA, luxS, srrA and icaR varied in different ica(+)aap(+)/BF(-) isolates. To further investigate the role of arlRS in biofilm formation, we analyzed icaA, sarA and rsbU transcription, PIA synthesis, Aap expression and biofilm formation in an arlRS deletion mutant of S. epidermidis strain 1457 and all were much less than in the wild type strain. This is consistent with the hypothesis that ArlRS may play an important role in regulating biofilm formation by the ica(+)aap(+)/BF(-)S. epidermidis clinical isolates and operate via both ica-dependent and Aap-dependent pathways.

Pfs promotes autolysis-dependent release of eDNA and biofilm formation in Staphylococcus aureus

Staphylococcus aureus is a major biofilm-forming pathogen, and biofilm formation remains an obstacle in the treatment of clinical S. aureus infection. Methylthioadenosine/S-adenosylhomocysteine nucleosidase (Pfs) has been implicated in methylation reactions, polyamine synthesis, vitamin synthesis, and quorum-sensing pathways. In this study, we observed that the deletion of pfs gene in S. aureus NCTC8325 reduced bacterial clumping ability and resulted in the decreased biofilm formation under both static and dynamic flow conditions in an autoinducer-2-independent manner. While the PIA amount was not affected, the pfs mutation significantly decreased the amount of eDNA present in the biofilm and the cell autolysis. Consistent with reduced autolysis, the transcription levels of the autolysin genes, lytM and atlE, were reduced in the absence of Pfs. These data suggest that Pfs promotes autolysis-dependent release of eDNA and biofilm formation in S. aureus, and our findings indicate that Pfs is a potential novel target for anti-biofilm therapy.

Altering gene expression by aminocoumarins: the role of DNA supercoiling in Staphylococcus aureus

Background

It has been shown previously that aminocoumarin antibiotics such as novobiocin lead to immediate downregulation of recA expression and thereby inhibit the SOS response, mutation frequency and recombination capacity in Staphylococcus aureus. Aminocoumarins function by inhibiting the ATPase activity of DNA gyrase subunit B with a severe impact on DNA supercoiling.

Results

Here, we have analysed the global impact of the DNA relaxing agent novobiocin on gene expression in S. aureus. Using a novobiocin-resistant mutant, it became evident that the change in recA expression is due to gyrase inhibition. Microarray analysis and northern blot hybridisation revealed that the expression levels of a distinct set of genes were increased (e.g., recF-gyrB-gyrA, the rib operon and the ure operon) or decreased (e.g., arlRS, recA, lukA, hlgC and fnbA) by novobiocin. The two-component ArlRS system was previously found to decrease the level of supercoiling in S. aureus. Thus, downregulation of arlRS might partially compensate for the relaxing effect of novobiocin. Global analysis and gene mapping of supercoiling-sensitive genes did not provide any indication that they are clustered in the genome. Promoter fusion assays confirmed that the responsiveness of a given gene is intrinsic to the promoter region but independent of the chromosomal location.

Conclusions

The results indicate that the molecular properties of a given promoter, rather than the chromosomal topology, dictate the responsiveness to changes in supercoiling in the pathogen Staphylococcus aureus.

The Staphylococcus aureus ArlRS two-component system is a novel regulator of agglutination and pathogenesis

Staphylococcus aureus is a prominent bacterial pathogen that is known to agglutinate in the presence of human plasma to form stable clumps. There is increasing evidence that agglutination aids S. aureus pathogenesis, but the mechanisms of this process remain to be fully elucidated. To better define this process, we developed both tube based and flow cytometry methods to monitor clumping in the presence of extracellular matrix proteins. We discovered that the ArlRS two-component system regulates the agglutination mechanism during exposure to human plasma or fibrinogen. Using divergent S. aureus strains, we demonstrated that arlRS mutants are unable to agglutinate, and this phenotype can be complemented. We found that the ebh gene, encoding the Giant Staphylococcal Surface Protein (GSSP), was up-regulated in an arlRS mutant. By introducing an ebh complete deletion into an arlRS mutant, agglutination was restored. To assess whether GSSP is the primary effector, a constitutive promoter was inserted upstream of the ebh gene on the chromosome in a wildtype strain, which prevented clump formation and demonstrated that GSSP has a negative impact on the agglutination mechanism. Due to the parallels of agglutination with infective endocarditis development, we assessed the phenotype of an arlRS mutant in a rabbit combined model of sepsis and endocarditis. In this model the arlRS mutant displayed a large defect in vegetation formation and pathogenesis, and this phenotype was partially restored by removing GSSP. Altogether, we have discovered that the ArlRS system controls a novel mechanism through which S. aureus regulates agglutination and pathogenesis.

Staphylococcus aureus is a bacterial pathogen that is responsible for causing significant disease in humans. The development of antibiotic resistant strains has made these infections more difficult to treat, and an improved understanding of how this pathogen causes infections will facilitate the development of new tools for treatment. It has long been recognized that S. aureus can bind human matrix proteins to form stable clumps in a process called agglutination, but the importance of agglutination during infection is only just becoming understood. In this work, we developed several techniques to investigate the S. aureus agglutination mechanism. We discovered that the ArlRS two-component regulatory system controls agglutination by regulating the expression of the ebh gene, which encodes the Giant Staphylococcal Surface Protein (GSSP). When ArlRS is non-functional, S. aureus agglutination is prevented through the action of GSSP. These phenotypes were confirmed in a rabbit model of sepsis and infective endocarditis, demonstrating that ArlRS is an important regulator of virulence. Taken together, the identification of ArlRS as a regulator of S. aureus agglutination and pathogenesis may lead to innovative directions for therapeutic development.

Evolution of community- and healthcare-associated methicillin-resistant Staphylococcus aureus

Staphylococcus aureus is a prominent cause of human infections globally. The high prevalence of infections is compounded by antibiotic resistance--a significant problem for treatment. Methicillin-resistant S. aureus (MRSA) is endemic in hospitals and healthcare facilities worldwide, and is an increasingly common cause of community-associated bacterial infections in industrialized countries. Although much focus is placed on the role of S. aureus as a human pathogen, it is in fact a human commensal organism that has had a relatively long coexistence with the human host. Many S. aureus infections can be explained by host susceptibility or other predisposing risk factors. On the other hand, the emergence/re-emergence of successful S. aureus clones (referred to as epidemic waves) suggests a rapid bacterial adaption and evolution, which includes the emergence of antibiotic resistance and increased virulence and/or transmissibility. It is within this context that we review our understanding of selected S. aureus epidemic waves, and highlight the use of genome sequencing as a means to better understand the evolution of each lineage.

Attenuating Staphylococcus aureus virulence gene regulation: a medicinal chemistry perspective

Virulence gene expression in Staphylococcus aureus is tightly regulated by intricate networks of transcriptional regulators and two-component signal transduction systems. There is now an emerging body of evidence to suggest that the blockade of S. aureus virulence gene expression significantly attenuates infection in experimental models. In this Perspective, we will provide insights into medicinal chemistry strategies for the development of chemical reagents that have the capacity to inhibit staphylococcal virulence expression. These reagents can be broadly grouped into four categories: (1) competitive inhibitors of the accessory gene regulator (agr) quorum sensing system, (2) inhibitors of AgrA–DNA interactions, (3) RNAIII transcription inhibitors, and (4) inhibitors of the SarA family of transcriptional regulators. We discuss the potential of specific examples of antivirulence agents for the management and treatment of staphylococcal infections.

Antibiotic-mediated selection of quorum-sensing-negative Staphylococcus aureus

Staphylococcus aureus is a human commensal that at times turns into a serious bacterial pathogen causing life-threatening infections. For the delicate control of virulence, S. aureus employs the agr quorum-sensing system that, via the intracellular effector molecule RNAIII, regulates virulence gene expression. We demonstrate that the presence of the agr locus imposes a fitness cost on S. aureus that is mediated by the expression of RNAIII. Further, we show that exposure to sublethal levels of the antibiotics ciprofloxacin, mupirocin, and rifampin, each targeting separate cellular functions, markedly increases the agr-mediated fitness cost by inducing the expression of RNAIII. Thus, the extensive use of antibiotics in hospitals may explain why agr-negative variants are frequently isolated from hospital-acquired S. aureus infections but rarely found among community-acquired S. aureus strains. Importantly, agr deficiency correlates with increased duration of and mortality due to bacteremia during antibiotic treatment and with a higher frequency of glycopeptide resistance than in agr-carrying strains. Our results provide an explanation for the frequent isolation of agr-defective strains from hospital-acquired S. aureus infections and suggest that the adaptability of S. aureus to antibiotics involves the agr locus.

Staphylococcus aureus is the most frequently isolated pathogen in intensive care units and a common cause of nosocomial infections, resulting in a high degree of morbidity and mortality. Surprisingly, a large fraction (15 to 60%) of hospital-isolated S. aureus strains are agr defective and lack the main quorum-sensing-controlled virulence regulatory system. This is a problem, as agr-defective strains are associated with a mortality level in bacteremic infections and a probability of glycopeptide resistance greater than those of other strains. We show here that agr-negative strains have a fitness advantage over agr-positive strains in the presence of sublethal concentrations of some antibiotics and that the fitness defect of agr-positive cells is caused by antibiotic-mediated expression of the agr effector molecule RNAIII. These results offer an explanation of the frequent isolation of agr-defective S. aureus strains in hospitals and will influence how we treat S. aureus infections.