Quinone skeleton as a new class of irreversible inhibitors against Staphylococcus aureus sortase A

The Complete Assessment of Small Molecule and Peptidomimetic Inhibitors of Sortase A Towards Antivirulence Treatment

This review covers the most recent advances in the development of inhibitors for the bacterial enzyme sortase A (SrtA). Sortase A (SrtA) is a critical virulence factor, present ubiquitously in Gram-positive bacteria of which many are pathogenic. Sortases are key enzymes regulating bacterial adherence to host cells, by anchoring extracellular matrix-binding proteins to the bacterial outer cell wall. By targeting virulence factors, effective treatment can be achieved, without inducing antibiotic resistance to the treatment. This is a potentially more sustainable, long-term approach to treating bacterial infections, including ones that display multiple resistance to current therapeutics. There are many promising approaches available for SrtA inhibition, some of which have the potential to advance into further clinical development, with peptidomimetic and in vivo active small molecules being among the most promising. There are currently no approved drugs on the market targeting SrtA, despite its promise, adding to the relevance of this review article, as it extends to the pharmaceutical industry additionally to academic researchers.

Fraxetin Targeting to Sortase A Decreases the Pathogenicity of Streptococcus agalactiae to Nile Tilapia

Sortase A (SrtA) is responsible for anchoring surface proteins to the cell wall, and has been identified as a promising target developing anti-infective drugs of Gram-positive bacteria. The aim of the study was to identify inhibitors of Streptococcus agalactiae (S. agalactiae) SrtA from natural compounds to overcome the spread of antibiotic resistance in aquaculture. Here, we found that the MIC of fraxetin against S. agalactiae was higher than 256 μg/mL, indicating that fraxetin had no anti- S. agalactiae activity. But fraxetin could dose-dependently decrease the activity of SrtA in vitro at concentrations ranging between 4-32 μg/mL by a fluorescence resonance energy transfer (FRET) assay. Moreover, the inhibition of SrtA by fraxetin decreased the anchoring of surface proteins with the LPXTG motif to the cell wall by detecting the immunofluorescence change of serine-rich repeat protein 1 (Srr1) on the bacterial cell surface. The results of fibronectin binding and cell adhesion assays indicated that fraxetin could significantly decrease the adhesion ability of S. agalactiae in a dose-dependent manner. The results were further proven by immunofluorescence staining. Animal challenge results showed that treatment with fraxetin could reduce the mortality of tilapia infected with S. agalactiae to 46.67%, indicating that fraxetin could provide a significant amount of protection to tilapia by inactivating SrtA. Taken together, these findings provided a novel inhibitor of S. agalactiae SrtA and a promising candidate for treating S. agalactiae infections in aquaculture.

Structure-Guided Design, Synthesis, and Antivirulence Assessment of Covalent Staphylococcus aureus Sortase A Inhibitors

Sortase A (SrtA) is a membrane-associated cysteine transpeptidase required for bacterial virulence regulation and anchors surface proteins to cell wall, thereby assisting biofilm formation. SrtA is targeted in antivirulence treatments against Gram-positive bacterial infections. However, the development of potent small-molecule SrtA inhibitors is constrained owing to the limited understanding of the mode of action of inhibitors in the SrtA binding pocket. Herein, we designed and synthesized a novel class of covalent SrtA inhibitors based on the binding mode detailed in the X-ray crystal structure of the ML346/Streptococcus pyogenes SrtA complex. ML346 analog Y40 exhibited 2-fold increased inhibitory activity on Staphylococcus aureus SrtA and showed superior inhibitory effects on biofilm formation in vitro. Y40 protected Galleria mellonella larvae fromS. aureusinfections in vivo while minimally attenuating staphylococcal growth in vitro. Our study indicates that the covalent SrtA inhibitor Y40 is an antivirulence agent that is effective againstS. aureusinfections.

Exploring the modulatory impact of isosakuranetin on Staphylococcus aureus: Inhibition of sortase A activity and α-haemolysin expression

The ubiquity of methicillin-resistant Staphylococcus aureus (MRSA) and the mounting prevalence of antibiotic resistance necessitate the identification of novel therapeutic approaches to reduce the selective pressure of antibiotics. Targeting bacterial virulence factors, such as the pivotal Sortase A (SrtA) in S. aureus for adhesion and invasion, and the salient toxin α-Hemolysin (Hla), offers a sophisticated approach to attenuate pathogenicity without bacterial elimination. Herein, we report the discovery of a flavonoid, isosakuranetin, which inhibits the activity of S. aureus SrtA. A fluorescence resonance energy transfer assay revealed that isosakuranetin exhibited a low IC50 of 21.20 μg/mL. Furthermore, isosakuranetin significantly inhibited SrtA-related virulence properties, such as bacterial adhesion to fibrinogen, biofilm formation, and invasion of A549 cells. We employed fluorescence quenching and molecular docking to determine the interactions between isosakuranetin and SrtA, revealing the key amino acid sites for binding. Importantly, isosakuranetin inhibited the haemolytic activity of S. aureus in vitro at a concentration of 32 μg/mL. Moreover, isosakuranetin effectively suppressed the transcription and expression of Hla in a dose-dependent manner and regulated the transcription of RNAIII, the upstream operator of Hla. Notably, isosakuranetin demonstrated in vivo efficacy in a mouse model of S. aureus-induced pneumonia by significantly improving survival rates and reducing lung damage. This is a valuable finding, as isosakuranetin's dual inhibitory effects on SrtA and haemolytic activity, as well as its anti-virulence activity against MRSA, make it an excellent candidate for therapeutic development.

Inhibitory Effect of Non-Saccharomyces Starmerella bacillaris CC-PT4 Isolated from Grape on MRSA Growth and Biofilm

Methicillin-resistant Staphylococcus aureus (MRSA) is a notorious pathogen with biofilm-forming and drug-resistant properties that make it difficult to eradicate. In this study, the inhibition of MRSA (ATCC 43300) by Starmerella bacillaris CC-PT4 (CGMCC No. 23573) was evaluated. The results showed that the inhibition of MRSA growth and biofilm was caused by S. bacillaris CC-PT4 cell-free supernatant (CFS). The CFS of S. bacillaris CC PT4 at different times can effectively inhibit the formation of MRSA biofilm, remove the preformed biofilm, and down-regulate the related genes that promote the formation of biofilm. Afterwards, untargeted metabolomics was performed to analyze the CFS of S. bacillaris CC-PT4. Several molecules with antibacterial and inhibitory biofilm effects from the CFS were found, one of which, 2-amino-1-phenylethanol (APE), has not been reported to have antiMRSA ability before. In this study, molecular docking analysis and in vitro experiments were used to verify the function of APE to inhibit MRSA. These results indicate that S. bacillaris CC-PT4 CFS can effectively inhibit MRSA which has potential application value in controlling MRSA.

An insight into novel therapeutic potentials of taxifolin

Taxifolin is a flavonoid compound, originally isolated from the bark of Douglas fir trees, which is often found in foods such as onions and olive oil, and is also used in commercial preparations, and has attracted the interest of nutritionists and medicinal chemists due to its broad range of health-promoting effects. It is a powerful antioxidant with excellent antioxidant, anti-inflammatory, anti-microbial and other pharmacological activities. This review focuses on the breakthroughs in taxifolin for the treatment of diseases from 2019 to 2022 according to various systems of the human body, such as the nervous system, immune system, and digestive system, and on the basis of this review, we summarize the problems of current research and try to suggest solutions and future research directions.

Development of new spiro[1,3]dithiine-4,11'-indeno[1,2-b]quinoxaline derivatives as S. aureus Sortase A inhibitors and radiosterilization with molecular modeling simulation

Multi-drug resistant microbes have become a severe threat to human health and arise a worldwide concern. A total of fifteen spiro-1,3-dithiinoindenoquinoxaline derivatives 2-7 were synthesized and evaluated for their biological activities against five standard and MDRB pathogens. The MIC and MBC/MFC for the most active derivatives were determined in vitro via broth microdilution assay. These derivatives showed significant activity against the tested strains with microbicidal behavior, with compound 4b as the most active compound (MIC range between 0.06 and 0.25 µg/mL for bacteria strains and MIC = 0.25 µg/mL for C. albicans). The most active spiro-1,3-dithiinoindenoquinoxaline derivatives were able to inhibit the activity of SrtA with IC₅₀ values ranging from 22.15 ± 0.4 µM to 37.12 ± 1.4 µM. In addition, the active spiro-1,3-dithiinoindenoquinoxaline attenuated the in vitro virulence-related phenotype of SrtA by weakening the adherence of S. aureus to fibrinogen and reducing the biofilm formation. Surprisingly, compound 4b revealed potent SrtA inhibitory activity with IC₅₀ = 22.15 µM, inhibiting the adhesion of S. aureus with 39.22 ± 0.15 % compared with untreated 9.43 ± 1.52 %, and showed a reduction in the biofilm biomass of S. aureus with 32.27 ± 0.52 %. We further investigated the effect of gamma radiation as a sterilization method on the microbial load and found that a dose of 5 kGy was sufficient to eradicate the microbial load. The quantum chemical studies exhibited that the tested derivatives have a small energy band gap (ΔE = -2.95 to -3.61 eV) and therefore exert potent bioactivity by interacting with receptors more stabilizing.

Covalent sortase A inhibitor ML346 prevents Staphylococcus aureus infection of Galleria mellonella

The housekeeping sortase A (SrtA), a membrane-associated cysteine transpeptidase, is responsible for anchoring surface proteins to the cell wall peptidoglycan in Gram-positive bacteria. This process is essential for the regulation of bacterial virulence and pathogenicity. Therefore, SrtA is considered to be an ideal target for antivirulence therapy. In this study, we report that ML346, a compound with a barbituric acid and cinnamaldehyde scaffold, functions as an irreversible inhibitor of Staphylococcus aureus SrtA (SaSrtA) and Streptococcus pyogenes SrtA (SpSrtA) in vitro at low micromolar concentrations. According to our X-ray crystal structure of the SpSrtAΔN81/ML346 complex (Protein Data Bank ID: 7V6K), ML346 covalently modifies the thiol group of Cys208 in the active site of SpSrtA. Importantly, ML346 significantly attenuated the virulence phenotypes of S. aureus and exhibited inhibitory effects on Galleria mellonella larva infection caused by S. aureus. Collectively, our results indicate that ML346 has potential for development as a covalent antivirulence agent for treating S. aureus infections, including methicillin-resistant S. aureus.

Identification of Novel Antistaphylococcal Hit Compounds Targeting Sortase A

Staphylococcus aureus (S. aureus) is a causative agent of many hospital- and community-acquired infections with the tendency to develop resistance to all known antibiotics. Therefore, the development of novel antistaphylococcal agents is of urgent need. Sortase A is considered a promising molecular target for the development of antistaphylococcal agents. The main aim of this study was to identify novel sortase A inhibitors. In order to find novel antistaphylococcal agents, we performed phenotypic screening of a library containing 15512 compounds against S. aureus ATCC43300. The molecular docking of hits was performed using the DOCK program and 10 compounds were selected for in vitro enzymatic activity inhibition assay. Two inhibitors were identified, N,N-diethyl-N′-(5-nitro-2-(quinazolin-2-yl)phenyl)propane-1,3-diamine (1) and acridin-9-yl-(1H-benzoimidazol-5-yl)-amine (2), which decrease sortase A activity with IC₅₀ values of 160.3 µM and 207.01 µM, respectively. It was found that compounds 1 and 2 possess antibacterial activity toward 29 tested multidrug resistant S. aureus strains with MIC values ranging from 78.12 to 312.5 mg/L. These compounds can be used for further structural optimization and biological research.

Chemical Biology of Sortase A Inhibition: A Gateway to Anti-infective Therapeutic Agents

Staphylococcus aureus is the leading cause of hospital-acquired infections. The enzyme sortase A, present on the cell surface of S. aureus, plays a key role in bacterial virulence without affecting the bacterial viability. Inhibition of sortase A activity offers a powerful but clinically less explored therapeutic strategy, as it offers the possibility of not inducing any selective pressure on the bacteria to evolve drug-resistant strains. In this Perspective, we offer a chemical space narrative for the design of sortase A inhibitors, as delineated into three broad domains: peptidomimetics, natural products, and synthetic small molecules. This provides immense opportunities for medicinal chemists to alleviate the ever-growing crisis of antibiotic resistance.

Taxifolin, an Inhibitor of Sortase A, Interferes With the Adhesion of Methicillin-Resistant Staphylococcal aureus

The evolution and spread of methicillin-resistant Staphylococcus aureus (MRSA) poses a significant hidden risk to human public health. The majority of antibiotics used clinically have become mostly ineffective, and so the development of novel anti-infection strategies is urgently required. Since Staphylococcus aureus (S. aureus) cysteine transpeptidase sortase A (SrtA) mediates the surface-anchoring of proteins to its surface, compounds that inhibit SrtA are considered potential antivirulence treatments. Herein, we report on the efficacy of the potent SrtA inhibitor taxifolin (Tax), a flavonoid compound isolated from Chinese herbs. It was able to reversibly block the activity of SrtA with an IC₅₀ of 24.53 ± 0.42 μM. Tax did not display toxicity toward mammalian cells or S. aureus at a concentration of 200 μM. In addition, Tax attenuated the virulence-related phenotype of SrtA in vitro by decreasing the adherence of S. aureus, reducing the formation of a biofilm, and anchoring of S. aureus protein A on its cell wall. The mechanism of the SrtA-Tax interaction was determined using a localized surface plasmon resonance assay. Subsequent mechanistic studies confirmed that Asp-170 and Gln-172 were the principal sites on SrtA with which it binds to Tax. Importantly, in vivo experiments demonstrated that Tax protects mice against pneumonia induced by lethal doses of MRSA, significantly improving their survival rate and reducing the number of viable S. aureus in the lung tissue. The present study indicates that Tax is a useful pioneer compound for the development of novel agents against S. aureus infections.

Targeting Bacterial Sortases in Search of Anti-Virulence Therapies with Low Risk of Resistance Development

Increasingly ineffective antibiotics and rapid spread of multi- and pan-resistant bacteria represent a global health threat; hence, the need of developing new antimicrobial medicines. A first step in this direction is identifying new molecular targets, such as virulence factors. Sortase A represents a virulence factor essential for the pathogenesis of Gram-positive pathogens, some of which have a high risk for human health. We present here an exhaustive collection of sortases inhibitors grouped by relevant chemical features: vinyl sulfones, 3-aryl acrylic acids and derivatives, flavonoids, naphtoquinones, anthraquinones, indoles, pyrrolomycins, isoquinoline derivatives, aryl β-aminoethyl ketones, pyrazolethiones, pyridazinones, benzisothiazolinones, 2-phenyl-benzoxazole and 2-phenyl-benzofuran derivatives, thiadiazoles, triazolothiadiazoles, 2-(2-phenylhydrazinylidene)alkanoic acids, and 1,2,4-thiadiazolidine-3,5-dione. This review focuses on highlighting their structure–activity relationships, using the half maximal inhibitory concentration (IC₅₀), when available, as an indicator of each compound effect on a specific sortase. The information herein is useful for acquiring knowledge on diverse natural and synthetic sortases inhibitors scaffolds and for understanding the way their structural variations impact IC₅₀. It will hopefully be the inspiration for designing novel effective and safe sortase inhibitors in order to create new anti-infective compounds and to help overcoming the current worldwide antibiotic shortage.

Sortase A (SrtA) inhibitors as an alternative treatment for superbug infections

Virulence factor, sortase A (SrtA), has crucial roles in the pathogenesis of Gram-positive superbugs. SrtA is a bacterial cell membrane enzyme that anchors crucial virulence factors to the cell wall surface of Gram-positive bacteria. SrtA is not necessary for bacterial growth and viability and is conveniently accessible in the cell membrane; therefore, it is an ideal target for antivirulence drug development. In this review, we focus on antimicrobial resistance (AMR)-expressing bacteria and SrtA as a potential target for overcoming AMR. The mechanism of action of SrtA and its inhibition by various types of inhibitors, such as synthetic small molecules, peptides, and natural products, are provided. Future SrtA research perspectives for alternative drug development to antibiotics are also proposed.

Eriodictyol as a Potential Candidate Inhibitor of Sortase A Protects Mice From Methicillin-Resistant Staphylococcus aureus-Induced Pneumonia

New anti-infective approaches are urgently needed to control multidrug-resistant (MDR) pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA). Sortase A (SrtA) is a membrane-bound cysteine transpeptidase that plays an essential role in the catalysis of covalent anchoring of surface proteins to the cell wall of Staphylococcus aureus (S. aureus). The present study reports identification of a flavonoid, eriodictyol, as a reversible inhibitor of SrtA with an IC50 of 2.229 ± 0.014 μg/mL that can be used as an innovative means to counter both resistance and virulence. The data indicated that eriodictyol inhibited the adhesion of the bacteria to fibrinogen and reduced the formation of biofilms and anchoring of staphylococcal protein A (SpA) on the cell wall. The results of fluorescence quenching experiments demonstrated a strong interaction between eriodictyol and SrtA. Subsequent mechanistic studies revealed that eriodictyol binds to SrtA by interacting with R197 amino acid residue. Importantly, eriodictyol reduced the adhesion-dependent invasion of A549 cells by S. aureus and showed a good therapeutic effect in a model of mouse pneumonia induced by S. aureus. Overall, the results indicated that eriodictyol can attenuate MRSA virulence and prevent the development of resistance by inhibiting SrtA, suggesting that eriodictyol may be a promising lead compound for the control of MRSA infections.

Tideglusib and Its Analogues As Inhibitors of Staphylococcus aureus SrtA

Sortase A (SrtA) anchors surface proteins to the cell wall envelope, and it has attracted increasing interesting as a potential antivirulence target. Several small-molecule inhibitors for SrtA have been developed, but target validation remains largely underexplored. Herein, we report a new class of SrtA inhibitors that supports antivirulence therapy through small-molecule targeting of SrtA. Tideglusib (TD), a drug candidate for myotonic dystrophy, was outstanding in high-throughput screening. A concise synthetic route quickly provided TD analogues, and the structure-activity relationships for SrtA inhibition have been established from those analogues. Several compounds largely retained the in vitro potency and exhibited a better solubility than TD. Additionally, TD attenuated virulence-related phenotypes in vitro and protected mice against lethal S. aureus USA300 bacteremia. Our study indicates that TD and its analogues could be new candidates as SrtA inhibitors with potential in the development of new antivirulence agents.

Capsaicin derivatives with nitrothiophene substituents: Design, synthesis and antibacterial activity against multidrug-resistant S. aureus

To address the emergency caused by multi-drug resistant Staphylococcus aureus, a series of novel capsaicin derivatives with nitrothiophene substituents have been designed and evaluated for the antibacterial activities against S. aureus Newman and multidrug-resistant strains (NRS-1, NRS-70, NRS-100, NRS-108, and NRS-271). The structure-activity relationship was further revealed. Compound 13c, 13f, and 13g were highly active against staphylococcal growth, with minimal inhibition concentration (MIC) values of 0.39-1.56 μg/mL. The oxadiazole-derived compound 21, a bioisostere of ester 13f, is the most potent candidate for anti-growth of five multidrug-resistant S. aureus strains with MICs of 0.20-0.78 μg/mL, which is more active compared with vancomycin in vitro. Notably, these anti-staphylococcal compounds are much less cytotoxic to the normal kidney epithelial cell line (HK293T).

Discovery of natural naphthoquinones as sortase A inhibitors and potential anti-infective solutions against Staphylococcus aureus

Three natural naphthoquinones were screened to find new anti-virulence agents as inhibitors against sortase A from Staphylococcus aureus (SaSrtA) by quantifying the increase in fluorescence intensity upon substrate cleavage at various concentrations. The 5-hydroxy-1,4-naphthalenedione derivatives, juglone and plumbagin, demonstrated a potent inhibitory effect, with IC₅₀ values of 1.78 μM, respectively, 16.71 μM. The related 2-hydroxy-1,4-naphthalenedione derivative, lawsone, demonstrated the selectivity of the chemical scaffold having no significant effect on SaSrtA. The experimental assay was reinforced by molecular docking experiments, antimicrobial, and toxicological studies. Molecular docking studies and the electrophilic character analysis suggest bonding to the enzyme active cysteine residue by a Michael addition reaction. None of the compounds had a significant effect on the concentration of total thiol proteins in the Daphnia magna toxicological assay after 24 hr exposure. Juglone and plumbagin moderately inhibited biofilm formation with no significant effect on bacterial growth of S. aureus, Enterococcus faecalis, and Staphylococcus epidermidis, indicating a selective anti-virulence profile.

Structural Insight into the Mechanism of Staphylococcus aureus Stp1 Phosphatase

Staphylococcus aureus Stp1, which belongs to the bacterial metal-dependent protein phosphatase (PPM) family, is a promising candidate for antivirulence targeting. How Stp1 recognizes the phosphorylated peptide remains unclear, however. In order to investigate the recognition mechanism of Stp1 in depth, we have determined a series of crystal structures of S. aureus Stp1 in different states and the structural complex of Stp1 bound with a phosphorylated peptide His12. Different phosphorylated peptides, including MgrA- and GraR-derived phosphopeptides, are substrates of Stp1, which supports the function of Stp1 as a selective Ser/Thr phosphatase. In addition, interestingly, the crystal structures of R161-Stp1 variants combined with the biochemical activity validations have uncovered that R161 residue plays a key role to control the conformation switches of the flap domain in order to facilitate substrate binding and the dephosphorylation process. Our findings provide crucial structural insight into the molecular mechanism of S. aureus Stp1 phosphatase and reveal the phosphorylated peptides for biochemistry study and inhibitor screening of Stp1.

Inhibitory effect of a natural phenolic compound, 3-p-trans-coumaroyl-2-hydroxyquinic acid against the attachment phase of biofilm formation of Staphylococcus aureus through targeting sortase A

3-p-trans-Coumaroyl-2-hydroxyquinic acid (CHQA), a natural phenolic compound, prevented Staphylococcus aureus biofilm formation due to the inhibition of the initial attachment stage of biofilm development by targeting sortase A.

Conversion of an amide to a high-energy thioester by Staphylococcus aureus sortase A is powered by variable binding affinity for calcium

Thioesters are key intermediates in biology, which often are generated from less energy-rich amide precursors. Staphylococcus aureus sortase A (SrtA) is an enzyme widely used in biotechnology for peptide ligation. The reaction proceeds in two steps, where the first step involves the conversion of an amide bond of substrate peptide into a thioester intermediate with the enzyme. Here we show that the free energy required for this step is matched by an about 30-fold increase in binding affinity of a calcium ion at the calcium binding site of SrtA, which is remote from the thioester bond. The magnitude of this allosteric effect highlights the importance of calcium for the activity of SrtA. The increase in calcium binding affinity upon binding of substrate not only achieves catalytic formation of an energy-rich intermediate in the absence of nucleotide triphosphates or any tight non-covalent enzyme-substrate interactions, but is also accompanied by accumulation of the labile thioester intermediate, which makes it directly observable in nuclear magnetic resonance (NMR) spectra.