Flipping the switch: tools for detecting small molecule inhibitors of staphylococcal virulence

Staphylococcus aureus Phenol-Soluble Modulins Mediate Interspecies Competition with Upper Respiratory Commensal Bacteria

In chronic rhinosinusitis (CRS) disease, microbial dysbiosis is considered a key contributor to inflammation and pathogenicity, with increased prevalence of upper respiratory tract (URT) pathogens concomitant with decreased abundance of commensal species. Staphylococcus aureus is a common URT pathobiont associated with higher carriage rates in CRS. S. aureus secreted toxins are implicated in CRS pathogenesis, and toxins and antibodies to S. aureus secreted factors have been observed in tissue from CRS subjects. CRS disease severity is positively correlated with immune reactivity to S. aureus proteins. Prior studies have examined polymicrobial interactions between S. aureus and URT commensals, however, no studies to date have described possible methods employed by S. aureus to outcompete commensals leading to a S. aureus- dominant microbiome as seen in CRS. This study addresses this gap in knowledge by characterizing how a CRS-associated secreted toxin from S. aureus can inhibit aggregation in commensal URT species. Using a model URT commensal, Corynebacterium pseudodiphtheriticum , we identified a CRS-associated secreted protein from S. aureus , δ-toxin (Hld), that can inhibit C. pseudodiphtheriticum aggregation at biologically relevant concentrations. Furthermore, we observed recombinant δ-toxin reduces C. pseudodiphtheriticum adherence and aggregation on human nasal epithelial cells in an air-liquid interface cell culture model. These results define a novel mechanism by which S. aureus can disrupt URT commensal lifestyles of microbial competitors, contributing to the establishment of microbial dysbiosis.

IMPORTANCE

Microbial dysbiosis in the upper respiratory tract (URT) is associated with disease pathogenicity in chronic rhinosinusitis (CRS). There are significant links between Staphylococcus aureus and worse CRS outcomes, but no studies to date have demonstrated if S. aureus outcompetes other URT microbes through direct interactions. Here, we report that S. aureus δ-toxin, a secreted protein found in CRS patient tissue, can inhibit the ability of commensal bacteria to aggregate, adhere to, and grow in association with human nasal epithelial cells. These results suggest a potential mechanism for S. aureus to establish dominance in the URT microbiome through direct antagonism of commensals with a disease-associated toxin.

Determinants of maturation of the Staphylococcus aureus autoinducing peptide

The accessory gene regulatory (Agr) system is required for virulence factor gene expression and pathogenesis of Staphylococcus aureus. The Agr system is activated in response to the accumulation of a cyclic autoinducing peptide (AIP), which is matured and secreted by the bacterium. The precursor of AIP, AgrD, consists of the AIP flanked by an N-terminal [Formula: see text]-helical Leader and a charged C-terminal tail. AgrD is matured to AIP by the action of two proteases, AgrB and MroQ. AgrB cleaves the C-terminal tail and promotes the formation of a thiolactone ring, whereas MroQ cleaves the N-terminal Leader in a manner that depends on the four-amino acid linker immediately following a conserved IG helix breaker motif. However, the attributes of AgrD that dictate the sequence of events in peptide maturation are not fully defined. Here, we used engineered AgrD peptide intermediates to ascertain the sufficiency of MroQ for N-terminal peptide cleavage, peptide export, and generation of mature AIP. We found that MroQ promotes the removal of the N-terminal Leader peptide from both linear and cyclic peptide intermediates, while peptide cyclization remained essential for signaling. The expression of the Leader peptide in isolation was sufficient for MroQ-dependent cleavage proximal to the four-amino-acid linker. In addition, active site mutations within AgrB destabilized full-length AgrD and thiolactone-containing intermediates and prevented the release of the Leader peptide. Altogether, our data support a tandem peptide maturation event involving both MroQ and AgrB that appears to couple protease activity and export of bioactive AIP.IMPORTANCEThe accessory gene regulatory (Agr) system is important for S. aureus pathogenesis. Activation of the Agr system requires recognition of a cyclic peptide pheromone, which must be fully matured to exert its biological activity. The complete events in cyclic peptide maturation and export from the bacterial cell remain to be fully defined. We and others recently discovered that the membrane peptidase MroQ is required for pheromone maturation. This study builds off the identification of MroQ and considers the attributes of the pheromone pro-peptide that are required for MroQ-mediated processing as well as uncovers features important for peptide stability and export. Overall, the findings in this study have implications for understanding bacterial pheromone maturation and virulence.

3D-printed porous titanium rods equipped with vancomycin-loaded hydrogels and polycaprolactone membranes for intelligent antibacterial drug release

Implant-related infections pose significant challenges to orthopedic surgeries due to the high risk of severe complications. The widespread use of bioactive prostheses in joint replacements, featuring roughened surfaces and tight integration with the bone marrow cavity, has facilitated bacterial proliferation and complicated treatment. Developing antibacterial coatings for orthopedic implants has been a key research focus in recent years to address this critical issue. Researchers have designed coatings using various materials and antibacterial strategies. In this study, we fabricated 3D-printed porous titanium rods, incorporated vancomycin-loaded mPEG750-b-PCL2500 gel, and coated them with a PCL layer. We then evaluated the antibacterial efficacy through both in vitro and in vivo experiments. Our coating passively inhibits bacterial biofilm formation and actively controls antibiotic release in response to bacterial growth, providing a practical solution for proactive and sustained infection control. This study utilized 3D printing technology to produce porous titanium rod implants simulating bioactive joint prostheses. The porous structure of the titanium rods was used to load a thermoresponsive gel, mPEG750-b-PCL2500 (PEG: polyethylene glycol; PCL: polycaprolactone), serving as a novel drug delivery system carrying vancomycin for controlled antibiotic release. The assembly was then covered with a PCL membrane that inhibits bacterial biofilm formation early in infection and degrades when exposed to lipase solutions, mimicking enzymatic activity during bacterial infections. This setup provides infection-responsive protection and promotes drug release. We investigated the coating's controlled release, antibacterial capability, and biocompatibility through in vitro experiments. We established a Staphylococcus aureus infection model in rabbits, implanting titanium rods in the femoral medullary cavity. We evaluated the efficacy and safety of the composite coating in preventing implant-related infections using imaging, hematology, and pathology. In vitro experiments demonstrated that the PCL membrane stably protects encapsulated vancomycin during PBS immersion. The PCL membrane rapidly degraded at a lipase concentration of 0.2 mg/mL. The mPEG750-b-PCL2500 gel ensured stable and sustained vancomycin release, inhibiting bacterial growth. We investigated the antibacterial effect of the 3D-printed titanium material, coated with PCL and loaded with mPEG750-b-PCL2500 hydrogel, using a rabbit Staphylococcus aureus infection model. Imaging, hematology, and histopathology confirmed that our composite antibacterial coating exhibited excellent antibacterial effects and infection prevention, with good safety in trials. Our results indicate that the composite antibacterial coating effectively protects vancomycin in the hydrogel from premature release in the absence of bacterial infection. The outer PCL membrane inhibits bacterial growth and prevents biofilm formation. Upon contact with bacterial lipase, the PCL membrane rapidly degrades, releasing vancomycin for antibacterial action. The mPEG750-b-PCL2500 gel provides stable and sustained vancomycin release, prolonging its antibacterial effects. Our composite antibacterial coating demonstrates promising potential for clinical application.

Altered quorum sensing and physiology of Staphylococcus aureus during spaceflight detected by multi-omics data analysis

Staphylococcus aureus colonizes the nares of approximately 30% of humans, a risk factor for opportunistic infections. To gain insight into S. aureus virulence potential in the spaceflight environment, we analyzed RNA-Seq, cellular proteomics, and metabolomics data from the "Biological Research in Canisters-23" (BRIC-23) GeneLab spaceflight experiment, a mission designed to measure the response of S. aureus to growth in low earth orbit on the international space station. This experiment used Biological Research in Canisters-Petri Dish Fixation Units (BRIC-PDFUs) to grow asynchronous ground control and spaceflight cultures of S. aureus for 48 h. RNAIII, the effector of the Accessory Gene Regulator (Agr) quorum sensing system, was the most highly upregulated gene transcript in spaceflight relative to ground controls. The agr operon gene transcripts were also highly upregulated during spaceflight, followed by genes encoding phenol-soluble modulins and secreted proteases, which are positively regulated by Agr. Upregulated spaceflight genes/proteins also had functions related to urease activity, type VII-like Ess secretion, and copper transport. We also performed secretome analysis of BRIC-23 culture supernatants, which revealed that spaceflight samples had increased abundance of secreted virulence factors, including Agr-regulated proteases (SspA, SspB), staphylococcal nuclease (Nuc), and EsxA (secreted by the Ess system). These data also indicated that S. aureus metabolism is altered in spaceflight conditions relative to the ground controls. Collectively, these data suggest that S. aureus experiences increased quorum sensing and altered expression of virulence factors in response to the spaceflight environment that may impact its pathogenic potential.

Development of a dual fluorescent reporter system to identify inhibitors of Staphylococcus aureus virulence factors

IMPORTANCE

Staphylococcus aureus is a formidable pathogen responsible for a wide range of infections, and the emergence of antibiotic-resistant strains has posed significant challenges in treating these infections. In this study, we have established a novel dual reporter system capable of concurrently monitoring the activities of two critical virulence regulators in S. aureus. By incorporating both reporters into a single screening platform, we provide a time- and cost-efficient approach for assessing the activity of compounds against two distinct targets in a single screening round. This innovative dual reporter system presents a promising strategy for the identification of molecules capable of modulating virulence gene expression in S. aureus, potentially expediting the development of antivirulence therapies.

Current State of Knowledge Regarding WHO High Priority Pathogens-Resistance Mechanisms and Proposed Solutions through Candidates Such as Essential Oils: A Systematic Review

Combating antimicrobial resistance (AMR) is among the 10 global health issues identified by the World Health Organization (WHO) in 2021. While AMR is a naturally occurring process, the inappropriate use of antibiotics in different settings and legislative gaps has led to its rapid progression. As a result, AMR has grown into a serious global menace that impacts not only humans but also animals and, ultimately, the entire environment. Thus, effective prophylactic measures, as well as more potent and non-toxic antimicrobial agents, are pressingly needed. The antimicrobial activity of essential oils (EOs) is supported by consistent research in the field. Although EOs have been used for centuries, they are newcomers when it comes to managing infections in clinical settings; it is mainly because methodological settings are largely non-overlapping and there are insufficient data regarding EOs' in vivo activity and toxicity. This review considers the concept of AMR and its main determinants, the modality by which the issue has been globally addressed and the potential of EOs as alternative or auxiliary therapy. The focus is shifted towards the pathogenesis, mechanism of resistance and activity of several EOs against the six high priority pathogens listed by WHO in 2017, for which new therapeutic solutions are pressingly required.

Chemical and biomolecular insights into the Staphylococcus aureus agr quorum sensing system: Current progress and ongoing challenges

Staphylococcus aureus is a ubiquitous bacterium that has become a major threat to human health due to its extensive toxin production and tremendous capacity for antibiotic resistance (e.g., MRSA "superbug" infections). Amid a worsening antibiotic resistance crisis, new strategies to combat this deadly microbe that remove the selective pressure of traditional approaches are in high demand. S. aureus utilizes an accessory gene regulator (agr) quorum sensing network to monitor its local cellular population and trigger a devastating communal attack, like an invading horde, once a threshold cell density has been reached. The role of the agr system in a range of disease types is still being unraveled. Herein, we discuss the present-day biochemical understanding of agr along with unresolved details, describe its connection to the progression of infection, and review how chemical strategies have been implemented to study and intercept this signaling pathway. This research is illuminating the potential of agr as an anti-virulence target in S. aureus and should inform the study of similar, yet less studied, agr systems in related bacterial pathogens.

Anti-virulence compounds against Staphylococcus aureus associated with bovine mastitis: A new therapeutic option?

Bovine mastitis represents a major economic burden faced by the dairy industry. S. aureus is an important and prevalent bovine mastitis-associated pathogen in dairy farms worldwide. The pathogenicity and persistence of S. aureus in the bovine mammary gland are associated with the expression of a range of virulence factors involved in biofilm formation and the production of several toxins. The traditional therapeutic approach to treating bovine mastitis includes the use of antibiotics, but the emergence of antibiotic-resistant strains has caused therapeutic failure. New therapeutic approaches targeting virulence factors of S. aureus rather than cell viability can have several advantages including lower selective pressure towards the development of resistance and little impact on the host commensal microbiota. This review summarizes the potential of anti-virulence therapies to control S. aureus associated with bovine mastitis focusing on anti-toxin, anti-biofilm, and anti-quorum sensing compounds. It also points to potential sources of new anti-virulence inhibitors and presents screening strategies for identifying these compounds.

An Explorative Review on Advanced Approaches to Overcome Bacterial Resistance by Curbing Bacterial Biofilm Formation

The continuous emergence of multidrug-resistant pathogens evoked the development of innovative approaches targeting virulence factors unique to their pathogenic cascade. These approaches aimed to explore anti-virulence or anti-infective therapies. There are evident concerns regarding the bacterial ability to create a superstructure, the biofilm. Biofilm formation is a crucial virulence factor causing difficult-to-treat, localized, and systemic infections. The microenvironments of bacterial biofilm reduce the efficacy of antibiotics and evade the host's immunity. Producing a biofilm is not limited to a specific group of bacteria; however, Pseudomonas aeruginosa, Acinetobacter baumannii, and Staphylococcus aureus biofilms are exemplary models. This review discusses biofilm formation as a virulence factor and the link to antimicrobial resistance. In addition, it explores insights into innovative multi-targeted approaches and their physiological mechanisms to combat biofilms, including natural compounds, phages, antimicrobial photodynamic therapy (aPDT), CRISPR-Cas gene editing, and nano-mediated techniques.

Characterization of MroQ-Dependent Maturation and Export of the Staphylococcus aureus Accessory Gene Regulatory System Autoinducing Peptide

Gram-positive bacteria produce small autoinducing peptides (AIPs), which act to regulate expression of genes that promote adaptive traits, including virulence. The Gram-positive pathogen Staphylococcus aureus generates a cyclic AIP that controls expression of virulence factors via the accessory gene regulatory (Agr) system. S. aureus strains belong to one of four Agr groups (Agr-I, -II, -III, and -IV); each group harbors allelic variants of AgrD, the precursor of AIP. In a prior screen for S. aureus virulence factors, we identified MroQ, a putative peptidase. A ΔmroQ mutant closely resembled a Δagr mutant and had significant defects in AIP production in an Agr-I strain. Here, we show that expression of AgrD-I in a ΔmroQ mutant leads to accumulation of an AIP processing intermediate at the membrane that coincides with a loss of secreted mature AIP, indicating that MroQ promotes maturation of AgrD-I. MroQ is conserved in all Agr sequence variants, suggesting either identical function among all Agr types or activity specific to Agr-I strains. Our data indicate that MroQ is required for AIP maturation and activity in Agr-I, -II, and -IV strains irrespective of background. However, MroQ is not required for Agr-III activity despite an identifiable role in peptide maturation. Isogenic Δagr and Δagr ΔmroQ strains complemented with Agr-I to -IV validated the critical role of MroQ in the generation of active AIP-I, -II, and -IV but not AIP-III. These findings were reinforced by skin infection studies with mice. Our data substantiate the prevailing model that MroQ is a mediator of cyclic peptide maturation.

Thymus zygis Essential Oil: Phytochemical Characterization, Bioactivity Evaluation and Synergistic Effect with Antibiotics against Staphylococcus aureus

Staphylococcus aureus is a nosocomial bacterium causing different infectious diseases, ranging from skin and soft-tissue infections to more serious and life-threatening infections such as sepsis, meningitis and endocarditis, which may be exacerbated by antibiotic resistance. Plant products may be seen as an alternative as antibacterial agents, namely, against S. aureus. Thus, the aim of this work was to characterize the chemical composition and evaluate the bioactive properties of the T. zygis essential oil (EO), with a focus on antimicrobial activity against S. aureus. Gas chromatography coupled with mass spectrometry was used to assess the chemical composition of the T. zygis EO, and the antioxidant activity was evaluated using the DPPH method and β-carotene-bleaching assay. The antimicrobial activity against S. aureus strains, the interaction with different antibiotics and the attenuation of this bacterium’s virulence were evaluated. The T. zygis EO showed antioxidant activity acting through two different mechanisms and antibacterial activity against S. aureus, with antibiofilm and antihaemolytic properties. This EO also demonstrated synergistic or additive interactions in combination with ampicillin, ciprofloxacin or vancomycin against S. aureus strains and, in some cases, changed the antibiotic-resistance phenotype from resistant to susceptible. Therefore, the present work demonstrates the good bioactive properties of the EO of T. zygis, mainly the antimicrobial activity against S. aureus, revealing its potential to be used as an antibacterial agent.

Castaneroxy A From the Leaves of Castanea sativa Inhibits Virulence in Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) represents one of the most serious infectious disease concerns worldwide, with the CDC labeling it a "serious threat" in 2019. The current arsenal of antibiotics works by targeting bacterial growth and survival, which exerts great selective pressure for the development of resistance. The development of novel anti-infectives that inhibit quorum sensing and thus virulence in MRSA has been recurrently proposed as a promising therapeutic approach. In a follow-up of a study examining the MRSA quorum sensing inhibitory activity of extracts of Italian plants used in local traditional medicine, 224C-F2 was reported as a bioactive fraction of a Castanea sativa (European chestnut) leaf extract. The fraction demonstrated high activity in vitro and effective attenuation of MRSA pathogenicity in a mouse model of skin infection. Through further bioassay-guided fractionation using reverse-phase high performance liquid chromatography, a novel hydroperoxy cycloartane triterpenoid, castaneroxy A (1), was isolated. Its structure was established by nuclear magnetic resonance, mass spectrometry and X-ray diffraction analyses. Isomers of 1 were also detected in an adjacent fraction. In a series of assays assessing inhibition of markers of MRSA virulence, 1 exerted activities in the low micromolar range. It inhibited agr::P3 activation (IC₅₀ = 31.72 µM), δ-toxin production (IC₅₀ = 31.72 µM in NRS385), supernatant cytotoxicity to HaCaT human keratinocytes (IC₅₀ = 7.93 µM in NRS385), and rabbit erythrocyte hemolytic activity (IC₅₀ = 7.93 µM in LAC). Compound 1 did not inhibit biofilm production, and at high concentrations it exerted cytotoxicity against human keratinocytes greater than that of 224C-F2. Finally, 1 reduced dermonecrosis in a murine model of MRSA infection. The results establish 1 as a promising antivirulence candidate for development against MRSA.

Essential Oils: A Natural Weapon against Antibiotic-Resistant Bacteria Responsible for Nosocomial Infections

The emergence of antibiotic-resistant bacteria has become a major concern worldwide. This trend indicates the need for alternative agents to antibiotics, such as natural compounds of plant origin. Using agar disc diffusion and minimum inhibitory concentration (MIC) assays, we investigated the antimicrobial activity of Citrus aurantium (AEO), Citrus x limon (LEO), Eucalyptus globulus (EEO), Melaleuca alternifolia (TTO), and Cupressus sempervirens (CEO) essential oils (EOs) against three representatives of antibiotic-resistant pathogens and respective biofilms: vancomycin-resistant enterococci (VRE), methicillin-resistant Staphylococcus aureus (MRSA), and extended-spectrum β-lactamase (ESBL)-producing Escherichia coli. Using the checkerboard method, the efficacy of the EOs alone, in an association with each other, or in combination with the reference antibiotics was quantified by calculating fractional inhibitory concentrations (FICs). All the EOs displayed antibacterial activity against all strains to different extents, and TTO was the most effective. The results of the EO–EO associations and EO–antibiotic combinations clearly showed a synergistic outcome in most tests. Lastly, the effectiveness of EOs both alone and in association or combination against biofilm formed by the antibiotic-resistant strains was comparable to, and sometimes better than, that of the reference antibiotics. In conclusion, the combination of EOs and antibiotics represents a promising therapeutic strategy against antibiotic-resistant bacteria, even protected inside biofilms, which can allow decreasing the concentrations of antibiotics used.

Increased Antibiotic Resistance of Methicillin-Resistant Staphylococcus aureus USA300 Δpsm Mutants and a Complementation Study of Δpsm Mutants Using Synthetic Phenol-Soluble Modulins

Phenol-soluble modulins (PSMs) are responsible for regulating biofilm formation, persister cell formation, pmtR expression, host cell lysis, and anti-bacterial effects. To determine the effect of psm deletion on methicillin-resistant Staphylococcus aureus, we investigated psm deletion mutants including Δpsmα, Δpsmβ, and Δpsmαβ;. These mutants exhibited increased β-lactam antibiotic resistance to ampicillin and oxacillin that was shown to be caused by increased Nacetylmannosamine kinase (nanK) mRNA expression, which regulates persister cell formation, leading to changes in the pattern of phospholipid fatty acids resulting in increased anteiso-C_15:0, and increased membrane hydrophobicity with the deletion of PSMs. When synthetic PSMs were applied to Δpsmα and Δpsmβ mutants, treatment of Δpsmα with PSMα1-4 and Δpsmβ with PSMβ1-2 restored the sensitivity to oxacillin and slightly reduced the biofilm formation. Addition of a single fragment showed that α1, α2, α3, and β2 had an inhibiting effect on biofilms in Δpsmα; however, β1 showed an enhancing effect on biofilms in Δpsmβ. This study demonstrates a possible reason for the increased antibiotic resistance in psm mutants and the effect of PSMs on biofilm formation.

2-(2-Methyl-2-nitrovinyl)furan but Not Furvina Interfere with Staphylococcus aureus Agr Quorum-Sensing System and Potentiate the Action of Fusidic Acid against Biofilms

Quorum sensing (QS) plays an essential role in the production of virulence factors, in biofilm formation and antimicrobial resistance. Consequently, inhibiting QS is being considered a promising target for antipathogenic/anti-virulence therapies. This study aims to screen 2-nitrovinylfuran derivatives structurally related to Furvina (a broad-spectrum antibiotic already used for therapeutic purposes) for their effects on QS and in biofilm prevention/control. Furvina and four 2-nitrovinylfuran derivatives (compounds 1–4) were tested to assess the ability to interfere with QS of Staphylococcus aureus using bioreporter strains (S. aureus ALC1742 and ALC1743). The activity of Furvina and the most promising quorum-sensing inhibitor (QSI) was evaluated in biofilm prevention and in biofilm control (combined with fusidic acid). The biofilms were further characterized in terms of biofilm mass, viability and membrane integrity. Compound 2 caused the most significant QS inhibition with reductions between 60% and 80%. Molecular docking simulations indicate that this compound interacts preferentially with the protein hydrophobic cleft in the LytTR domain of AgrA pocket. Metabolic inactivations of 40% for S. aureus ALC1742 and 20% for S. aureus ALC1743 were reached. A 24 h-old biofilm formed in the presence of the QSI increased the metabolic inactivation by fusidic acid to 80%, for both strains. The overall results highlight the effects of compound 2 as well as the potential of combining QSI with in-use antibiotics for the management of skin and soft tissues infections.

Transcriptomic analysis of stress response to novel antimicrobial coatings in a clinical MRSA strain

Multi-drug resistant pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) cause nosocomial infections that can have deleterious effects on human health. Thus, it is imperative to find solutions to treat these detrimental infections as well as to control their spread. We tested the effect of two different antimicrobial materials, functionalised graphene oxide (GOX), and AGXX® coated on cellulose fibres, on the growth and transcriptome of the clinical MRSA strain S. aureus 04-02981. In addition, we investigated the effect of a third material as a combination of GOX and AGXX® fibres on S. aureus 04-02981. Standard plate count assay revealed that the combination of fibres, GOX-AGXX® inhibited the growth of S. aureus 04-02981 by 99.98%. To assess the effect of these antimicrobials on the transcriptome of our strain, cultures of S. aureus 04-02981 were incubated with GOX, AGXX®, or GOX-AGXX® fibres for different time periods and then subjected to RNA-sequencing. Uncoated cellulose fibres were used as a negative control. The antimicrobial fibres had a huge impact on the transcriptome of S. aureus 04-02981 affecting the expression of 2650 genes. Primarily genes related to biofilm formation and virulence (such as agr, sarA, and those of the two-component system SaeRS), and genes crucial for survival in biofilms (like arginine metabolism arc genes) were repressed. In contrast, the expression of siderophore biosynthesis genes (sbn) was induced, a probable response to stress imposed by the antimicrobials and the conditions of iron-deficiency. Genes associated with potassium transport, intracellular survival and pathogenesis (kdp) were also differentially expressed. Our data suggest that the combination of GOX and AGXX® acts as an efficient antimicrobial against S. aureus 04-02981. Thus, these materials are potential candidates for applications in antimicrobial surface coatings.

Staphylococcal Biofilm Development: Structure, Regulation, and Treatment Strategies

In many natural and clinical settings, bacteria are associated with some type of biotic or abiotic surface that enables them to form biofilms, a multicellular lifestyle with bacteria embedded in an extracellular matrix. Staphylococcus aureus and Staphylococcus epidermidis, the most frequent causes of biofilm-associated infections on indwelling medical devices, can switch between an existence as single free-floating cells and multicellular biofilms. During biofilm formation, cells first attach to a surface and then multiply to form microcolonies. They subsequently produce the extracellular matrix, a hallmark of biofilm formation, which consists of polysaccharides, proteins, and extracellular DNA. After biofilm maturation into three-dimensional structures, the biofilm community undergoes a disassembly process that leads to the dissemination of staphylococcal cells. As biofilms are dynamic and complex biological systems, staphylococci have evolved a vast network of regulatory mechanisms to modify and fine-tune biofilm development upon changes in environmental conditions. Thus, biofilm formation is used as a strategy for survival and persistence in the human host and can serve as a reservoir for spreading to new infection sites. Moreover, staphylococcal biofilms provide enhanced resilience toward antibiotics and the immune response and impose remarkable therapeutic challenges in clinics worldwide. This review provides an overview and an updated perspective on staphylococcal biofilms, describing the characteristic features of biofilm formation, the structural and functional properties of the biofilm matrix, and the most important mechanisms involved in the regulation of staphylococcal biofilm formation. Finally, we highlight promising strategies and technologies, including multitargeted or combinational therapies, to eradicate staphylococcal biofilms.

Staphylococcal Biofilm Development: Structure, Regulation, and Treatment Strategies

In many natural and clinical settings, bacteria are associated with some type of biotic or abiotic surface that enables them to form biofilms, a multicellular lifestyle with bacteria embedded in an extracellular matrix. Staphylococcus aureus and Staphylococcus epidermidis, the most frequent causes of biofilm-associated infections on indwelling medical devices, can switch between an existence as single free-floating cells and multicellular biofilms. During biofilm formation, cells first attach to a surface and then multiply to form microcolonies. They subsequently produce the extracellular matrix, a hallmark of biofilm formation, which consists of polysaccharides, proteins, and extracellular DNA. After biofilm maturation into three-dimensional structures, the biofilm community undergoes a disassembly process that leads to the dissemination of staphylococcal cells. As biofilms are dynamic and complex biological systems, staphylococci have evolved a vast network of regulatory mechanisms to modify and fine-tune biofilm development upon changes in environmental conditions. Thus, biofilm formation is used as a strategy for survival and persistence in the human host and can serve as a reservoir for spreading to new infection sites. Moreover, staphylococcal biofilms provide enhanced resilience toward antibiotics and the immune response and impose remarkable therapeutic challenges in clinics worldwide. This review provides an overview and an updated perspective on staphylococcal biofilms, describing the characteristic features of biofilm formation, the structural and functional properties of the biofilm matrix, and the most important mechanisms involved in the regulation of staphylococcal biofilm formation. Finally, we highlight promising strategies and technologies, including multitargeted or combinational therapies, to eradicate staphylococcal biofilms.

Nusbiarylins Inhibit Transcription and Target Virulence Factors in Bacterial Pathogen Staphylococcus aureus

The emergence of multidrug resistance in the clinically significant pathogen Staphylococcus aureus is a global health burden, compounded by a diminishing drug development pipeline, and a lack of approved novel antimicrobials. Our previously reported first-in-class bacterial transcription inhibitors “nusbiarylins” presented a promising prospect towards the discovery of novel antimicrobial agents with a novel mechanism. Here we investigated and characterised the lead nusbiarylin compound, MC4, and several of its chemical derivatives in both methicillin-resistant S. aureus (MRSA) and the S. aureus type strains, demonstrating their capacity for the arrest of growth and cellular respiration, impairment of RNA and intracellular protein levels at subinhibitory concentrations. In some instances, derivatives of MC4 were also shown to attenuate the production of staphylococcal virulence factors in vitro, such as the exoproteins α-toxin and Panton–Valentine Leukocidin (PVL). Trends observed from quantitative PCR assays suggested that nusbiarylins elicited these effects possibly by acting via but not limited to the modulation of global regulatory pathways, such as the agr regulon, which coordinates the expression of S. aureus genes associated with virulence. Our findings encourage the continued development of more potent compounds within this novel family of bacterial transcription inhibitors.

Targeting ESKAPE pathogens with anti-infective medicinal plants from the Greater Mpigi region in Uganda

Antibiotic resistance poses one of the greatest threats to global health today; conventional drug therapies are becoming increasingly inefficacious and limited. We identified 16 medicinal plant species used by traditional healers for the treatment of infectious and inflammatory diseases in the Greater Mpigi region of Uganda. Extracts were evaluated for their ability to inhibit growth of clinical isolates of multidrug-resistant ESKAPE pathogens. Extracts were also screened for quorum quenching activity against S. aureus, including direct protein output assessment (δ-toxin), and cytotoxicity against human keratinocytes (HaCaT). Putative matches of compounds were elucidated via LC–FTMS for the best-performing extracts. These were extracts of Zanthoxylum chalybeum (Staphylococcus aureus: MIC: 16 μg/mL; Enterococcus faecium: MIC: 32 μg/mL) and Harungana madagascariensis (S. aureus: MIC: 32 μg/mL; E. faecium: MIC: 32 μg/mL) stem bark. Extracts of Solanum aculeastrum root bark and Sesamum calycinum subsp. angustifolium leaves exhibited strong quorum sensing inhibition activity against all S. aureus accessory gene regulator (agr) alleles in absence of growth inhibition (IC₅₀ values: 1–64 μg/mL). The study provided scientific evidence for the potential therapeutic efficacy of these medicinal plants in the Greater Mpigi region used for infections and wounds, with 13 out of 16 species tested being validated with in vitro studies.

Targeted and untargeted analysis of secondary metabolites to monitor growth and quorum sensing inhibition for methicillin-resistant Staphylococcus aureus (MRSA)

Drug resistant infections are an increasing problem world-wide, responsible for an estimated 700,000 annual mortalities. The use of antibiotics to treat such infections has resulted in the development of resistant bacterial pathogens such as methicillin-resistant Staphylococcus aureus (MRSA). One potential alternative strategy for treating drug resistant bacterial infections is to inhibit the production of toxins, thereby making the bacteria less harmful to the host, a so called "anti-virulence" approach. In MRSA, the agr quorum sensing system is one of the major regulators of toxin production, and quorum sensing inhibitors that target this system are a promising anti-virulence strategy. With this study, we developed a method that enables the activity of quorum sensing inhibitors to be measured using ultra-performance liquid chromatography coupled to mass spectrometry (UPLC-MS). This method is an improvement over existing methods because it can be employed to distinguish antimicrobial activity from quorum sensing inhibition activity based on the UPLC-MS data. This is possible by simultaneously tracking production of metabolites regulated by the agr quorum sensing system (AIP-I and formylated δ-toxin) and a metabolite that appears not to be agr regulated under the conditions of this study (aureusimine B). The newly developed method provides more nuanced indication of how metabolite production changes over time and in response to quorum sensing or growth inhibition than is possible with commonly employed spectroscopic methods.

In Vitro Activity of Essential Oils Against Planktonic and Biofilm Cells of Extended-Spectrum β-Lactamase (ESBL)/Carbapenamase-Producing Gram-Negative Bacteria Involved in Human Nosocomial Infections

The aim of this study was to analyze the antibacterial activity of four essential oils (EOs), Melaleuca alternifolia, Eucalyptus globulus, Mentha piperita, and Thymus vulgaris, in preventing the development and spread of extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae, metallo-beta-lactamase (MBL)-producing Pseudomonas aeruginosa and carbapenemase (KPC)-producing Klebsiella pneumoniae. A total of 60 strains were obtained from the stock collection from the Microbiology Laboratory of Hesperia Hospital, Modena, Italy. Twenty ESBL-producing E. coli, 5 K. pneumoniae, 13 KPC-producing K. pneumoniae, and 20 MBL-producing P. aeruginosa were cultured and reconfirmed as ESBL and carbapenamase producers. Polymerase chain reaction was used for the detection of genes responsible for antibiotic resistance (ESBL and KPC/MBL). Antibacterial activity of the EOs was determined using the agar disk diffusion assay, and minimal inhibitory concentrations (MICs) were also evaluated. Lastly, adhesion capability and biofilm formation on polystyrene and glass surfaces were studied in 24 randomly selected strains. M. alternifolia and T. vulgaris EOs showed the best antibacterial activity against all tested strains and, as revealed by agar disk diffusion assay, M. alternifolia was the most effective, even at low concentrations. This effect was also confirmed by MICs, with values ranging from 0.5 to 16 µg/mL and from 1 to 16 µg/mL, for M. alternifolia and T. vulgaris EOs, respectively. The EOs' antibacterial activity compared to antibiotics confirmed M. alternifolia EO as the best antibacterial agent. T. vulgaris EO also showed a good antibacterial activity with MICs lower than both reference antibiotics. Lastly, a significant anti-biofilm activity was observed for the two EOs (*P < 0.05 and **P < 0.01 for M. alternifolia and T. vulgaris EOs, respectively). A good antibacterial and anti-biofilm activity of M. alternifolia and T. vulgaris EOs against all selected strains was observed, thus demonstrating a future possible use of these EOs to treat infections caused by ESBL/carbapenemase-producing strains, even in association with antibiotics.

Apicidin Attenuates MRSA Virulence through Quorum-Sensing Inhibition and Enhanced Host Defense

Recurrent epidemics of drug-resistant Staphylococcus aureus illustrate the rapid lapse of antibiotic efficacy following clinical implementation. Over the last decade, community-associated methicillin-resistant S. aureus (MRSA) has emerged as a dominant cause of infections, and this problem is amplified by the hyper-virulent nature of these isolates. Herein, we report the discovery of a fungal metabolite, apicidin, as an innovative means to counter both resistance and virulence. Owing to its breadth and specificity as a quorum-sensing inhibitor, apicidin antagonizes all MRSA agr systems in a non-biocidal manner. In skin challenge experiments, the apicidin-mediated abatement of MRSA pathogenesis corresponds with quorum-sensing inhibition at in vivo sites of infection. Additionally, we show that apicidin attenuates MRSA-induced disease by potentiating innate effector responses, particularly through enhanced neutrophil accumulation and function at cutaneous challenge sites. Together, these results indicate that apicidin treatment represents a strategy to limit MRSA virulence and promote host defense.

Interference With Quorum-Sensing Signal Biosynthesis as a Promising Therapeutic Strategy Against Multidrug-Resistant Pathogens

Faced with the global health threat of increasing resistance to antibiotics, researchers are exploring interventions that target bacterial virulence factors. Quorum sensing is a particularly attractive target because several bacterial virulence factors are controlled by this mechanism. Furthermore, attacking the quorum-sensing signaling network is less likely to select for resistant strains than using conventional antibiotics. Strategies that focus on the inhibition of quorum-sensing signal production are especially attractive because the enzymes involved are expressed in bacterial cells but are not present in their mammalian counterparts. We review here various approaches that are being taken to interfere with quorum-sensing signal production via the inhibition of autoinducer-2 synthesis, PQS synthesis, peptide autoinducer synthesis, and N-acyl-homoserine lactone synthesis. We expect these approaches will lead to the discovery of new quorum-sensing inhibitors that can help to stem the tide of antibiotic resistance.

Novel antibacterial modalities against methicillin resistant Staphylococcus aureus derived from plants

Methicillin-resistant Staphylococcus aureus (MRSA) is a notorious bacterial pathogen that induces high mortality and morbidity. Due to the emergence of multiple resistance, antibiotic treatments are rapidly becoming ineffective for the related infections. Natural products, especially those derived from plants, have been proven to be effective agents with unique antibacterial properties through different mechanisms. This review interprets the resistance mechanisms of MRSA with the aim to conquer public health threat. Further, recent researches about plant antimicrobials that showed remarkable antibacterial activity against MRSA are recorded, including the crude plant extracts and purified plant-derived bioactive compounds. Novel anti-MRSA modalities of plant antimicrobials such as alteration in efflux pump, inhibition of pyruvate kinase, and disturbance of quorum sensing in MRSA are also summarized which may be promising alternatives to antibacterial drug development in future.

Antibacterial Properties of Medicinal Plants From Pakistan Against Multidrug-Resistant ESKAPE Pathogens

Local people in the Sudhnoti district of Pakistan share a rich practice of traditional medicine for the treatment of a variety of ailments. We selected nine plants from the Sudhnoti ethnopharmacological tradition used for the treatment of infectious and inflammatory disease. Our aim was to evaluate the in vitro anti-infective potential of extracts from these species against multidrug-resistant (MDR) ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa, and Enterobacter species) pathogens. Plant specimens were collected in the Sudhnoti district of Pakistan and vouchers deposited in Pakistan and the USA. Dried bulk specimens were ground into a fine powder and extracted by aqueous decoction and maceration in ethanol. Extracts were assessed for growth inhibitory activity against ESKAPE pathogens and biofilm and quorum sensing activity was assessed in Staphylococcus aureus. Cytotoxicity to human cells was assessed via a lactate dehydrogenase assay of treated human keratinocytes (HaCaTs). Four ethanolic extracts (Zanthoxylum armatum, Adiantum capillus-venaris, Artemisia absinthium, and Martynia annua) inhibited the growth of MDR strains of ESKAPE pathogens (IC₅₀: 256 μg mL^-1). All extracts, with the exception of Pyrus pashia and M. annua, exhibited significant quorum quenching in a reporter strain for S. aureus agr I. The ethanolic extract of Z. armatum fruits (Extract 1290) inhibited quorum sensing (IC₅₀ 32-256 μg mL^-1) in S. aureus reporter strains for agr I-III. The quorum quenching activity of extract 1290 was validated by detection of δ-toxin in the bacterial supernatant, with concentrations of 64-256 μg mL^-1 sufficient to yield a significant drop in δ-toxin production. None of the extracts inhibited S. aureus biofilm formation at sub-inhibitory concentrations for growth. All extracts were well tolerated by human keratinocytes (LD₅₀ ≥ 256 μg mL^-1). Chemical analysis of extract 1290 by liquid chromatography-Fourier transform mass spectrometry (LC-FTMS) revealed the presence of 29 compounds, including eight with putative structural matches. In conclusion, five out of the nine selected anti-infective medicinal plants exhibited growth inhibitory activity against at least one MDR ESKAPE pathogen at concentrations not harmful to human keratinocytes. Furthermore, Z. armatum was identified as a source of quorum quenching natural products and further bioassay-guided fractionation of this species is merited.

A Novel Antimicrobial Coating Represses Biofilm and Virulence-Related Genes in Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) has become an important cause of hospital-acquired infections worldwide. It is one of the most threatening pathogens due to its multi-drug resistance and strong biofilm-forming capacity. Thus, there is an urgent need for novel alternative strategies to combat bacterial infections. Recently, we demonstrated that a novel antimicrobial surface coating, AGXX®, consisting of micro-galvanic elements of the two noble metals, silver and ruthenium, surface-conditioned with ascorbic acid, efficiently inhibits MRSA growth. In this study, we demonstrated that the antimicrobial coating caused a significant reduction in biofilm formation (46%) of the clinical MRSA isolate, S. aureus 04-02981. To understand the molecular mechanism of the antimicrobial coating, we exposed S. aureus 04-02981 for different time-periods to the coating and investigated its molecular response via next-generation RNA-sequencing. A conventional antimicrobial silver coating served as a control. RNA-sequencing demonstrated down-regulation of many biofilm-associated genes and of genes related to virulence of S. aureus. The antimicrobial substance also down-regulated the two-component quorum-sensing system agr suggesting that it might interfere with quorum-sensing while diminishing biofilm formation in S. aureus 04-02981.

Targeting Virulence in Staphylococcus aureus by Chemical Inhibition of the Accessory Gene Regulator System In Vivo

Methicillin-resistant Staphylococcus aureus (MRSA) presents one of the most serious health concerns worldwide. The WHO labeled it as a “high-priority” pathogen in 2017, also citing the more recently emerged vancomycin-intermediate and -resistant strains.

Methicillin-resistant Staphylococcus aureus (MRSA) presents one of the most serious health concerns worldwide. The WHO labeled it as a “high-priority” pathogen in 2017, also citing the more recently emerged vancomycin-intermediate and -resistant strains. With the spread of antibiotic resistance due in large part to the selective pressure exerted by conventional antibiotics, the use of antivirulence strategies has been recurrently proposed as a promising therapeutic approach. In MRSA, virulence is chiefly controlled by quorum sensing (QS); inhibitors of QS are called quorum quenchers (QQ). In S. aureus, the majority of QS components are coded for by the accessory gene regulator (Agr) system. Although much work has been done to develop QQs against MRSA, only a few studies have progressed to in vivo models. Those studies include both prophylactic and curative models of infection as well as combination treatments with antibiotic. For most, high efficacy is seen at attenuating MRSA virulence and pathogenicity, with some studies showing effects such as synergy with antibiotics and antibiotic resensitization. This minireview aims to summarize and derive conclusions from the literature on the in vivo efficacy of QQ agents in MRSA infection models. In vitro data are also summarized to provide sufficient background on the hits discussed. On the whole, the reported in vivo effects of the reviewed QQs against MRSA represent positive progress at this early stage in drug development. Follow-up studies that thoroughly examine in vitro and in vivo activity are needed to propel the field forward and set the stage for lead optimization.

Coagulase-Negative Staphylococcal Strain Prevents Staphylococcus aureus Colonization and Skin Infection by Blocking Quorum Sensing

Coagulase-negative staphylococci (CoNS) and Staphylococcus aureus are part of the natural flora of humans and other mammals. We found that spent media from the CoNS species Staphylococcus caprae can inhibit agr-mediated quorum sensing by all classes of S. aureus. A biochemical assessment of the inhibitory activity suggested that the S. caprae autoinducing peptide (AIP) was responsible, and mass spectrometric analysis identified the S. caprae AIP as an eight-residue peptide (YSTCSYYF). Using a murine model of intradermal MRSA infection, the therapeutic efficacy of synthetic S. caprae AIP was evident by a dramatic reduction in both dermonecrotic injury and cutaneous bacterial burden relative to controls. Competition experiments between S. caprae and MRSA demonstrated a significant reduction in MRSA burden using murine models of both skin colonization and intradermal infection. Our findings indicate that important interactions occur between commensals that can impact disease outcomes and potentially shape the composition of the natural flora.

The Chemical and Antibacterial Evaluation of St. John's Wort Oil Macerates Used in Kosovar Traditional Medicine

Hypericum perforatum L. (Hypericaceae), or St. John's Wort, is a well-known medicinal herb often associated with the treatment of anxiety and depression. Additionally, an oil macerate (Oleum Hyperici) of its flowering aerial parts is widely used in traditional medicine across the Balkans as a topical wound and ulcer salve. Other studies have shown that Oleum Hyperici reduces both wound size and healing time. Of its active constituents, the naphthodianthrone hypericin and phloroglucinol hyperforin are effective antibacterial compounds against various Gram-positive bacteria. However, hyperforin is unstable with light and heat, and thus should not be present in the light-aged oil macerate. Additionally, hypericin can cause phototoxic skin reactions if ingested or absorbed into the skin. Therefore, the established chemistry presents a paradox for this H. perforatum oil macerate: the hyperforin responsible for the antibacterial bioactivity should degrade in the sunlight as the traditional oil is prepared; alternately, if hypericin is present in established bioactive levels, then the oil macerate should cause photosensitivity, yet none is reported. In this research, various extracts of H. perforatum were compared to traditional oil macerates with regards to chemical composition and antibacterial activity (inhibition of growth, biofilm formation, and quorum sensing) vs. several strains of Staphylococcus aureus in order to better understand this traditional medicine. It was found that four Kosovar-crafted oil macerates were effective at inhibiting biofilm formation (MBIC₅₀ active range of 0.004-0.016% v/v), exhibited moderate inhibition of quorum sensing (QSIC₅₀ active range of 0.064-0.512% v/v), and contained detectable amounts of hyperforin, but not hypericin. Overall, levels of hypericin were much higher in the organic extracts, and these also exhibited more potent growth inhibitory activity. In conclusion, these data confirm that oil macerates employed in traditional treatments of skin infection lack the compound credited with phototoxic reactions in H. perforatum use and exhibit anti-biofilm and modest quorum quenching effects, rather than growth inhibitory properties against S. aureus.

Virulence Inhibitors from Brazilian Peppertree Block Quorum Sensing and Abate Dermonecrosis in Skin Infection Models

Widespread antibiotic resistance is on the rise and current therapies are becoming increasingly limited in both scope and efficacy. Methicillin-resistant Staphylococcus aureus (MRSA) represents a major contributor to this trend. Quorum sensing controlled virulence factors include secreted toxins responsible for extensive damage to host tissues and evasion of the immune system response; they are major contributors to morbidity and mortality. Investigation of botanical folk medicines for wounds and infections led us to study Schinus terebinthifolia (Brazilian Peppertree) as a potential source of virulence inhibitors. Here, we report the inhibitory activity of a flavone rich extract "430D-F5" against all S. aureus accessory gene regulator (agr) alleles in the absence of growth inhibition. Evidence for this activity is supported by its agr-quenching activity (IC50 2-32 μg mL-1) in transcriptional reporters, direct protein outputs (α-hemolysin and δ-toxin), and an in vivo skin challenge model. Importantly, 430D-F5 was well tolerated by human keratinocytes in cell culture and mouse skin in vivo; it also demonstrated significant reduction in dermonecrosis following skin challenge with a virulent strain of MRSA. This study provides an explanation for the anti-infective activity of peppertree remedies and yields insight into the potential utility of non-biocide virulence inhibitors in treating skin infections.

Fluorescent sensing arrays for cations and anions

A review of fluorescent sensing arrays for anions and cations, highlighting promising strategies and directions for future research.

Prototype Development of the Intelligent Hydrogel Wound Dressing and Its Efficacy in the Detection of Model Pathogenic Wound Biofilms

The early detection of wound infection in situ can dramatically improve patient care pathways and clinical outcomes. There is increasing evidence that within an infected wound the main bacterial mode of living is a biofilm: a confluent community of adherent bacteria encased in an extracellular polymeric matrix. Here we have reported the development of a prototype wound dressing, which switches on a fluorescent color when in contact with pathogenic wound biofilms. The dressing is made of a hydrated agarose film in which the fluorescent dye containing vesicles were mixed with agarose and dispersed within the hydrogel matrix. The static and dynamic models of wound biofilms, from clinical strains of Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Enterococcus faecalis, were established on nanoporous polycarbonate membrane for 24, 48, and 72 h, and the dressing response to the biofilms on the prototype dressing evaluated. The dressing indicated a clear fluorescent/color response within 4 h, only observed when in contact with biofilms produced by a pathogenic strain. The sensitivity of the dressing to biofilms was dependent on the species and strain types of the bacterial pathogens involved, but a relatively higher response was observed in strains considered good biofilm formers. There was a clear difference in the levels of dressing response, when dressings were tested on bacteria grown in biofilm or in planktonic cultures, suggesting that the level of expression of virulence factors is different depending of the growth mode. Colorimetric detection on wound biofilms of prevalent pathogens (S. aureus, P. aeruginosa, and E. faecalis) is also demonstrated using an ex vivo porcine skin model of burn wound infection.

Hybrid Quadrupole-Orbitrap mass spectrometry for quantitative measurement of quorum sensing inhibition

Drug resistant bacterial infections cause significant morbidity and mortality worldwide, and new strategies are needed for the treatment of these infections. The anti-virulence approach, which targets non-essential virulence factors in bacteria, has been proposed as one way to combat the problem of antibiotic resistance. Virulence in methicillin-resistant Staphylococcus aureus (MRSA) and many other Gram-positive bacterial pathogens is controlled by the quorum sensing system. Thus, there is excellent therapeutic potential for compounds that target this system. With this project, we have developed and validated a novel approach for measuring quorum sensing inhibition in vitro. Ultraperformance liquid chromatography coupled to mass spectrometry (UPLC-MS) was employed to directly measure one of the important outputs of the quorum sensing system in MRSA, auto-inducing peptide I (AIP I) in bacterial cultures. The method for AIP detection was validated and demonstrated limits of detection and quantification of range of 0.0035μM and 0.10μM, respectively. It was shown that the known quorum sensing inhibitor ambuic acid inhibited AIP I production by a clinically relevant strain of MRSA, with an IC50 value of 2.6±0.2μM. The new method performed similarly to previously published methods using GFP reporter assays, but has the advantage of being applicable without the need for engineering of a reporter strain. Additionally, the mass spectrometry-based method could be applicable in situations where interference by the inhibitor prevents the application of fluorescence-based methods.

Impact of Environmental Cues on Staphylococcal Quorum Sensing and Biofilm Development

Staphylococci are commensal bacteria that colonize the epithelial surfaces of humans and many other mammals. These bacteria can also attach to implanted medical devices and develop surface-associated biofilm communities that resist clearance by host defenses and available chemotherapies. These communities are often associated with persistent staphylococcal infections that place a tremendous burden on the healthcare system. Understanding the regulatory program that controls staphylococcal biofilm development, as well as the environmental conditions that modulate this program, has been a focal point of research in recent years. A central regulator controlling biofilm development is a peptide quorum-sensing system, also called the accessory gene regulator or agr system. In the opportunistic pathogen Staphylococcus aureus, the agr system controls production of exo-toxins and exo-enzymes essential for causing infections, and simultaneously, it modulates the ability of this pathogen to attach to surfaces and develop a biofilm, or to disperse from the biofilm state. In this review, we explore advances on the interconnections between the agr quorum-sensing system and biofilm mechanisms, and topics covered include recent findings on how different environmental conditions influence quorum sensing, the impact on biofilm development, and ongoing questions and challenges in the field. As our understanding of the quorum sensing and biofilm interconnection advances, there are growing opportunities to take advantage of this knowledge and develop therapeutic approaches to control staphylococcal infections.

Castanea sativa (European Chestnut) Leaf Extracts Rich in Ursene and Oleanene Derivatives Block Staphylococcus aureus Virulence and Pathogenesis without Detectable Resistance

The Mediterranean is home to a rich history of medical traditions that have developed under the influence of diverse cultures over millennia. Today, many such traditions are still alive in the folk medical practices of local people. Investigation of botanical folk medicines used in the treatment of skin and soft tissue infections led us to study Castanea sativa (European Chestnut) for its potential antibacterial activity. Here, we report the quorum sensing inhibitory activity of refined and chemically characterized European Chestnut leaf extracts, rich in oleanene and ursene derivatives (pentacyclic triterpenes), against all Staphylococcus aureus accessory gene regulator (agr) alleles. We present layers of evidence of agr blocking activity (IC₅₀ 1.56–25 μg mL^-1), as measured in toxin outputs, reporter assays hemolytic activity, cytotoxicity studies, and an in vivo abscess model. We demonstrate the extract’s lack of cytotoxicity to human keratinocytes and murine skin, as well as lack of growth inhibitory activity against S. aureus and a panel of skin commensals. Lastly, we demonstrate that serial passaging of the extract does not result in acquisition of resistance to the quorum quenching composition. In conclusion, through disruption of quorum sensing in the absence of growth inhibition, this study provides insight into the role that non-biocide inhibitors of virulence may play in future antibiotic therapies.