Antiinfective therapy with a small molecule inhibitor of Staphylococcus aureus sortase

Structural and functional insights of sortases and their interactions with antivirulence compounds

Sortase proteins play a crucial role as integral membrane proteins in anchoring bacterial surface proteins by recognizing them through a Cell-Wall Sorting (CWS) motif and cleaving them at specific sites before initiating pilus assembly. Both sortases and their substrate proteins are major virulence factors in numerous Gram-positive pathogens, making them attractive targets for antimicrobial intervention. Recognizing the significance of virulence proteins, a comprehensive exploration of their structural and functional characteristics is essential to enhance our understanding of pilus assembly in diverse Gram-positive bacteria. Therefore, this review article discusses the structural features of different classes of sortases and pilin proteins, primarily serving as substrates for sortase-assembled pili. Moreover, it thoroughly examines the molecular-level interactions between sortases and their inhibitors, providing insights from both structural and functional perspectives. In essence, this review article will provide a contemporary and complete understanding of both sortase pathways and various strategies to target them effectively to counteract the virulence.

Halenaquinol Blocks Staphylococcal Protein A Anchoring on Cell Wall Surface by Inhibiting Sortase A in Staphylococcus aureus

Sortase A (SrtA) is a cysteine transpeptidase that binds to the periplasmic membrane and plays a crucial role in attaching surface proteins, including staphylococcal protein A (SpA), to the peptidoglycan cell wall. Six pentacyclic polyketides (1-6) were isolated from the marine sponge Xestospongia sp., and their structures were elucidated using spectroscopic techniques and by comparing them to previously reported data. Among them, halenaquinol (2) was found to be the most potent SrtA inhibitor, with an IC50 of 13.94 μM (4.66 μg/mL). Semi-quantitative reverse transcription PCR data suggest that halenaquinol does not inhibit the transcription of srtA and spA, while Western blot analysis and immunofluorescence microscopy images suggest that it blocks the cell wall surface anchoring of SpA by inhibiting the activity of SrtA. The onset and magnitude of the inhibition of SpA anchoring on the cell wall surface in S. aureus that has been treated with halenaquinol at a value 8× that of the IC50 of SrtA are comparable to those for an srtA-deletion mutant. These findings contribute to the understanding of the mechanism by which marine-derived pentacyclic polyketides inhibit SrtA, highlighting their potential as anti-infective agents targeting S. aureus virulence.

Harnessing the combined effect of antivirulence agent trans-chalcone with bactericidal curcumin against sortase A enzyme to tackle Gram-positive bacterial infections

Gram-positive bacteria are responsible for a wide range of infections in humans. In most Gram-positive bacteria, sortase A plays a significant role in attaching virulence factors to the bacteria's cell wall. These cell surface proteins play a significant role in virulence and pathogenesis. Even though antibiotics are available to treat these infections, there is a continuous search for an alternative strategy due to an increase in antibiotic resistance. Thus, using anti-sortase drugs to combat these bacterial infections may be a promising approach. Here, we describe a method for targeting Gram-positive bacterial infection by combining curcumin and trans-chalcone as sortase A inhibitors. We have used curcumin and trans-chalcone alone and in combination as a sortase A inhibitor. We have seen ~78%, ~43%, and ~94% inhibition when treated with curcumin, trans-chalcone, and a combination of both compounds, respectively. The compounds have also shown a significant effect on biofilm formation, IgG binding, protein A recruitment, and IgG deposition. We discovered that combining curcumin and trans-chalcone is more effective against Gram-positive bacteria than either compound alone. The present work demonstrated that a combination of these natural compounds could be used as an antivirulence therapy against Gram-positive bacterial infection.

Structure optimizing of flavonoids against both MRSA and VRE

Methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococci (VRE) cause more than 100,000 deaths each year, which need efficient and non-resistant antibacterial agents. SAR analysis of 162 flavonoids from the plant in this paper suggested that lipophilic group at C-3 was crucial, and then 63 novel flavonoid derivatives were designed and total synthesized. Among them, the most promising K15 displayed potent bactericidal activity against clinically isolated MRSA and VRE (MICs = 0.25-1.00 μg/mL) with low toxicity and high membrane selectivity. Moreover, mechanism insights revealed that K15 avoided resistance by disrupting biofilm and targeting the membrane, while vancomycin caused 256 times resistance against MRSA, and ampicillin caused 16 times resistance against VRE by the same 20 generations inducing. K15 eliminated residual bacteria in mice skin MRSA-infected model (>99 %) and abdominal VRE-infected model (>92 %), which was superior to vancomycin and ampicillin.

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.

A unique binding mode of P1' Leu-containing target sequences for Streptococcus pyogenes sortase A results in alternative cleavage

Sortase enzymes are cysteine transpeptidases that attach environmental sensors, toxins, and other proteins to the cell surface in Gram-positive bacteria. The recognition motif for many sortases is the cell wall sorting signal (CWSS), LPXTG, where X = any amino acid. Recent work from ourselves and others has described recognition of additional amino acids at a number of positions in the CWSS, specifically at the Thr (or P1) and Gly (or P1') positions. In addition, although standard cleavage occurs between these two residues (P1/P1'), we previously observed that the SrtA enzyme from Streptococcus pneumoniae will cleave after the P1' position when its identity is a Leu or Phe. The stereochemical basis of this alternative cleavage is not known, although homologs, e.g., SrtA from Listeria monocytogenes or Staphylococcus aureus do not show alternative cleavage to a significant extent. Here, we use protein biochemistry, structural biology, and computational biochemistry to predict an alternative binding mode that facilitates alternative cleavage. We use Streptococcus pyogenes SrtA (spySrtA) as our model enzyme, first confirming that it shows similar standard/alternative cleavage ratios for LPATL, LPATF, and LPATY sequences. Molecular dynamics simulations suggest that when P1' is Leu, this amino acid binds in the canonical S1 pocket, pushing the P1 Thr towards solvent. The P4 Leu (L̲PATL) binds as it does in standard binding, resulting in a puckered binding conformation. We use P1 Glu-containing peptides to support our hypotheses, and present the complex structure of spySrtA-LPALA to confirm favorable accommodation of Leu in the S1 pocket. Overall, we structurally characterize an alternative binding mode for spySrtA and specific target sequences, expanding the potential protein engineering possibilities in sortase-mediated ligation applications.

A dual mechanism with H2S inhibition and membrane damage of morusin from Morus alba Linn. against MDR-MRSA

It's urgent to discover new antibiotics along with the increasing emergence and dissemination of multidrug resistant (MDR) bacterial pathogens. In the present investigation, morusin exhibited rapid bactericidal activity against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) by targeting the phospholipid of bacterial inner membrane, increasing membrane rigidity and disrupting bacterial homeostasis together with the membrane permeability, which caused fundamental metabolic disorders. Furthermore, morusin can also accumulate ROS, suppress H2S production, and aggravate oxidative damage in bacteria. Importantly, morusin also inhibited the spread of wounds and reduced the bacterial burden in the mouse model of skin infection caused by MRSA. It's a chance to meet the challenge of existing antibiotic resistance and avoid the development of bacterial resistance, given the multiple targets of morusin.

Echinacoside, a promising sortase A inhibitor, combined with vancomycin against murine models of MRSA-induced pneumonia

Methicillin-resistant Staphylococcus aureus (MRSA) is a pathogenic bacterium responsible for a range of severe infections, such as skin infections, bacteremia, and pneumonia. Due to its antibiotic-resistant nature, current research focuses on targeting its virulence factors. Sortase A (SrtA) is a transpeptidase that anchors surface proteins to the bacterial cell wall and is involved in adhesion and invasion to host cells. Through fluorescence resonance energy transfer (FRET), we identified echinacoside (ECH), a natural polyphenol, as a potential SrtA inhibitor with an IC50 of 38.42 μM in vitro. It was demonstrated that ECH inhibited SrtA-mediated S. aureus fibrinogen binding, surface protein A anchoring, and biofilm formation. The fluorescence quenching assay determined the binding mode of ECH to SrtA and calculated the KA-binding constant of 3.09 × 105 L/mol, demonstrating the direct interaction between the two molecules. Molecular dynamics simulations revealed that ECH-SrtA interactions occurred primarily at the binding sites of A92G, A104G, V168A, G192A, and R197A. Importantly, the combination of ECH and vancomycin offered protection against murine models of MRSA-induced pneumonia. Therefore, ECH may serve as a potential antivirulence agent against S. aureus infections, either alone or in combination with vancomycin.

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.

An idea to explore: Engaging high school students in structure-function studies of bacterial sortase enzymes and inhibitors - A comprehensive computational experimental pipeline

High school science fairs provide an exceptional opportunity for students to gain experience with scientific research, and participation has positive outcomes with respect to chosen careers in the sciences. However, it can be challenging to engage high school students in university-level research outside of formal internship programs. Here, we describe an experimental pipeline for a computational structural biology project that engages high school students. Students are involved at every step of the investigation and utilize freely available software to dock inhibitors onto protein homologues, and then analyze the resulting complexes. Bacterial sortases are transpeptidases on the cell surface of Gram-positive bacteria and are a potential target for the development of antibiotics. Students modeled inhibitors bound to sortases from several organisms, asking questions about affinity and selectivity. Their project was ranked in the top 10% at both regional and state science fairs. This project design is easily adaptable to countless other protein systems and provides a pipeline for collaborative high school student/university professor inquiry.

Using next generation antimicrobials to target the mechanisms of infection

The remarkable impact of antibiotics on human health is being eroded at an alarming rate by the emergence of multidrug resistant pathogens. There is a recognised consensus that new strategies to tackle infection are urgently needed to limit the devasting impact of antibiotic resistance on our global healthcare infrastructure. Next generation antimicrobials (NGAs) are compounds that target bacterial virulence factors to disrupt pathogenic potential without impacting bacterial viability. By disabling the key virulence factors required to establish and maintain infection, NGAs make pathogens more vulnerable to clearance by the immune system and can potentially render them more susceptible to traditional antibiotics. In this review, we discuss the developing field of NGAs and how advancements in this area could offer a viable standalone alternative to traditional antibiotics or an effective means to prolong antibiotic efficacy when used in combination.

Understanding mechanisms of virulence in MRSA: implications for antivirulence treatment strategies

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA) is a widespread pathogen, often causing recurrent and deadly infections in the hospital and community. Many S. aureus virulence factors have been suggested as potential targets for antivirulence therapy to decrease the threat of diminishing antibiotic availability. Antivirulence methods hold promise due to their adjunctive and prophylactic potential and decreased risk for selective pressure.

AREAS COVERED

This review describes the dominant virulence mechanisms exerted by MRSA and antivirulence therapeutics that are currently undergoing testing in clinical or preclinical stages. We also discuss the advantages and downsides of several investigational antivirulence approaches, including the targeting of bacterial transporters, host-directed therapy, and quorum-sensing inhibitors. For this review, a systematic search of literature on PubMed, Google Scholar, and Web of Science for relevant search terms was performed in April and May 2023.

EXPERT OPINION

Vaccine and antibody strategies have failed in clinical trials and could benefit from more basic science-informed approaches. Antivirulence-targeting approaches need to be set up better to meet the requirements of drug development, rather than only providing limited results to provide 'proof-of-principle' translational value of pathogenesis research. Nevertheless, there is great potential of such strategies and potential particular promise for novel probiotic approaches.

Functional diversity of staphylococcal surface proteins at the host-microbe interface

Surface proteins of Gram-positive pathogens are key determinants of virulence that substantially shape host-microbe interactions. Specifically, these proteins mediate host invasion and pathogen transmission, drive the acquisition of heme-iron from hemoproteins, and subvert innate and adaptive immune cell responses to push bacterial survival and pathogenesis in a hostile environment. Herein, we briefly review and highlight the multi-facetted roles of cell wall-anchored proteins of multidrug-resistant Staphylococcus aureus, a common etiological agent of purulent skin and soft tissue infections as well as severe systemic diseases in humans. In particular, we focus on the functional diversity of staphylococcal surface proteins and discuss their impact on the variety of clinical manifestations of S. aureus infections. We also describe mechanistic and underlying principles of staphylococcal surface protein-mediated immune evasion and coupled strategies S. aureus utilizes to paralyze patrolling neutrophils, macrophages, and other immune cells. Ultimately, we provide a systematic overview of novel therapeutic concepts and anti-infective strategies that aim at neutralizing S. aureus surface proteins or sortases, the molecular catalysts of protein anchoring in Gram-positive bacteria.

Integrating in Silico and In Vitro Studies to Screen Anti-Staphylococcus aureus Activity From Vietnamese Ganoderma multiplicatum and Ganoderma sinense

Background: Staphylococcus aureus is a nosocomial pathogen responsible for many serious infectious diseases in humans. Finding the anti- S. aureus agents is a time-consuming and costly process. Recently, computational methods have provided a better understanding of the interactions between herbal medicine drug targets to help clinical practitioners rationally design herbal formulae. Methods: In this study, molecular docking simulation was applied to screen a list of natural secondary metabolites from Ganoderma sp. on the protein target S. aureus sortase A. Molecular dynamics models were used to assess the stability of protein–ligand complexes during the first 100 ns. To validate the computational results, 2 Ganoderma species, G. multiplicatum VNKKK1901 and G. sinense VNKKK1902, were tested for antibacterial activity against S. aureus using the disk diffusion method. Results: The results showed that, among the selected compounds, ganosinensin B and ganosinoside A generated the highest binding energy on S. aureus sortase A, and demonstrated strong and stable binding capacity to proteins. In addition, the extracts of G. sinense VNKKK1902 and G. multiplicatum VNKKK1901 were bactericidal, with minimum bactericidal concentration (MBC)/minimum inhibitory concentration (MIC) ratios of 2. Conclusion: Our findings provide the first scientific report on the antibacterial activity of Ganoderma sp., which contain 2 promising compounds, ganosinensin B and ganosinoside A, as potential hits for developing novel drugs capable of supporting treatment of S. aureus infection.

Oligopeptide Sortase Inhibitor Modulates Staphylococcus aureus Cell Adhesion and Biofilm Formation

Prevention of bacterial adhesion is one of the most important antivirulence strategies for meeting the global challenge posed by antimicrobial resistance. We aimed to investigate the influence of a peptidic S. aureus sortase A inhibitor on bacterial adhesion to eukaryotic cells and biofilm formation as a potential method for reducing S. aureus virulence. The pentapeptide LPRDA was synthesized and characterized as a pure individual organic compound. Incubation of MSSA and MRSA strains with LPRDA induced a subsequent reduction in staphylococcal adhesion to Vero cells and biofilm formation, as visualized by microscopic and spectrophotometric methods, respectively. LPRDA did not have a cytotoxic effect on eukaryotic or bacterial cells. The pentapeptide LPRDA deserves further investigation using in vitro and in vivo models of Gram-positive bacteriemia as a potential antibacterial agent with an antiadhesive mechanism of action.

Cyanidin chloride protects mice from methicillin-resistant Staphylococcus aureus-induced pneumonia by targeting Sortase A

Methicillin-resistant Staphylococcus aureus (MRSA) has been developing rapidly in recent years. It poses a severe peril to global health care, and the new strategies to against the MRSA is urgently needed. Sortase A (SrtA) regulates the anchoring of many surface proteins. Compounds repress Staphylococcus aureus (S. aureus) cysteine transpeptidase SrtA are considered adequate potent virulence inhibitors. Then, we describe the identification of an effective SrtA inhibitor, cyanidin chloride, a bioflavonoid compound isolated from various plants. It has a reversible inhibitory effect on SrtA activity at an IC₅₀ of 21.91 μg/mL. As a SrtA inhibitor, cyanidin chloride antagonizes SrtA-related virulence phenotypes due to its breadth and specificity, including fibrinogen adhesion, A549 cell invasion, biofilm formation, and surface protein (SpA) anchoring. Subsequently, molecular docking and fluorescence quenching revealed that SrtA and cyanidin chloride had robust mutual affinity. Further mechanistic studies revealed that Arg-197, Gly-167, and Sep-116 were the key-binding sites mediating the interaction between SrtA and cyanidin chloride. Notably, a significant therapeutic effect of cyanidin chloride in vivo was also observed on the mouse pneumonia model induced by MRSA. In conclusion, our study indicates that cyanidin chloride potentially represents a new candidate SrtA inhibitor for S. aureus and potentially be developed as a new antivirulence agent.

Anti-infective therapy using species-specific activators of Staphylococcus aureus ClpP

The emergence of methicillin-resistant Staphylococcus aureus isolates highlights the urgent need to develop more antibiotics. ClpP is a highly conserved protease regulated by ATPases in bacteria and in mitochondria. Aberrant activation of  bacterial ClpP is an alternative method of discovering antibiotics, while it remains difficult to develop selective  Staphylococcus aureus ClpP activators that can avoid disturbing Homo sapiens ClpP functions. Here, we use a structure-based design to identify (R)- and (S)-ZG197 as highly selective Staphylococcus aureus ClpP activators. The key structural elements in Homo sapiens ClpP, particularly W146 and its joint action with the C-terminal motif, significantly contribute to the discrimination of the activators. Our selective activators display wide antibiotic properties towards an array of multidrug-resistant staphylococcal strains in vitro, and demonstrate promising antibiotic efficacy in zebrafish and murine skin infection models. Our findings indicate that the species-specific activators of Staphylococcus aureus ClpP are exciting therapeutic agents to treat staphylococcal infections.

Structure Activity Relationship of the Stem Peptide in Sortase A Mediated Ligation from Staphylococcus aureus

The surfaces of most Gram-positive bacterial cells, including that of Staphylococcus aureus (S. aureus), are heavily decorated with proteins that coordinate cellular interactions with the extracellular space. In S. aureus, sortase A is the principal enzyme responsible for covalently anchoring proteins, which display the sorting signal LPXTG, onto the peptidoglycan (PG) matrix. Considerable efforts have been made to understand the role of this signal peptide in the sortase-mediated reaction. In contrast, much less is known about how the primary structure of the other substrate involved in the reaction (PG stem peptide) could impact sortase activity. To assess the sortase activity, a library of synthetic analogs of the stem peptide that mimic naturally existing variations found in the S. aureus PG primary sequence were evaluated. Using a combination of two unique assays, we showed that there is broad tolerability of substrate variations that are effectively processed by sortase A. While some of these stem peptide derivatives are naturally found in mature PG, they are not known to be present in the PG precursor, lipid II. These results suggest that sortase A could process both lipid II and mature PG as acyl-acceptor strands that might reside near the membrane, which has not been previously described.

Lysine-Targeted Reversible Covalent Ligand Discovery for Proteins via Phage Display

Binding via reversible covalent bond formation presents a novel and powerful mechanism to enhance the potency of synthetic inhibitors for therapeutically important proteins. Work on this front has yielded the anticancer drug bortezomib as well as the antisickling drug voxelotor. However, the rational design of reversible covalent inhibitors remains difficult even when noncovalent inhibitors are available as a scaffold. Herein, we report chemically modified phage libraries, both linear and cyclic, that incorporate 2-acetylphenylboronic acid (APBA) as a warhead to bind lysines via reversible iminoboronate formation. To demonstrate their utility, these APBA-presenting phage libraries were screened against sortase A of Staphylococcus aureus, as well as the spike protein of SARS-CoV-2. For both protein targets, peptide ligands were readily identified with single-digit micromolar potency and excellent specificity, enabling live-cell sortase inhibition and highly sensitive spike protein detection, respectively. Furthermore, our structure-activity studies unambiguously demonstrate the benefit of the APBA warhead for protein binding. Overall, this contribution shows for the first time that reversible covalent inhibitors can be developed via phage display for a protein of interest. The phage display platform should be widely applicable to proteins including those involved in protein-protein interactions.

Structures of Streptococcus pyogenes Class A sortase in complex with substrate and product mimics provide key details of target recognition

The cell wall is a critical extracellular barrier for bacteria and many other organisms. In bacteria, this structural layer consists of peptidoglycan, which maintains cell shape and structural integrity and provides a scaffold for displaying various protein factors. To attach proteins to the cell wall, Gram-positive bacteria utilize sortase enzymes, which are cysteine transpeptidases that recognize and cleave a specific sorting signal, followed by ligation of the sorting signal–containing protein to the peptidoglycan precursor lipid II (LII). This mechanism is the subject of considerable interest as a target for therapeutic intervention and as a tool for protein engineering, where sortases have enabled sortase-mediated ligation or sortagging strategies. Despite these uses, there remains an incomplete understanding of the stereochemistry of substrate recognition and ligation product formation. Here, we solved the first structures of sortase A from Streptococcus pyogenes bound to two substrate sequences, LPATA and LPATS. In addition, we synthesized a mimetic of the product of sortase-mediated ligation involving LII (LPAT-LII) and solved the complex structure in two ligand conformations. These structures were further used as the basis for molecular dynamics simulations to probe sortase A-ligand dynamics and to construct a model of the acyl–enzyme intermediate, thus providing a structural view of multiple key states in the catalytic mechanism. Overall, this structural information provides new insights into the recognition of the sortase substrate motif and LII ligation partner and will support the continued development of sortases for protein engineering applications.

Hibifolin, a Natural Sortase A Inhibitor, Attenuates the Pathogenicity of Staphylococcus aureus and Enhances the Antibacterial Activity of Cefotaxime

This study aimed to identify hibifolin as a sortase A (SrtA) inhibitor and to determine whether it could attenuate the virulence of methicillin-resistant Staphylococcus aureus (MRSA). We employed a fluorescence resonance energy transfer (FRET) assay to screen a library of natural molecules to identify compounds that inhibit SrtA activity. Fluorescence quenching assay and molecular docking were performed to verify the direct binding interaction between SrtA and hibifolin. The pneumonia model was established using C57BL/6J mice by MRAS nasal administration for evaluating the effect of hibifolin on the pathogenicity of MRSA. Herein, we found that hibifolin was able to inhibit SrtA activity with an IC₅₀ of 31.20 μg/mL. Further assays showed that the capacity of adhesion of bacteria to the host cells and biofilm formation was decreased in hibifolin-treated USA300. Results obtained from fluorescence quenching assay and molecular docking indicated that hibifolin was capable of targeting SrtA protein directly. This interaction was further confirmed by the finding that the inhibition activities of hibifolin on mutant SrtA were substantially reduced after mutating the binding sites (TRP-194, ALA-104, THR-180, ARG-197, ASN-114). The in vivo study showed that hibifolin in combination with cefotaxime protected mice from USA300 infection-induced pneumonia, which was more potent than cefotaxime alone, and no significant cytotoxicity of hibifolin was observed. Taken together, we identified that hibifolin attenuated the pathogenicity of S. aureus by directly targeting SrtA, which may be utilized in the future as adjuvant therapy for S. aureus infections.

IMPORTANCE We identified hibifolin as a sortase A (SrtA) inhibitor by screening the natural compounds library, which effectively inhibited the activity of SrtA with an IC₅₀ value of 31.20 μg/mL. Hibifolin attenuated the pathogenic behavior of Staphylococcus aureus, including adhesion, invasion, and biofilm formation. Binding assays showed that hibifolin bound to SrtA protein directly. Hibifolin improved the survival of pneumonia induced by S. aureus USA300 in mice and alleviated the pathological damage. Moreover, hibifolin showed a synergistic antibacterial effect with cefotaxime in USA300-infected mice.

N-Terminal cysteine mediated backbone-side chain cyclization for chemically enhanced phage display

Phage display, an ingenious invention for evaluating peptide libraries, has been limited to natural peptides that are ribosomally assembled with proteinogenic amino acids. Recently, there has been growing interest in chemically modifying phage libraries to create nonnatural cyclic and multicyclic peptides, which are appealing for use as inhibitors of protein-protein interactions. While earlier reports largely focused on side-chain side-chain cyclization, we report herein a novel strategy for creating backbone-side chain cyclized peptide libraries on phage. Our strategy capitalizes on the unique reactivity of an N-terminal cysteine (NCys) with 2-cyanobenzothiazole (CBT) which, in conjugation with another thiol-reactive group, can elicit rapid cyclization between an NCys and an internal cysteine. The resulting library was screened against two model proteins, namely Keap1 and Sortase A. The screening readily revealed potent inhibitors for both proteins with certain Keap1 ligands reaching low nanomolar potency. The backbone-side chain cyclization strategy described herein presents a significant addition to the toolkit of creating nonnatural macrocyclic peptide libraries for phage display.

Punicalagin, an Inhibitor of Sortase A, Is a Promising Therapeutic Drug to Combat Methicillin-Resistant Staphylococcus aureus Infections

Antimicrobial resistance (AMR) poses a major threat to human health globally. Staphylococcus aureus is recognized as a cause of disease worldwide, especially methicillin-resistant S. aureus (MRSA) and vancomycin-resistant S. aureus (VRSA). The enzyme sortase A (SrtA), present on the cell surface of S. aureus, plays a key role in bacterial virulence without affecting the bacterial viability, and SrtA-deficient S. aureus strains do not affect the growth of bacteria. Here, we found that punicalagin, a natural compound, was able to inhibit SrtA activity with a very low half maximal inhibitory concentration (IC₅₀) value of 4.23 μg/mL, and punicalagin is a reversible inhibitor of SrtA. Moreover, punicalagin has no distinct cytotoxicity toward A549, HEK293T, or HepG2 cells at a much higher concentration than the IC₅₀ detected by MTT [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide] assays. In addition, punicalagin visibly attenuated the virulence-related phenotype of SrtA in vitro by decreasing adhesion of S. aureus to fibrinogen, reducing the ability of protein A (SpA) displayed on the surface of the bacteria and biofilm formation. Fluorescence quenching elucidated the interaction between punicalagin and SrtA. Molecular docking further implied that the inhibitory activity lay in the bond between punicalagin and SrtA residues LYS190, TYR187, ALA104, and GLU106. In In vivo studies, we surprisingly found that punicalagin had a more effective curative effect combined with cefotaxime when mice were infected with pneumonia caused by MRSA. Essentially, punicalagin, a therapeutic compound targeting SrtA, demonstrates great potential for combating MRSA infections.

Identification of novel small-molecular inhibitors of Staphylococcus aureus sortase A using hybrid virtual screening

Staphylococcus aureus is one of the most dangerous pathogens commonly associated with high levels of morbidity and mortality. Sortase A is considered as a promising molecular target for the development of antistaphylococcal agents. Using hybrid virtual screening approach and FRET analysis, we have identified five compounds able to decrease the activity of sortase A by more than 50% at the concentration of 200 µM. The most promising compound was 2-(2-amino-3-chloro-benzoylamino)-benzoic acid which was able to inhibit S. aureus sortase A at the IC₅₀ value of 59.7 µM. This compound was selective toward sortase A compared to other four cysteine proteases - cathepsin L, cathepsin B, rhodesain, and the SARS-CoV2 main protease. Microscale thermophoresis experiments confirmed that this compound bound sortase A with K_D value of 189 µM. Antibacterial and antibiofilm assays also confirmed high specificity of the hit compound against two standard and three wild-type, S. aureus hospital infection isolates. The effect of the compound on biofilms produced by two S. aureus ATCC strains was also observed suggesting that the compound reduced biofilm formation by changing the biofilm structure and thickness.

Targeting the BspC-vimentin interaction to develop anti-virulence therapies during Group B streptococcal meningitis

Bacterial infections are a major cause of morbidity and mortality worldwide and the rise of antibiotic resistance necessitates development of alternative treatments. Pathogen adhesins that bind to host cells initiate disease pathogenesis and represent potential therapeutic targets. We have shown previously that the BspC adhesin in Group B Streptococcus (GBS), the leading cause of bacterial neonatal meningitis, interacts with host vimentin to promote attachment to brain endothelium and disease development. Here we determined that the BspC variable (V-) domain contains the vimentin binding site and promotes GBS adherence to brain endothelium. Site directed mutagenesis identified a binding pocket necessary for GBS host cell interaction and development of meningitis. Using a virtual structure-based drug screen we identified compounds that targeted the V-domain binding pocket, which blocked GBS adherence and entry into the brain in vivo. These data indicate the utility of targeting the pathogen-host interface to develop anti-virulence therapeutics.

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.

Lysine-Targeting Reversible Covalent Inhibitors with Long Residence Time

We report a new reversible lysine conjugation that features a novel diazaborine product and much slowed dissociation kinetics in comparison to the previously known iminoboronate chemistry. Incorporating the diazaborine-forming warhead RMR1 to a peptide ligand gives potent and long-acting reversible covalent inhibitors of the staphylococcal sortase. The efficacy of sortase inhibition is demonstrated via biochemical and cell-based assays. A comparative study of RMR1 and an iminoboronate-forming warhead highlights the significance and potential of modulating bond dissociation kinetics in achieving long-acting reversible covalent inhibitors.

2-Sulfonylpyrimidines as Privileged Warheads for the Development of S. aureus Sortase A Inhibitors

Staphylococcus aureus is one of the most frequent causes of nosocomial and community-acquired infections, with emerging multiresistant isolates causing a significant burden to public health systems. We identified 2-sulfonylpyrimidines as a new class of potent inhibitors against S. aureus sortase A acting by covalent modification of the active site cysteine 184. Series of derivatives were synthesized to derive structure-activity relationship (SAR) with the most potent compounds displaying low micromolar K_I values. Studies on the inhibition selectivity of homologous cysteine proteases showed that 2-sulfonylpyrimidines reacted efficiently with protonated cysteine residues as found in sortase A, though surprisingly, no reaction occurred with the more nucleophilic cysteine residue from imidazolinium-thiolate dyads of cathepsin-like proteases. By means of enzymatic and chemical kinetics as well as quantum chemical calculations, it could be rationalized that the S_NAr reaction between protonated cysteine residues and 2-sulfonylpyrimidines proceeds in a concerted fashion, and the mechanism involves a ternary transition state with a conjugated base. Molecular docking and enzyme inhibition at variable pH values allowed us to hypothesize that in sortase A this base is represented by the catalytic histidine 120, which could be substantiated by QM model calculation with 4-methylimidazole as histidine analog.

AMPing Up the Search: A Structural and Functional Repository of Antimicrobial Peptides for Biofilm Studies, and a Case Study of Its Application to Corynebacterium striatum, an Emerging Pathogen

Antimicrobial peptides (AMPs) have been recognized for their ability to target processes important for biofilm formation. Given the vast array of AMPs, identifying potential anti-biofilm candidates remains a significant challenge, and prompts the need for preliminary in silico investigations prior to extensive in vitro and in vivo studies. We have developed Biofilm-AMP (B-AMP), a curated 3D structural and functional repository of AMPs relevant to biofilm studies. In its current version, B-AMP contains predicted 3D structural models of 5544 AMPs (from the DRAMP database) developed using a suite of molecular modeling tools. The repository supports a user-friendly search, using source, name, DRAMP ID, and PepID (unique to B-AMP). Further, AMPs are annotated to existing biofilm literature, consisting of a vast library of over 10,000 articles, enhancing the functional capabilities of B-AMP. To provide an example of the usability of B-AMP, we use the sortase C biofilm target of the emerging pathogen Corynebacterium striatum as a case study. For this, 100 structural AMP models from B-AMP were subject to in silico protein-peptide molecular docking against the catalytic site residues of the C. striatum sortase C protein. Based on docking scores and interacting residues, we suggest a preference scale using which candidate AMPs could be taken up for further in silico, in vitro and in vivo testing. The 3D protein-peptide interaction models and preference scale are available in B-AMP. B-AMP is a comprehensive structural and functional repository of AMPs, and will serve as a starting point for future studies exploring AMPs for biofilm studies. B-AMP is freely available to the community at https://b-amp.karishmakaushiklab.com and will be regularly updated with AMP structures, interaction models with potential biofilm targets, and annotations to biofilm literature.

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.

Detection of Sortase A and Identification of Its Inhibitors by Paramagnetic Nanoparticle-Assisted Nuclear Relaxation

Sortase A is a virulence factor responsible for the attachment of surface proteins to Staphylococcus aureus and other Gram-positive bacteria. Inhibitors of this enzyme are potential anti-infective agents. Herein, a new highly selective magnetic relaxation-based method for screening potential sortase A inhibitors is described. A 13-amino acid-long peptide substrate of sortase A is conjugated to SiO₂-EDTA-Gd NPs. In the presence of sortase A, the LPXTG motif on the peptide strand is cleaved resulting in a shortened peptide as well as a reduced water T₂ value whose magnitude is dependent on the concentration of sortase A. The detection limit is determined to be 76 pM. In contrast, the presence of sortase A inhibitors causes the T₂ to remain at a higher value. The proposed method is used to characterize inhibition of sortase A by curcumin and 4-(hydroxymercuri)benzoic acid with an IC₅₀ value of 12.9 ± 1.6 μM and 130 ± 1.76 μM, respectively. Furthermore, this method was successfully applied to detect sortase A activity in bacterial suspensions. The feasibility to screen different inhibitors in Escherichia coli and S. aureus suspensions was demonstrated. This method is fast and potentially useful to rapidly screen possible inhibitors of sortase A in bacterial suspensions, thereby aiding in the development of antibacterial agents targeting Gram-positive bacteria.

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.

Target-based screening for natural products against Staphylococcus aureus biofilms

As a notorious food-borne pathogen, Staphylococcus aureus can readily cause diseases in humans via contaminated food. Biofilm formation on various surfaces can increase the capacity of viable S. aureus cells for self-protection due to the stubborn structure of the biofilm matrix. Increased disease risk and economic losses caused by biofilm contamination in the food industry necessitate the urgent development of effective strategies for the inhibition and removal of S. aureus biofilms. Natural products have been extensively used as important sources of "eco-friendly" antibiofilm agents to avoid the side effects of conventional strategies on human health and the environment. This review discusses biofilm formation of S. aureus in food industries and focuses on providing an overview of potential promising target-oriented natural products and their mechanisms of S. aureus biofilm inhibition or removal. Hoping to provide valuable information of attractive research targets or potential undeveloped targets to screen potent natural anti-biofilm agents in food industries.

Bacteriophages as drivers of bacterial virulence and their potential for biotechnological exploitation

Bacteria-infecting viruses (phages) and their hosts maintain an ancient and complex relationship. Bacterial predation by lytic phages drives an ongoing phage-host arms race, whereas temperate phages initiate mutualistic relationships with their hosts upon lysogenization as prophages. In human pathogens, these prophages impact bacterial virulence in distinct ways: by secretion of phage-encoded toxins, modulation of the bacterial envelope, mediation of bacterial infectivity and the control of bacterial cell regulation. This review builds the argument that virulence-influencing prophages hold extensive, unexplored potential for biotechnology. More specifically, it highlights the development potential of novel therapies against infectious diseases, to address the current antibiotic resistance crisis. First, designer bacteriophages may serve to deliver genes encoding cargo proteins which repress bacterial virulence. Secondly, one may develop small molecules mimicking phage-derived proteins targeting central regulators of bacterial virulence. Thirdly, bacteria equipped with phage-derived synthetic circuits which modulate key virulence factors could serve as vaccine candidates to prevent bacterial infections. The development and exploitation of such antibacterial strategies will depend on the discovery of other prophage-derived, virulence control mechanisms and, more generally, on the dissection of the mutualistic relationship between temperate phages and bacteria, as well as on continuing developments in the synthetic biology field.

Transpeptidation-mediated single-particle imaging assay for sensitive and specific detection of sortase with dark-field optical microscopy

Transpeptidation of surface proteins catalyzed by the transpeptidase sortase plays a critical role in the infection process of Gram-positive pathogen. Monitoring sortase activity and screening its inhibitors are of great significance to fundamental understanding of the infection mechanism and pharmaceutical development. Herein, we developed a digital single-particle imaging method to quantify sortase A (SrtA) activity based on transpeptidation-mediated assembly and enumeration of gold nanoparticles (GNPs). The assay utilizes two peptide stands, in which one has the SrtA recognition sequence LPXTG motif while the other carries an oligoglycine nucleophile at the one end and a biotin group at the other. The presence of SrtA enables the ligation of two peptides and allows for the immobilization of streptavidin-functionalized GNPs. Thus, SrtA activity can be quantified by imaging and enumeration of the surface-assembled GNPs at the single-particle level via dark-field microscopy. The single-particle method was highly sensitive to SrtA activity with a low detection limit of 7.9 pM and a wide linear dynamic range from 0.05 to 50 nM. Besides detection of SrtA in complex biological samples such as Gram-positive pathogen lysates, the proposed method was also successfully applied to estimate the half-maximal inhibitory concentration (IC₅₀) values of SrtA inhibitors (curcumin, berberine hydrochloride and quercetin). The present method, combining single-GNP counting and dark-field imaging, provides a facile and novel analytical tool for SrtA activity and its inhibitor screening.

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.

Sorting out the Superbugs: Potential of Sortase A Inhibitors among Other Antimicrobial Strategies to Tackle the Problem of Antibiotic Resistance

Rapid spread of antibiotic resistance throughout the kingdom bacteria is inevitably bringing humanity towards the "post-antibiotic" era. The emergence of so-called "superbugs"-pathogen strains that develop resistance to multiple conventional antibiotics-is urging researchers around the globe to work on the development or perfecting of alternative means of tackling the pathogenic bacteria infections. Although various conceptually different approaches are being considered, each comes with its advantages and drawbacks. While drug-resistant pathogens are undoubtedly represented by both Gram(+) and Gram(-) bacteria, possible target spectrum across the proposed alternative approaches of tackling them is variable. Numerous anti-virulence strategies aimed at reducing the pathogenicity of target bacteria rather than eliminating them are being considered among such alternative approaches. Sortase A (SrtA) is a membrane-associated cysteine protease that catalyzes a cell wall sorting reaction by which surface proteins, including virulence factors, are anchored to the bacterial cell wall of Gram(+) bacteria. Although SrtA inhibition seems perspective among the Gram-positive pathogen-targeted antivirulence strategies, it still remains less popular than other alternatives. A decrease in virulence due to inactivation of SrtA activity has been extensively studied in Staphylococcus aureus, but it has also been demonstrated in other Gram(+) species. In this manuscript, results of past studies on the discovery of novel SrtA inhibitory compounds and evaluation of their potency were summarized and commented on. Here, we discussed the rationale behind the inhibition of SrtA, raised some concerns on the comparability of the results from different studies, and touched upon the possible resistance mechanisms as a response to implementation of such therapy in practice. The goal of this article is to encourage further studies of SrtA inhibitory compounds.

Design and Synthesis of Small Molecules as Potent Staphylococcus aureus Sortase A Inhibitors

The widespread and uncontrollable emergence of antibiotic-resistant bacteria, especially methicillin-resistant Staphylococcus aureus, has promoted a wave of efforts to discover a new generation of antibiotics that prevent or treat bacterial infections neither as bactericides nor bacteriostats. Due to its crucial role in virulence and its nonessentiality in bacterial survival, sortase A has been considered as a great target for new antibiotics. Sortase A inhibitors have emerged as promising alternative antivirulence agents against bacteria. Herein, the structural and preparative aspects of some small synthetic organic compounds that block the pathogenic action of sortase A have been described.

Challenges and New Therapeutic Approaches in the Management of Chronic Wounds

Chronic non-healing wounds are estimated to cost the US healthcare $28-$31 billion per year. Diabetic ulcers, arterial and venous ulcers, and pressure ulcers are some of the most common types of chronic wounds. The burden of chronic wounds continues to rise due to the current epidemic of obesity and diabetes and the increase in elderly adults in the population who are more vulnerable to chronic wounds than younger individuals. This patient population is also highly vulnerable to debilitating infections caused by opportunistic and multi-drug resistant pathogens. Reduced microcirculation, decreased availability of cytokines and growth factors that promote wound closure and healing, and infections by multi-drug resistant and biofilm forming microbes are some of the critical factors that contribute to the development of chronic non-healing wounds. This review discusses novel approaches to understand chronic wound pathology and methods to improve chronic wound care, particularly when chronic wounds are infected by multi-drug resistant, biofilm forming microbes.

Gene Duplications in the Genomes of Staphylococci and Enterococci

Gene duplications are a feature of bacterial genomes. In the present work we analyze the extent of gene duplications in the genomes of three microorganisms that belong to the Firmicutes phylum and that are etiologic agents of several nosocomial infections: Staphylococcus aureus, Enterococcus faecium, and Enterococcus faecalis. In all three groups, there is an irregular distribution of duplications in the genomes of the strains analyzed. Whereas in some of the strains duplications are scarce, hundreds of duplications are present in others. In all three species, mobile DNA accounts for a large percentage of the duplicated genes: phage DNA in S. aureus, and plasmid DNA in the enterococci. Duplicates also include core genes. In all three species, a reduced group of genes is duplicated in all strains analyzed. Duplication of the deoC and rpmG genes is a hallmark of S. aureus genomes. Duplication of the gene encoding the PTS IIB subunit is detected in all enterococci genomes. In E. faecalis it is remarkable that the genomes of some strains encode duplicates of the prgB and prgU genes. They belong to the prgABCU cluster, which responds to the presence of the peptide pheromone cCF10 by expressing the surface adhesins PrgA, PrgB, and PrgC.

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.

A Cell-based Screen in Actinomyces oris to Identify Sortase Inhibitors

Sortase enzymes are attractive antivirulence drug targets that attach virulence factors to the surface of Staphylococcus aureus and other medically significant bacterial pathogens. Prior efforts to discover a useful sortase inhibitor have relied upon an in vitro activity assay in which the enzyme is removed from its native site on the bacterial surface and truncated to improve solubility. To discover inhibitors that are effective in inactivating sortases in vivo, we developed and implemented a novel cell-based screen using Actinomyces oris, a key colonizer in the development of oral biofilms. A. oris is unique because it exhibits sortase-dependent growth in cell culture, providing a robust phenotype for high throughput screening (HTS). Three molecules representing two unique scaffolds were discovered by HTS and disrupt surface protein display in intact cells and inhibit enzyme activity in vitro. This represents the first HTS for sortase inhibitors that relies on the simple metric of cellular growth and suggests that A. oris may be a useful platform for discovery efforts targeting sortase.

Asymmetric Disulfanylbenzamides as Irreversible and Selective Inhibitors of Staphylococcus aureus Sortase A

Staphylococcus aureus is one of the most frequent causes of nosocomial and community-acquired infections, with drug-resistant strains being responsible for tens of thousands of deaths per year. S. aureus sortase A inhibitors are designed to interfere with virulence determinants. We have identified disulfanylbenzamides as a new class of potent inhibitors against sortase A that act by covalent modification of the active-site cysteine. A broad series of derivatives were synthesized to derive structure-activity relationships (SAR). In vitro and in silico methods allowed the experimentally observed binding affinities and selectivities to be rationalized. The most active compounds were found to have single-digit micromolar Ki values and caused up to a 66 % reduction of S. aureus fibrinogen attachment at an effective inhibitor concentration of 10 μM. This new molecule class exhibited minimal cytotoxicity, low bacterial growth inhibition and impaired sortase-mediated adherence of S. aureus cells.

Amentoflavone Attenuates Clostridium perfringens Gas Gangrene by Targeting Alpha-Toxin and Perfringolysin O

Clostridium perfringens (C. perfringens) type A strains are the main cause of gas gangrene in humans and animals. Treatment of this lethal disease is limited, and the prognosis is not good. Alpha-toxin (CPA) and perfringolysin O (PFO) secreted by C. perfringens play irreplaceable roles in cytotoxicity to host cells, persistence in host tissues, and lethality of gas gangrene pathology. This work determined the influence of amentoflavone, a biflavonoid isolated from Selaginella tamariscina and other plants, on hemolysis and cytotoxicity mediated by CPA and PFO and evaluated the in vivo therapeutic effect on gas gangrene. Our data showed that amentoflavone could block the hemolysis and cytotoxicity induced by CPA and PFO in vitro, thereby mediating significant protection against mortality of infected mice in a mouse gas gangrene model, efficient bacterial clearance in tissues and alleviation of histological damage in vivo. Based on the above results, amentoflavone may be a potential candidate against C. perfringens infection by reducing CPA and PFO-mediated virulence.

The combination of salvianolic acid A with latamoxef completely protects mice against lethal pneumonia caused by methicillin-resistant Staphylococcus aureus

Staphylococcus aureus (S. aureus), especially methicillin-resistant Staphylococcus aureus (MRSA), is a major cause of pneumonia, resulting in severe morbidity and mortality in adults and children. Sortase A (SrtA), which mediates the anchoring of cell surface proteins in the cell wall, is an important virulence factor of S. aureus. Here, we found that salvianolic acid A (Sal A), which is a natural product that does not affect the growth of S. aureus, could inhibit SrtA activity (IC₅₀ = 5.75 μg/ml) and repress the adhesion of bacteria to fibrinogen, the anchoring of protein A to cell wall, the biofilm formation, and the ability of S. aureus to invade A549 cells. Furthermore, in vivo studies demonstrated that Sal A treatment reduced inflammation and protected mice against lethal pneumonia caused by MRSA. More significantly, full protection (a survival rate of 100%) was achieved when Sal A was administered in combination with latamoxef. Together, these results indicate that Sal A could be developed into a promising therapeutic drug to combat MRSA infections while limiting resistance development.

Synthesis and spectral characteristics of N-(1-([1,2,4]triazolo[3,4-b][1,3,4]thiadiazol-6-ylamino)-2,2,2-trichloroethyl)carboxamides

Based on readily available N-(2,2,2-trichloro-1-hydroxyethyl)carboxamides, N-(2,2,2-trichloro-1-(3-(3-mercapto-4H-1,2,4-triazol-4-yl)thioureido)ethyl) carboxamides, dehydrosulfurization–under the influence of excess HgO–led to the formation of N-(1-([1,2,4] triazolo[3,4-b][1,3,4]thiadiazol-6-ylamino)-2,2,2-trichloroethyl)carboxamides. The reaction was carried out in boiling glacial acetic acid for 1-1.5 hours. The cyclization products were obtained in 42-62% yields and easily isolated from the reaction mixture. The structure of all synthesized compounds was confirmed by complex spectral studies.

Bacterial Anti-adhesives: Inhibition of Staphylococcus aureus Nasal Colonization

Bacterial adhesion to the skin and mucosa is often a fundamental and early step in host colonization, the establishment of bacterial infections, and pathology. This process is facilitated by adhesins on the surface of the bacterial cell that recognize host cell molecules. Interfering with bacterial host cell adhesion, so-called anti-adhesive therapeutics, offers promise for the development of novel approaches to control bacterial infections. In this review, we focus on the discovery of anti-adhesives targeting the high priority pathogen Staphylococcus aureus. This organism remains a major clinical burden, and S. aureus nasal colonization is associated with poor clinical outcomes. We describe the molecular basis of nasal colonization and highlight potentially efficacious targets for the development of novel nasal decolonization strategies.

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.