Selective chemical inhibition of agr quorum sensing in Staphylococcus aureus promotes host defense with minimal impact on resistance

The importance of the PapR7 C-terminus and amide protons in mediating quorum sensing in Bacilluscereus

The opportunistic human pathogen Bacillus cereus controls the expression of key infection-promoting phenotypes using bacterial quorum sensing (QS). QS signal transduction within the species is controlled by an autoinducing peptide, PapR7, and its cognate receptor, PlcR, indicating that the PlcR:PapR interface is a prime target for QS inhibitor development. The C-terminal region of the peptide (PapR7; ADLPFEF) has been successfully employed as a scaffold to develop potent QS modulators. Despite the noted importance of the C-terminal carboxylate and amide protons in crystallographic data, their role in QS activity has yet to be explored. In this study, an N-methyl scan of PapR7 was conducted in conjunction with a C-terminal modification of previously identified B. cereus QS inhibitors. The results indicate that the amide proton at Glu6 and the C-terminal carboxylate are important for effective QS inhibition of the PlcR regulon. Through β-galactosidase and hemolysis assays, a series of QS inhibitors were discovered, including several capable of inhibiting QS with nanomolar potency. These inhibitors, along with the structure-activity data reported, will serve as valuable tools for disrupting the B. cereus QS pathway towards developing novel anti-infective strategies.

Application of nanomaterials as potential quorum quenchers for disease: Recent advances and challenges

Chemical signal molecules are used by bacteria to interact with one another. Small hormone-like molecules known as autoinducers are produced, released, detected, and responded to during chemical communication. Quorum Sensing (QS) is the word for this procedure; it allows bacterial populations to communicate and coordinate group behavior. Several research has been conducted on using inhibitors to prevent QS and minimize the detrimental consequences. Through the enzymatic breakdown of the autoinducer component, by preventing the formation of autoinducers, or by blocking their reception by adding some compounds (inhibitors) that can mimic the autoinducers, a technique known as "quorum quenching" (QQ) disrupts microbial communication. Numerous techniques, including colorimetry, electrochemistry, bioluminescence, chemiluminescence, fluorescence, chromatography-mass spectroscopy, and many more, can be used to test QS/QQ. They all permit quantitative and qualitative measurements of QS/QQ molecules. The mechanism of QS and QQ, as well as the use of QQ in the prevention of biofilms, are all elaborated upon in this writing, along with the fundamental study of nanoparticle (NP)in QQ. Q.

Identification of an antivirulence agent targeting the master regulator of virulence genes in Staphylococcus aureus

The emergence of bactericidal antibiotic-resistant strains has increased the demand for alternative therapeutic agents, such as antivirulence agents targeting the virulence regulators of pathogens. Staphylococcus aureus exoprotein expression (sae) locus, the master regulator of virulence gene expression in multiple drug-resistant S. aureus, is a promising therapeutic target. In this study, we screened a small-molecule library using a SaeRS green fluorescent protein (GFP)-reporter that responded to transcription controlled by the sae locus. We identified the compound, N-(2-methylcyclohexyl)-11-oxo-10,11-dihydrodibenzo[b,f][1,4]thiazepine-8-carboxamide (SKKUCS), as an efficient repressor of sae-regulated GFP activity. SKKUCS inhibited hemolysin production and reduced α-hemolysin-mediated cell lysis. Moreover, SKKUCS substantially reduced the expression levels of various virulence genes controlled by the master regulators, sae, and the accessory gene regulator (agr), demonstrating its potential as an antivirulence reagent targeting the key virulence regulators. Furthermore, autokinase inhibition assay and molecular docking suggest that SKKUCS inhibits the kinase activity of SaeS and potentially targets the active site of SaeS kinase, possibly inhibiting ATP binding. Next, we evaluated the efficacy and toxicity of SKKUCS in vivo using murine models of staphylococcal intraperitoneal and skin infections. Treatment with SKKUCS markedly increased animal survival and significantly decreased the bacterial burden in organs and skin lesion sizes. These findings highlight SKKUCS as a potential antivirulence drug for drug-resistant staphylococcal infections.

Anti-Biofilm Activity of Carnosic Acid from Salvia rosmarinus against Methicillin-Resistant Staphylococcus aureus

The Salvia rosmarinus "Eretto Liguria" ecotype was studied as a source of valuable bioactive compounds. LC-MS analysis of the methanolic extract underlined the presence of diterpenoids, triterpenoids, polyphenolic acids, and flavonoids. The anti-virulence activity of carnosic acid along with the other most abundant compounds against methicillin-resistant Staphylococcus aureus (MRSA) was evaluated. Only carnosic acid induced a significant reduction in the expression of agrA and rnaIII genes, which encode the key components of quorum sensing (QS), an intracellular signaling mechanism controlling the virulence of MRSA. At a concentration of 0.05 mg/mL, carnosic acid inhibited biofilm formation by MRSA and the expression of genes involved in toxin production and made MRSA more susceptible to intracellular killing, with no toxic effects on eukaryotic cells. Carnosic acid did not affect biofilm formation by Pseudomonas aeruginosa, a human pathogen that often coexists with MRSA in complex infections. The selected ecotype showed a carnosic acid content of 94.3 ± 4.3 mg/g. In silico analysis highlighted that carnosic acid potentially interacts with the S. aureus AgrA response regulator. Our findings suggest that carnosic acid could be an anti-virulence agent against MRSA infections endowed with a species-specific activity useful in multi-microbial infections.

Silver-Containing Biomaterials for Biomedical Hard Tissue Implants

Bacterial infection caused by biomaterials is a very serious problem in the clinical treatment of implants. The emergence of antibiotic resistance has prompted other antibacterial agents to replace traditional antibiotics. Silver is rapidly developing as an antibacterial candidate material to inhibit bone infections due to its significant advantages such as high antibacterial timeliness, high antibacterial efficiency, and less susceptibility to bacterial resistance. However, silver has strong cytotoxicity, which can cause inflammatory reactions and oxidative stress, thereby destroying tissue regeneration, making the application of silver-containing biomaterials extremely challenging. In this paper, the application of silver in biomaterials is reviewed, focusing on the following three issues: 1) how to ensure the excellent antibacterial properties of silver, and not easy to cause bacterial resistance; 2) how to choose the appropriate method to combine silver with biomaterials; 3) how to make silver-containing biomaterials in hard tissue implants have further research. Following a brief introduction, the discussion focuses on the application of silver-containing biomaterials, with an emphasis on the effects of silver on the physicochemical properties, structural properties, and biological properties of biomaterials. Finally, the review concludes with the authors' perspectives on the challenges and future directions of silver in commercialization and in-depth research.

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.

Drug delivery strategies for antibiofilm therapy

Although new antibiofilm agents have been developed to prevent and eliminate pathogenic biofilms, their widespread clinical use is hindered by poor biocompatibility and bioavailability, unspecific interactions and insufficient local concentrations. The development of innovative drug delivery strategies can facilitate penetration of antimicrobials through biofilms, promote drug dispersal and synergistic bactericidal effects, and provide novel paradigms for clinical application. In this Review, we discuss the potential benefits of such emerging techniques for improving the clinical efficacy of antibiofilm agents, as well as highlighting the existing limitations and future prospects for these therapies in the clinic.

The Arabinose 5-Phosphate Isomerase KdsD Is Required for Virulence in Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, which is endemic primarily in Southeast Asia and northern Australia but is increasingly being seen in other tropical and subtropical regions of the world. Melioidosis is associated with high morbidity and mortality rates, which is mediated by the wide range of virulence factors encoded by B. pseudomallei. These virulence determinants include surface polysaccharides such as lipopolysaccharide (LPS) and capsular polysaccharides (CPS). Here, we investigated a predicted arabinose-5-phosphate isomerase (API) similar to KdsD in B. pseudomallei strain K96243. KdsD is required for the production of the highly conserved 3-deoxy-d-manno-octulosonic acid (Kdo), a key sugar in the core region of LPS. Recombinant KdsD was expressed and purified, and API activity was determined. Although a putative API paralogue (KpsF) is also predicted to be encoded, the deletion of kdsD resulted in growth defects, loss of motility, reduced survival in RAW 264.7 murine macrophages, and attenuation in a BALB/c mouse model of melioidosis. Suppressor mutations were observed during a phenotypic screen for motility, revealing single nucleotide polymorphisms or indels located in the poorly understood CPS type IV cluster. Crucially, suppressor mutations did not result in reversion of attenuation in vivo. This study demonstrates the importance of KdsD for B. pseudomallei virulence and highlights further the complex nature of the polysaccharides it produces. IMPORTANCE The intrinsic resistance of B. pseudomallei to many antibiotics complicates treatment. This opportunistic pathogen possesses a wide range of virulence factors, resulting in severe and potentially fatal disease. Virulence factors as targets for drug development offer an alternative approach to combat pathogenic bacteria. Prior to initiating early drug discovery approaches, it is important to demonstrate that disruption of the target gene will prevent the development of disease. This study highlights the fact that KdsD is crucial for virulence of B. pseudomallei in an animal model of infection and provides supportive phenotypic characterization that builds a foundation for future therapeutic development.

Identification of inhibitors for Agr quorum sensing system of Staphylococcus aureus by machine learning, pharmacophore modeling, and molecular dynamics approaches

CONTEXT

Staphylococcus aureus is a highly pathogenic organism that is the most common cause of postoperative complications as well as severe infections like bacteremia and infective endocarditis. By mediating the formation of biofilms and the expression of virulent genes, the quorum sensing (QS) mechanism is a major contributor to the development of these diseases. By hindering its QS network, an innovative approach to avoiding this bacterial infection is taken. Targeting the AgrA of the Agr system serves as beneficial in holding the top position in the QS system cascade.

METHODS

Using known AgrA inhibitors, the machine learning algorithms (artificial neural network, naïve Bayes, random forest, and support vector machine) and pharmacophore model were developed. The potential lead compounds were screened against the Zinc and COCONUT databases using the best pharmacophore hypothesis. The hits were then subjected second screening process using the best machine learning model. The predicted active compounds were then reranked based on the docking score. The stability of AgrA-lead compounds was studied using molecular dynamics approaches, and an ADME profile was also carried out. Five lead compounds, namely, CNP02386963,4,5-trihydroxy-2-[({7,13,14-trihydroxy-3,10-dioxo-2,9-dioxatetracyclo[6.6.2.04,16.011,15]hexadeca-1(14),4,6,8(16),11(15),12-hexaen-6-yl}oxy)methyl]benzoic acid, CNP0129274 4-(dimethylamino)-1,5,6,10,12,12a-hexahydroxy-6-methyl-3,11-dioxo-3,4,4a,5,5a,6,11,12a-octahydrotetracene-2-carboxamide, CNP0242717 3-Hydroxyasebotin, CNP0361624 3,4,5-trihydroxy-6-[(2,4,5,6,7-pentahydroxy-1-oxooctan-3-yl)oxy]oxane-2-carboxylic acid, and CNP0285058 2-{[4,5-dihydroxy-6-(hydroxymethyl)-3-[(3,4,5-trihydroxy-6-methyloxan-2-yl)oxy]oxan-2-yl]oxy}-2-(4-hydroxyphenyl)acetonitrile were obtained using the two-step virtual screening process. The molecular dynamics study revealed that the CNP0238696 was found to be stable in the binding pocket of AgrA. ADME profiles show that this compound has two Lipinski violations and low bioavailability. Further studies should be performed to assess the anti-biofilm activity of the lead compound in vitro.

Polymer based dual drug delivery system for targeted treatment of fluoroquinolone resistant Staphylococcus aureus mediated infections

The present study attempts to treat S. aureus-induced soft skin infections using a combinatorial therapy with an antibiotic, Ciprofloxacin (CIP), and an efflux pump inhibitor 5-Nitro-2-(3-phenylpropoxy) pyridine (5-NPPP) through a smart hydrogel delivery system. The study aims to reduce the increasing rates of infections and antimicrobial resistance; therefore, an efflux pump inhibitor molecule is synthesized and delivered along with an antibiotic to re-sensitize the pathogen towards antibiotics and treat the infections. CIP-loaded polyvinyl alcohol (PVA) hydrogels at varying concentrations were fabricated and optimized by a chemical cross-linking process, which exhibited sustained drug release for 5 days. The compound 5-NPPP loaded hydrogels provided linear drug release for 2 days, necessitating the need for the development of polymeric nanoparticles to alter the release drug pattern. 5-NPPP loaded Eudragit RSPO nanoparticles were prepared by modified nanoprecipitation-solvent evaporation method, which showed optimum average particle size of 230-280 nm with > 90% drug entrapment efficiency. The 5-NPPP polymeric nanoparticles loaded PVA hydrogels were fabricated to provide a predetermined sustained release of the compound to provide a synergistic effect. The selected 7% PVA hydrogels loaded with the dual drugs were evaluated using Balb/c mice models induced with S. aureus soft skin infections. The results of in vivo studies were evidence that the dual drugs loaded hydrogels were non-toxic and reduced the bacterial load causing re-sensitization towards antibiotics, which could initiate re-epithelization. The research concluded that the PVA hydrogels loaded with CIP and 5-NPPP nanoparticles could be an ideal and promising drug delivery system for treating S. aureus-induced skin infections.

Diflunisal and Analogue Pharmacophores Mediating Suppression of Virulence Phenotypes in Staphylococcus aureus

Invasive methicillin-resistant Staphylococcus aureus (MRSA) infections are leading causes of morbidity and mortality that are complicated by increasing resistance to conventional antibiotics. Thus, minimizing virulence and enhancing antibiotic efficacy against MRSA is a public health imperative. We originally demonstrated that diflunisal (DIF; [2-hydroxy-5-(2,4-difluorophenyl) benzoic acid]) inhibits S. aureus virulence factor expression. To investigate pharmacophores that are active in this function, we evaluated a library of structural analogues for their efficacy to modulate virulence phenotypes in a panel of clinically relevant S. aureus isolates in vitro. Overall, the positions of the phenyl, hydroxyl, and carboxylic moieties and the presence or type of halogen (F vs. Cl) influenced the efficacy of compounds in suppressing hemolysis, proteolysis, and biofilm virulence phenotypes. Analogues lacking halogens inhibited proteolysis to an extent similar to DIF but were ineffective at reducing hemolysis or biofilm production. In contrast, most analogues lacking the hydroxyl or carboxylic acid groups did not suppress proteolysis but did mitigate hemolysis and biofilm production to an extent similar to DIF. Interestingly, chirality and the substitution of fluorine with chlorine resulted in a differential reduction in virulence phenotypes. Together, this pattern of data suggests virulence-suppressing pharmacophores of DIF and structural analogues integrate halogen, hydroxyl, and carboxylic acid moiety stereochemistry. The anti-virulence effects of DIF were achieved using concentrations that are safe in humans, do not impair platelet antimicrobial functions, do not affect S. aureus growth, and do not alter the efficacy of conventional antibiotics. These results offer proof of concept for using novel anti-virulence strategies as adjuvants to antibiotic therapy to address the challenge of MRSA infection.

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.

Cinnamaldehyde Targets the LytTR DNA-Binding Domain of the Response Regulator AgrA to Attenuate Biofilm Formation of Listeria monocytogenes

The Agr quorum sensing (QS) system is known to contribute to biofilm formation in Listeria monocytogenes. Cinnamaldehyde, a natural food preservative, is considered an inhibitor of Agr-mediated QS in L. monocytogenes. However, the exact mechanism by which cinnamaldehyde acts on Agr remains unclear. In this study, we assessed the effects of cinnamaldehyde on the histidine kinase AgrC and the response regulator AgrA in the Agr system. AgrC kinase activity was not influenced by cinnamaldehyde, and binding between AgrC and cinnamaldehyde was not observed when microscale thermophoresis (MST) was performed, indicating that AgrC was not the target of cinnamaldehyde. AgrA is specifically bound to the agr promoter (P2) to activate the transcription of the Agr system. However, AgrA-P2 binding was prevented by cinnamaldehyde. The interaction between cinnamaldehyde and AgrA was further confirmed with MST. Two conserved amino acids, Asn-178 and Arg-179, located in the LytTR DNA-binding domain of AgrA, were identified as the key sites for cinnamaldehyde-AgrA binding by alanine mutagenesis and MST. Coincidentally, Asn-178 was also involved in the AgrA-P2 interaction. Taken together, these results suggest that cinnamaldehyde acts as a competitive inhibitor of AgrA in AgrA-P2 binding, which leads to suppressed transcription of the Agr system and reduced biofilm formation in L. monocytogenes. IMPORTANCE Listeria monocytogenes can form biofilms on various food contact surfaces, posing a serious threat to food safety. Biofilm formation of L. monocytogenes is positively regulated by the Agr quorum sensing system. Thus, an alternative strategy for controlling L. monocytogenes biofilms is interfering with the Agr system. Cinnamaldehyde is considered an inhibitor of the L. monocytogenes Agr system; however, its exact mechanism of action is still unclear. Here, we found that AgrA (response regulator), rather than AgrC (histidine kinase), was the target of cinnamaldehyde. The conserved Asn-178 in the LytTR DNA-binding domain of AgrA was involved in cinnamaldehyde-AgrA and AgrA-P2 binding. Therefore, the occupation of Asn-178 by cinnamaldehyde suppressed transcription of the Agr system and reduced biofilm formation in L. monocytogenes. Our findings could provide a better understanding of the mechanism by which cinnamaldehyde inhibits L. monocytogenes biofilm formation.

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

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

Molecular mechanism of green tea polyphenol epicatechin gallate attenuating Staphylococcus aureus pathogenicity by targeting Ser/Thr phosphatase Stp1

In this study, through virtual screening and in vitro bioactivity assays, we discovered that (-)-epicatechin gallate (ECG), a polyphenol compound extracted from green tea, demonstrated marked anti-Ser/Thr phosphatase (Stp1) activity towards Staphylococcus aureus (S. aureus) with an IC₅₀ value of 8.35 μM. By targeting S. aureus Stp1, ECG prevented the up-regulation of virulence gene and the formation of antibody membrane and protected the mice from S. aureus infection. Through MD simulation, the allosteric inhibitory mechanism of ECG on Stp1 was determined. The Stp1-ECG complex model underwent a significant change in conformation; its flap subdomain changed from opening to closing, whereas Stp1 activity was lost when bound to ECG. In addition, the MD simulation results of Stp1 and several tea polyphenol compounds showed that gallate groups and fewer adjacent phenolic hydroxyl groups contributed to the binding of Stp1 and inhibitors. As an inhibitor targeting S. aureus Stp1, ECG reduced the pathogenicity of S. aureus without inhibiting S. aureus, which largely reduced the possibility of drug resistance. Our findings demonstrated a novel molecular mechanism of green tea as the usual drink against S. aureus infection and elucidated the future design of allosteric inhibitors targeting Stp1.

High-Throughput Screening of Natural Product and Synthetic Molecule Libraries for Antibacterial Drug Discovery

Due to the continued emergence of resistance and a lack of new and promising antibiotics, bacterial infection has become a major public threat. High-throughput screening (HTS) allows rapid screening of a large collection of molecules for bioactivity testing and holds promise in antibacterial drug discovery. More than 50% of the antibiotics that are currently available on the market are derived from natural products. However, with the easily discoverable antibiotics being found, finding new antibiotics from natural sources has seen limited success. Finding new natural sources for antibacterial activity testing has also proven to be challenging. In addition to exploring new sources of natural products and synthetic biology, omics technology helped to study the biosynthetic machinery of existing natural sources enabling the construction of unnatural synthesizers of bioactive molecules and the identification of molecular targets of antibacterial agents. On the other hand, newer and smarter strategies have been continuously pursued to screen synthetic molecule libraries for new antibiotics and new druggable targets. Biomimetic conditions are explored to mimic the real infection model to better study the ligand-target interaction to enable the designing of more effective antibacterial drugs. This narrative review describes various traditional and contemporaneous approaches of high-throughput screening of natural products and synthetic molecule libraries for antibacterial drug discovery. It further discusses critical factors for HTS assay design, makes a general recommendation, and discusses possible alternatives to traditional HTS of natural products and synthetic molecule libraries for antibacterial drug discovery.

The Effects of Antiperspirant Aluminum Chlorohydrate on the Development of Antibiotic Resistance in Staphylococcus epidermidis

This study investigates the effects of the antiperspirant aluminum chlorohydrate on the development of antibiotic resistance in commensal Staphylococcus epidermidis isolates. The isolates were exposed to aluminum chlorohydrate for 30 days. The bacteria that developed resistance to oxacillin and ciprofloxacin were isolated, and the expression levels of some antibiotic resistance genes were determined using quantitative reverse transcriptase PCR. Before and after exposure, the minimum inhibitory concentration (MIC) values of the bacteria were determined using the microdilution method. A time-dependent increase was observed in the number of bacteria that developed resistance and increased MIC values. Consistent with the ciprofloxacin resistance observed after exposure, an increase in norA, norB/C, gyrA, gyrB, parC, and parE gene expression was observed. In addition to aluminum chlorohydrate exposure, oxacillin resistance was observed in all test bacteria in the group only subcultured in the medium, suggesting that phenotypic resistance cannot be correlated with chemical exposure in light of these data. The increase in mecA gene expression in selected test bacteria that acquired resistance to oxacillin after exposure compared with control groups suggests that the observed resistance may have been related to aluminum chlorohydrate exposure. To our knowledge, this is the first time in the literature that the effects of aluminum chlorohydrate as an antiperspirant on the development of antibiotic resistance in Staphylococcus epidermidis have been reported.

Computational tools for exploring peptide-membrane interactions in gram-positive bacteria

The vital cellular functions in Gram-positive bacteria are controlled by signaling molecules known as quorum sensing peptides (QSPs), considered promising therapeutic interventions for bacterial infections. In the bacterial system QSPs bind to membrane-coupled receptors, which then auto-phosphorylate and activate intracellular response regulators. These response regulators induce target gene expression in bacteria. One of the most reliable trends in drug discovery research for virulence-associated molecular targets is the use of peptide drugs or new functionalities. In this perspective, computational methods act as auxiliary aids for biologists, where methodologies based on machine learning and in silico analysis are developed as suitable tools for target peptide identification. Therefore, the development of quick and reliable computational resources to identify or predict these QSPs along with their receptors and inhibitors is receiving considerable attention. The databases such as Quorumpeps and Quorum Sensing of Human Gut Microbes (QSHGM) provide a detailed overview of the structures and functions of QSPs. The tools and algorithms such as QSPpred, QSPred-FL, iQSP, EnsembleQS and PEPred-Suite have been used for the generic prediction of QSPs and feature representation. The availability of compiled key resources for utilizing peptide features based on amino acid composition, positional preferences, and motifs as well as structural and physicochemical properties, including biofilm inhibitory peptides, can aid in elucidating the QSP and membrane receptor interactions in infectious Gram-positive pathogens. Herein, we present a comprehensive survey of diverse computational approaches that are suitable for detecting QSPs and QS interference molecules. This review highlights the utility of these methods for developing potential biomarkers against infectious Gram-positive pathogens.

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

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

Virtual Screening and In Vitro Experimental Verification of LuxS Inhibitors for Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli O157:H7 is an important foodborne pathogen that forms biofilms. In this study, three quorum-sensing (QS) inhibitors (M414-3326, 3254-3286, and L413-0180) were obtained through virtual screening, and their in vitro antibiofilm activities were validated. Briefly, the three-dimensional structure model of LuxS was constructed and characterized using the SWISS-MODEL. High-affinity inhibitors were screened from the ChemDiv database (1,535,478 compounds) using LuxS as a ligand. Five compounds (L449-1159, L368-0079, M414-3326, 3254-3286, and L413-0180) with a good inhibitory effect (50% inhibitory concentration <10 μM) on type II QS signal molecule autoinducer-2 (AI-2) were obtained using a AI-2 bioluminescence assay. The absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties predicated that the five compounds had high intestinal absorption levels (high) and plasma protein binding (absorbent strong) and did not inhibit the metabolism of CYP2D6 metabolic enzymes. In addition, molecular dynamics simulation showed that compounds L449-1159 and L368-0079 could not stably bind with LuxS. Thus, these compounds were excluded. Furthermore, surface plasmon resonance results showed that the three compounds could specifically bind to LuxS. IN addition, the three compounds could effectively inhibit the biofilm formation without affecting the growth and metabolism of the bacteria. Finally, the reverse transcription-quantitative PCR results showed that the three compounds downregulated the expression of the LuxS gene. Overall, these results revealed that the three compounds obtained through virtual screening could inhibit biofilm formation of E. coli O157:H7 and are potential LuxS inhibitors that can be used to treat E. coli O157:H7 infections. IMPORTANCE E. coli O157:H7 is a foodborne pathogen of public health importance. Quorum sensing (QS) is a form of bacterial communication that can regulate various group behaviors, including biofilm formation. Here, we identified three QS AI-2 inhibitors (M414-3326, 3254-3286, and L413-0180) that can stably and specifically bind to LuxS protein. The three QS AI-2 inhibitors inhibited biofilm formation without affecting the growth and metabolic activity of E. coli O157:H7. The three QS AI-2 inhibitors are promising agents for treating E. coli O157:H7 infections. Further studies to identify the mechanism of the three QS AI-2 inhibitors are needed to develop new drugs to overcome antibiotic resistance.

Critical Assessment of the Prospects of Quorum-Quenching Therapy for Staphylococcus aureus Infection

Staphylococcus aureus is an important pathogen that causes a high number of infections and is one of the leading causes of death in hospitalized patients. Widespread antibiotic resistance such as in methicillin-resistant S. aureus (MRSA) has prompted research into potential anti-virulence-targeted approaches. Targeting the S. aureus accessory gene regulator (Agr) quorum-sensing system, a master regulator of virulence, is the most frequently proposed anti-virulence strategy for S. aureus. While much effort has been put into the discovery and screening for Agr inhibitory compounds, in vivo analysis of their efficacy in animal infection models is still rare and reveals various shortcomings and problems. These include (i) an almost exclusive focus on topical skin infection models, (ii) technical problems that leave doubt as to whether observed in vivo effects are due to quorum-quenching, and (iii) the discovery of counterproductive biofilm-increasing effects. Furthermore, potentially because of the latter, invasive S. aureus infection is associated with Agr dysfunctionality. Altogether, the potential of Agr inhibitory drugs is nowadays seen with low enthusiasm given the failure to provide sufficient in vivo evidence for their potential after more than two decades since the initiation of such efforts. However, current Agr inhibition-based probiotic approaches may lead to a new application of Agr inhibition strategies in preventing S. aureus infections by targeting colonization or for otherwise difficult-to-treat skin infections such as atopic dermatitis.

Genetic and Environmental Investigation of a Novel Phenylamino Acetamide Inhibitor of the Pseudomonas aeruginosa Type III Secretion System

Traditional antibiotics target essential cellular components or metabolic pathways conserved in both pathogenic and nonpathogenic bacteria. Unfortunately, long-term antibiotic use often leads to antibiotic resistance and disruption of the overall microbiota. In this work, we identified a phenylamino acetamide compound, named 187R, that strongly inhibited the expression of the type III secretion system (T3SS) encoding genes and the secretion of the T3SS effector proteins in Pseudomonas aeruginosa. T3SS is an important virulence factor, as T3SS-deficient strains of P. aeruginosa are greatly attenuated in virulence. We further showed that 187R had no effect on bacterial growth, implying a reduced selective pressure for the development of resistance. 187R-mediated repression of T3SS was dependent on ExsA, the master regulator of T3SS in P. aeruginosa. The impact of 187R on the host-associated microbial community was also tested using the Arabidopsis thaliana phyllosphere as a model. Both culture-independent (Illumina sequencing) and culture-dependent (Biolog) methods showed that the application of 187R had little impact on the composition and function of microbial community compared to the antibiotic streptomycin. Together, these results suggested that compounds that target virulence factors could serve as an alternative strategy for disease management caused by bacterial pathogens. IMPORTANCE New antimicrobial therapies are urgently needed, since antibiotic resistance in human pathogens has become one of the world's most urgent public health problems. Antivirulence therapy has been considered a promising alternative for the management of infectious diseases, as antivirulence compounds target only the virulence factors instead of the growth of bacteria, and they are therefore unlikely to affect commensal microorganisms. However, the impacts of antivirulence compounds on the host microbiota are not well understood. We report a potent synthetic inhibitor of the P. aeruginosa T3SS, 187R, and its effect on the host microbiota of Arabidopsis. Both culture-independent (Illumina sequencing) and culture-dependent (Biolog) methods showed that the impacts of the antivirulence compound on the composition and function of host microbiota were limited. These results suggest that antivirulence compounds can be a potential alternative method to antibiotics.

Antimicrobial Resistance and Recent Alternatives to Antibiotics for the Control of Bacterial Pathogens with an Emphasis on Foodborne Pathogens

Antimicrobial resistance (AMR) is one of the most important global public health problems. The imprudent use of antibiotics in humans and animals has resulted in the emergence of antibiotic-resistant bacteria. The dissemination of these strains and their resistant determinants could endanger antibiotic efficacy. Therefore, there is an urgent need to identify and develop novel strategies to combat antibiotic resistance. This review provides insights into the evolution and the mechanisms of AMR. Additionally, it discusses alternative approaches that might be used to control AMR, including probiotics, prebiotics, antimicrobial peptides, small molecules, organic acids, essential oils, bacteriophage, fecal transplants, and nanoparticles.

Effect of Savirin or Ticagrelor Treatment on the Expression of Commonly Used Reference Genes in Staphylococcus aureus

Reference genes are frequently used for the normalization of quantitative reverse transcriptase PCR (qRTPCR) data in gene expression studies. Staphylococcus aureus is one of the most common causes of biofilm-related infections. Savirin and ticagrelor show in vitro as well as in vivo antibiofilm activity against S. aureus. The main aim of this study was to identify the most stably expressed reference genes to study the effect of these molecules on genes in a strong biofilm producing S. aureus isolate isolated from biofilm-related infection. Quantitative real-time PCR was performed by using relative quantification method. Four different algorithms, delta Ct, normfinder, bestkeeper, and genorm, followed by a comprehensive analysis was used to identify the most stable reference genes from a list of sixteen different candidate reference genes. All four algorithms reported different results, with some comparable findings among some methods. In the comprehensive analysis of the results of all the algorithms used, the most stable reference genes found were spa, rpoD, and pyk for savirin treatment experiment and gapdH, gyrA, and gmk for ticagrelor treatment experiment. The optimal number of reference genes required was two for both the experimental conditions. Despite having some drawbacks, each algorithm can reliably determine an appropriate reference gene independently. However, based on consensus ranking and the required optimal number of reference genes reported, spa and rpoD were the most appropriate reference genes for savirin treatment experiment, and gapdH and gyrA were most appropriate for ticagrelor treatment experiment. This study provides baseline data on reference genes to study the effect of savirin or ticagrelor treatment on the expression of potential reference genes in S. aureus. We recommend prior re-validation of reference genes on a case-by-case basis before they can be used.

The Cell Wall, Cell Membrane and Virulence Factors of Staphylococcus aureus and Their Role in Antibiotic Resistance

Antibiotic resistant strains of bacteria are a serious threat to human health. With increasing antibiotic resistance in common human pathogens, fewer antibiotics remain effective against infectious diseases. Staphylococcus aureus is a pathogenic bacterium of particular concern to human health as it has developed resistance to many of the currently used antibiotics leaving very few remaining as effective treatment. Alternatives to conventional antibiotics are needed for treating resistant bacterial infections. A deeper understanding of the cellular characteristics of resistant bacteria beyond well characterized resistance mechanisms can allow for increased ability to properly treat them and to potentially identify targetable changes. This review looks at antibiotic resistance in S aureus in relation to its cellular components, the cell wall, cell membrane and virulence factors. Methicillin resistant S aureus bacteria are resistant to most antibiotics and some strains have even developed resistance to the last resort antibiotics vancomycin and daptomycin. Modifications in cell wall peptidoglycan and teichoic acids are noted in antibiotic resistant bacteria. Alterations in cell membrane lipids affect susceptibility to antibiotics through surface charge, permeability, fluidity, and stability of the bacterial membrane. Virulence factors such as adhesins, toxins and immunomodulators serve versatile pathogenic functions in S aureus. New antimicrobial strategies can target cell membrane lipids and virulence factors including anti-virulence treatment as an adjuvant to traditional antibiotic therapy.

Novel quorum sensing inhibitor Echinatin as an antibacterial synergist against Escherichia coli

A new antibacterial strategy based on inhibiting bacterial quorum sensing (QS) has emerged as a promising method of attenuating bacterial pathogenicity and preventing bacterial resistance to antibiotics. In this study, we screened Echinatin (Ech) with high-efficiency anti-QS from 13 flavonoids through the AI-2 bioluminescence assay. Additionally, crystal violet (CV) staining combined with confocal laser scanning microscopy (CLSM) was used to evaluate the effect of anti-biofilm against Escherichia coli (E. coli). Further, the antibacterial synergistic effect of Ech and marketed antibiotics were measured by broth dilution and Alamar Blue Assay. It was found that Ech interfered with the phenotype of QS, including biofilm formation, exopolysaccharide (EPS) production, and motility, without affecting bacterial growth and metabolic activity. Moreover, qRT-PCR exhibited that Ech significantly reduced the expression of QS-regulated genes (luxS, pfs, lsrB, lsrK, lsrR, flhC, flhD, fliC, csgD, and stx2). More important, Ech with currently marketed colistin antibiotics (including colistin B and colistin E) showed significantly synergistically increased antibacterial activity in overcoming antibiotic resistance of E. coli. In summary, these results suggested the potent anti-QS and novel antibacterial synergist candidate of Ech for treating E. coli infections.

Towards the human nasal microbiome: Simulating D. pigrum and S. aureus

The human nose harbors various microbes that decisively influence the wellbeing and health of their host. Among the most threatening pathogens in this habitat is Staphylococcus aureus. Multiple epidemiological studies identify Dolosigranulum pigrum as a likely beneficial bacterium based on its positive association with health, including negative associations with S. aureus. Carefully curated GEMs are available for both bacterial species that reliably simulate their growth behavior in isolation. To unravel the mutual effects among bacteria, building community models for simulating co-culture growth is necessary. However, modeling microbial communities remains challenging. This article illustrates how applying the NCMW fosters our understanding of two microbes' joint growth conditions in the nasal habitat and their intricate interplay from a metabolic modeling perspective. The resulting community model combines the latest available curated GEMs of D. pigrum and S. aureus. This uses case illustrates how to incorporate genuine GEM of participating microorganisms and creates a basic community model mimicking the human nasal environment. Our analysis supports the role of negative microbe-microbe interactions involving D. pigrum examined experimentally in the lab. By this, we identify and characterize metabolic exchange factors involved in a specific interaction between D. pigrum and S. aureus as an in silico candidate factor for a deep insight into the associated species. This method may serve as a blueprint for developing more complex microbial interaction models. Its direct application suggests new ways to prevent disease-causing infections by inhibiting the growth of pathogens such as S. aureus through microbe-microbe interactions.

Effect of savirin in the prevention of biofilm-related Staphylococcus aureus prosthetic joint infection

Background: Most of the arthroplasty surgery failure due to prosthetic joint infections (PJI) is caused by biofilm-associated Staphylococcus aureus. In a recent experimental study, savirin has been used to prevent and treat S. aureus skin infections in animal models. We explored the application of savirin in a PJI mouse model to determine its utility as an adjunct therapy to prevent PJI.

Materials and methods: The in-vitro antibacterial and antibiofilm activity of savirin, with or without antibiotics (cefazolin, rifampicin, and vancomycin), against S. aureus were investigated using broth microdilution and crystal violet staining method, respectively. The effect of savirin treatment on the expression of the key biofilm-related genes (icaA, icaD, eno, fib, ebps, and agr) in S. aureus was studied using quantitative reverse transcriptase polymerase chain reaction (qRTPCR). The in-vivo efficacy of savirin alone and with cefazolin to prevent S. aureus PJI was determined using a clinically relevant PJI mouse model. Mice were randomized into five groups (n = 8/group): 1) infected K-wire savirin treated group, 2) infected K-wire cefazolin treated group, 3) infected K-wire savirin plus cefazolin treated group, 4) infected K-wire PBS treated group, 5) sterile K-wire group. Savirin was administered subcutaneously immediately post-surgery and intravenous cefazolin was given on day seven.

Results: Savirin inhibited planktonic and biofilm in-vitro growth of S. aureus, showed enhanced inhibitory activity when combined with antibiotics, and down-regulated the expression of key S. aureus biofilm-related genes (icaA, icaD, eno, fib, ebps, and agr). Savirin significantly reduced bacterial counts on joint implants in comparison with the PBS treated control, while savirin plus cefazolin reduced bacterial counts on both implants and peri-prosthetic tissues.

Conclusion: Savirin adjuvant therapy may prevent biofilm formation and S. aureus PJI. This study gives baseline data for using savirin for the prevention as well as treatment of S. aureus PJI in future animal studies.

Subinhibitory Concentrations of Antibiotics Exacerbate Staphylococcal Infection by Inducing Bacterial Virulence

Antibiotics are widely used for the treatment of bacterial infections. However, injudicious use of antibiotics based on an empirical method may lead to the emergence of resistant strains. Despite appropriate administration of antibiotics, their concentrations may remain subinhibitory in the body, due to individual variations in tissue distribution and metabolism rates. This may promote bacterial virulence and complicate the treatment strategies. To investigate whether the administration of certain classes of antibiotics will induce bacterial virulence and worsen the infection under in vivo conditions. Different classes of antibiotics were tested in vitro for their ability to induce virulence in a methicillin-resistant S. aureus strain Mu3 and clinical isolates. Antibiotic-induced pathogenicity was assessed in vivo using mouse peritonitis and bacteremia models. In vitro, β-lactam antibiotics and tetracyclines induced the expression of multiple surface-associated virulence factors as well as the secretion of toxins. In peritonitis and bacteremia models, mice infected with MRSA and treated with ampicillin, ceftazidime, or tetracycline showed enhanced bacterial pathogenicity. The release of induced virulence factors in vivo was confirmed in a histological examination. Subinhibitory concentrations of antibiotics belonging to β-lactam and tetracycline aggravated infection by inducing staphylococcal virulence in vivo. Thus, when antibiotics are required, it is preferable to employ combination therapy and to initiate the appropriate treatment plan, following diagnosis. Our findings emphasize the risks associated with antibiotic-based therapy and underline the need for alternative therapeutic options. IMPORTANCE Antibiotics are widely applied to treat infectious diseases. Empirically treatment with incorrect antibiotics, or even correct antibiotics always falls into subinhibitory concentrations, due to dosing, distribution, or secretion. In this study, we have systematically evaluated in vitro virulence induction effect of antibiotics and in vivo exacerbated infection. The major highlight of this work is to prove the β-lactam and tetracyclines antibiotics exacerbated disease is due to their induction effect on staphylococcal virulence. This phenomenon is common and suggests that if β-lactam antibiotics remain the first line of defense during empirical therapy, we either need to increase patient reliability or the treatment approach may improve in the future when paired with anti-virulence drugs.

The Structures and Binding Modes of Small-Molecule Inhibitors of Pseudomonas aeruginosa Elastase LasB

Elastase B (LasB) is a zinc metalloprotease and a crucial virulence factor of Pseudomonas aeruginosa. As the need for new strategies to fight antimicrobial resistance (AMR) constantly rises, this protein has become a key target in the development of novel antivirulence agents. The extensive knowledge of the structure of its active site, containing two subpockets and a zinc atom, led to various structure-based medicinal chemistry programs and the optimization of several chemical classes of inhibitors. This review provides a brief reminder of the structure of the active site and a summary of the disclosed P. aeruginosa LasB inhibitors. We specifically focused on the analysis of their binding modes with a detailed representation of them, hence giving an overview of the strategies aiming at targeting LasB by small molecules.

Role of sodium salicylate in Staphylococcus aureus quorum sensing, virulence, biofilm formation and antimicrobial susceptibility

The widespread threat of antibiotic resistance requires new treatment options. Disrupting bacterial communication, quorum sensing (QS), has the potential to reduce pathogenesis by decreasing bacterial virulence. The aim of this study was to investigate the influence of sodium salicylate (NaSa) on Staphylococcus aureus QS, virulence production and biofilm formation. In S. aureus ATCC 25923 (agr III), with or without serum, NaSa (10 mM) downregulated the agr QS system and decreased the secretion levels of alpha-hemolysin, staphopain A and delta-hemolysin. Inhibition of agr expression caused a downregulation of delta-hemolysin, decreasing biofilm dispersal and increasing biofilm formation on polystyrene and titanium under static conditions. In contrast, NaSa did not increase biofilm biomass under flow but caused one log₁₀ reduction in biofilm viability on polystyrene pegs, resulting in biofilms being twice as susceptible to rifampicin. A concentration-dependent effect of NaSa was further observed, where high concentrations (10 mM) decreased agr expression, while low concentrations (≤0.1 mM) increased agr expression. In S. aureus 8325-4 (agr I), a high concentration of NaSa (10 mM) decreased hla expression, and a low concentration of NaSa (≤1 mM) increased rnaIII and hla expression. The activity of NaSa on biofilm formation was dependent on agr type and material surface. Eight clinical strains isolated from prosthetic joint infection (PJI) or wound infection belonging to each of the four agr types were evaluated. The four PJI S. aureus strains did not change their biofilm phenotype with NaSa on the clinically relevant titanium surface. Half of the wound strains (agr III and IV) did not change the biofilm phenotype in the 3D collagen wound model. In addition, compared to the control, ATCC 25923 biofilms formed with 10 mM NaSa in the collagen model were more susceptible to silver. It is concluded that NaSa can inhibit QS in S. aureus, decreasing the levels of toxin production with certain modulation of biofilm formation. The effect on biofilm formation was dependent on the strain and material surface. It is suggested that the observed NaSa inhibition of bacterial communication is a potential alternative or adjuvant to traditional antibiotics.

Antivirulence Agent as an Adjuvant of β-Lactam Antibiotics in Treating Staphylococcal Infections

Staphylococcus aureus can cause a plethora of life-threatening infections. Antibiotics have been extensively used to treat S. aureus infections. However, when antibiotics are used at sub-inhibitory concentrations, especially for β-lactam antibiotics, they may enhance staphylococcal pathogenicity and exacerbate the infection. The combination of antivirulence agents and antibiotics may be a novel approach to controlling antibiotic-induced S. aureus pathogenicity. We have illustrated that under in vitro conditions, antivirulence agent M21, when administered concurrently with ampicillin, suppressed the expression and production of virulence factors induced by ampicillin. In a mouse peritonitis model, M21 reduced bacterial load irrespective of administration of ampicillin. In a bacteremia model, combinatorial treatment consisting of ampicillin or ceftazidime and M21 increased the survival rate of mice and reduced cytokine abundance, suggesting the suppression of antibiotic-induced virulence by M21. Different from traditional antibiotic adjuvants, an antivirulence agent may not synergistically inhibit bacterial growth in vitro, but effectively benefit the host in vivo. Collectively, our findings from this study demonstrated the benefits of antivirulence–antibiotic combinatorial treatment against S. aureus infections and provide a new perspective on the development of antibiotic adjuvants.

Evolution-proof inhibitors of public good cooperation: a screening strategy inspired by social evolution theory

Interference with public good cooperation provides a promising novel antimicrobial strategy since social evolution theory predicts that resistant mutants will be counter-selected if they share the public benefits of their resistance with sensitive cells in the population. Although this hypothesis is supported by a limited number of pioneering studies, an extensive body of more fundamental work on social evolution describes a multitude of mechanisms and conditions that can stabilize public behaviour, thus potentially allowing resistant mutants to thrive. In this paper we theorize on how these different mechanisms can influence the evolution of resistance against public good inhibitors. Based hereon, we propose an innovative 5-step screening strategy to identify novel evolution-proof public good inhibitors, which involves a systematic evaluation of the exploitability of public goods under the most relevant experimental conditions, as well as a careful assessment of the most optimal way to interfere with their action. Overall, this opinion paper is aimed to contribute to long-term solutions to fight bacterial infections.

Thymol Reduces agr-Mediated Virulence Factor Phenol-Soluble Modulin Production in Staphylococcus aureus

Staphylococcus aureus is a major human bacterial pathogen that carries a large number of virulence factors. Many virulence factors of S. aureus are regulated by the accessory gene regulator (agr) quorum-sensing system. Phenol-soluble modulins (PSMs) are one of the agr-mediated virulence determinants known to play a significant role in S. aureus pathogenesis. In the present study, the efficacy of thymol to inhibit PSM production including δ-toxin in S. aureus was explored. We employed liquid chromatography-mass spectrometry (LC-MS) to quantify the PSMsα1-PSMα4, PSMβ1 and PSMβ2, and δ-toxin production from culture supernatants. We found that thymol at 0.5 MIC (128 μg/mL) significantly reduced the PSMα and δ-toxin production in S. aureus WKZ-1, WKZ-2, LAC USA300, and ATCC29213. Downregulation in transcription by quantitative real-time (qRT) PCR analysis of response regulator agrA and receptor histidine kinase agrC upon 0.5 MIC thymol treatment affirmed the results of LC-MS quantification of PSMs. In silico molecular docking analysis demonstrated the binding affinity of thymol with receptors AgrA and AgrC. Transmission electron microscopy images revealed no ultrastructural alterations (cell wall and membrane) in thymol-treated WKZ-1 and WKZ-2 S. aureus strains. Here, we demonstrated that thymol reduces various PSM production in S. aureus clinical isolates and reference strains with mass spectrometry.

Anti-Quorum Sensing Activities of Gliptins against Pseudomonas aeruginosa and Staphylococcus aureus

The development of bacterial resistance to traditional antibiotics constitutes an emerging public health issue. Promising approaches have been innovated to conquer bacterial resistance, and targeting bacterial virulence is one of these approaches. Bacterial virulence mitigation offers several merits, as antivirulence agents do not affect the growth of bacteria and hence do not induce bacteria to develop resistance. In this direction, numerous drugs have been repurposed as antivirulence agents prior to their clinical use alone or in combination with traditional antibiotics. Quorum sensing (QS) plays a key role in controlling bacterial virulence. In the current study, dipeptidase inhibitor-4 (DPI-4) antidiabetic gliptins were screened for their antivirulence and anti-quorum sensing (anti-QS) activities against Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus. Upon assessing their antibiofilm activities, the ten tested gliptins significantly diminished biofilm formation. In particular, sitagliptin exhibited the most efficient antibiofilm activity, so it was chosen as a representative of all gliptins to further investigate its antivirulence activity. Sitagliptin significantly protected mice from P. aeruginosa and S. aureus pathogenesis. Furthermore, sitagliptin downregulated QS-encoding genes in P. aeruginosa and S. aureus. To test the anti-QS activities of gliptins, a detailed molecular docking study was conducted to evaluate the gliptins’ binding affinities to P. aeruginosa and S. aureus QS receptors, which helped explain the anti-QS activities of gliptins, particularly sitagliptin and omarigliptin. In conclusion, this study evaluates the possible antivirulence and anti-QS activities of gliptins that could be promising novel candidates for the treatment of aggressive Gram-negative or -positive bacterial infections either alone or as adjuvants to other antibiotics.

Anti-Quorum Sensing Activities of Gliptins against Pseudomonas aeruginosa and Staphylococcus aureus

The development of bacterial resistance to traditional antibiotics constitutes an emerging public health issue. Promising approaches have been innovated to conquer bacterial resistance, and targeting bacterial virulence is one of these approaches. Bacterial virulence mitigation offers several merits, as antivirulence agents do not affect the growth of bacteria and hence do not induce bacteria to develop resistance. In this direction, numerous drugs have been repurposed as antivirulence agents prior to their clinical use alone or in combination with traditional antibiotics. Quorum sensing (QS) plays a key role in controlling bacterial virulence. In the current study, dipeptidase inhibitor-4 (DPI-4) antidiabetic gliptins were screened for their antivirulence and anti-quorum sensing (anti-QS) activities against Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus. Upon assessing their antibiofilm activities, the ten tested gliptins significantly diminished biofilm formation. In particular, sitagliptin exhibited the most efficient antibiofilm activity, so it was chosen as a representative of all gliptins to further investigate its antivirulence activity. Sitagliptin significantly protected mice from P. aeruginosa and S. aureus pathogenesis. Furthermore, sitagliptin downregulated QS-encoding genes in P. aeruginosa and S. aureus. To test the anti-QS activities of gliptins, a detailed molecular docking study was conducted to evaluate the gliptins' binding affinities to P. aeruginosa and S. aureus QS receptors, which helped explain the anti-QS activities of gliptins, particularly sitagliptin and omarigliptin. In conclusion, this study evaluates the possible antivirulence and anti-QS activities of gliptins that could be promising novel candidates for the treatment of aggressive Gram-negative or -positive bacterial infections either alone or as adjuvants to other antibiotics.

Peptidomimetics as Potential Anti-Virulence Drugs Against Resistant Bacterial Pathogens

The uncontrollable spread of superbugs calls for new approaches in dealing with microbial-antibiotic resistance. Accordingly, the anti-virulence approach has arisen as an attractive unconventional strategy to face multidrug-resistant pathogens. As an emergent strategy, there is an imperative demand for discovery, design, and development of anti-virulence drugs. In this regard, peptidomimetic compounds could be a valuable source of anti-virulence drugs, since these molecules circumvent several shortcomings of natural peptide-based drugs like proteolytic instability, immunogenicity, toxicity, and low bioavailability. Some emerging evidence points to the feasibility of peptidomimetics to impair pathogen virulence. Consequently, in this review, we shed some light on the potential of peptidomimetics as anti-virulence drugs to overcome antibiotic resistance. Specifically, we address the anti-virulence activity of peptidomimetics against pathogens' secretion systems, biofilms, and quorum-sensing systems.

The role of bacterial signaling networks in antibiotics response and resistance regulation

Excessive use of antibiotics poses a threat to public health and the environment. In ecosystems, such as the marine environment, antibiotic contamination has led to an increase in bacterial resistance. Therefore, the study of bacterial response to antibiotics and the regulation of resistance formation have become an important research field. Traditionally, the processes related to antibiotic responses and resistance regulation have mainly included the activation of efflux pumps, mutation of antibiotic targets, production of biofilms, and production of inactivated or passivation enzymes. In recent years, studies have shown that bacterial signaling networks can affect antibiotic responses and resistance regulation. Signaling systems mostly alter resistance by regulating biofilms, efflux pumps, and mobile genetic elements. Here we provide an overview of how bacterial intraspecific and interspecific signaling networks affect the response to environmental antibiotics. In doing so, this review provides theoretical support for inhibiting bacterial antibiotic resistance and alleviating health and ecological problems caused by antibiotic contamination.

Two Novel Phenylpropanoid Trimers From Ligusticum chuanxiong Hort With Inhibitory Activities on Alpha-Hemolysin Secreted by Staphylococcus aureus

The emergence of antibiotic resistance in Staphylococcus aureus has necessitated the development of innovative anti-infective agents acting on novel targets. Alpha-hemolysin (Hla), a key virulence factor of S. aureus, is known to cause various cell damage and death. In this study, with bioassay-guided fractionation, a pair of unusual epimeric lignan trimers, ligustchuanes A and B (1 and 2), were isolated from the rhizomes of Ligusticum chuanxiong Hort, together with two known phthalides being identified by UPLC-QTOF-MS. To the best of our knowledge, trimers with rare C8-C9″-type neolignan and ferulic acid fragments have not been identified in any natural product. Both of them were isolated as racemic mixtures, and their absolute configurations were determined by comparing experimental and calculated ECD spectra after enantioseparation. Ligustchuane B exhibited an outstanding inhibitory effect on α-hemolysin expression in both MRSA USA300 LAC and MSSA Newman strains at concentrations of 3 and 6 μM, respectively. Notably, a mouse model of infection further demonstrated that ligustchuane B could attenuate MRSA virulence in vivo.

Sustained Release of a Synthetic Autoinducing Peptide Mimetic Blocks Bacterial Communication and Virulence In Vivo

A synthetic peptide was found to block cell-to-cell signalling, or quorum sensing, in bacteria and be highly bioavailable in mouse tissue. The controlled release of this agent from degradable polymeric microparticles strongly inhibited skin infection in a wound model at levels that far surpassed the potency of the peptide when delivered conventionally.

Antibiotic Discovery and Resistance: The Chase and the Race

The history of antimicrobial resistance (AMR) evolution and the diversity of the environmental resistome indicate that AMR is an ancient natural phenomenon. Acquired resistance is a public health concern influenced by the anthropogenic use of antibiotics, leading to the selection of resistant genes. Data show that AMR is spreading globally at different rates, outpacing all efforts to mitigate this crisis. The search for new antibiotic classes is one of the key strategies in the fight against AMR. Since the 1980s, newly marketed antibiotics were either modifications or improvements of known molecules. The World Health Organization (WHO) describes the current pipeline as bleak, and warns about the scarcity of new leads. A quantitative and qualitative analysis of the pre-clinical and clinical pipeline indicates that few antibiotics may reach the market in a few years, predominantly not those that fit the innovative requirements to tackle the challenging spread of AMR. Diversity and innovation are the mainstays to cope with the rapid evolution of AMR. The discovery and development of antibiotics must address resistance to old and novel antibiotics. Here, we review the history and challenges of antibiotics discovery and describe different innovative new leads mechanisms expected to replenish the pipeline, while maintaining a promising possibility to shift the chase and the race between the spread of AMR, preserving antibiotic effectiveness, and meeting innovative leads requirements.

Nonconformity of biofilm formation in vivo and in vitro based on Staphylococcus aureus accessory gene regulator status

Staphylococcus aureus is an opportunistic, pathogenic bacteria that causes significant morbidity and mortality. As antibiotic resistance by S. aureus continues to be a serious concern, developing novel drug therapies to combat these infections is vital. Quorum sensing inhibitors (QSI) dampen S. aureus virulence and facilitate clearance by the host immune system by blocking quorum sensing signaling that promotes upregulation of virulence genes controlled by the accessory gene regulator (agr) operon. While QSIs have shown therapeutic promise in mouse models of S. aureus skin infection, their further development has been hampered by the suggestion that agr inhibition promotes biofilm formation. In these studies, we investigated the relationship between agr function and biofilm growth across various S. aureus strains and experimental conditions, including in a mouse model of implant-associated infection. We found that agr deletion was associated with the presence of increased biofilm only under narrow in vitro conditions and, crucially, was not associated with enhanced biofilm development or enhanced morbidity in vivo.

Targeting of Regulators as a Promising Approach in the Search for Novel Antimicrobial Agents

Since the discovery of penicillin in the first half of the last century, antibiotics have become the pillars of modern medicine for fighting bacterial infections. However, pathogens resistant to antibiotic treatment have increased in recent decades, and efforts to discover new antibiotics have decreased. As a result, it is becoming increasingly difficult to treat bacterial infections successfully, and we look forward to more significant efforts from both governments and the scientific community to research new antibacterial drugs. This perspective article highlights the high potential of bacterial transcriptional and posttranscriptional regulators as targets for developing new drugs. We highlight some recent advances in the search for new compounds that inhibit their biological activity and, as such, appear very promising for treating bacterial infections.

Evolution of two-component quorum sensing systems

Quorum sensing (QS) is a cell-to-cell communication system that enables bacteria to coordinate their gene expression depending on their population density, via the detection of small molecules called autoinducers. In this way bacteria can act collectively to initiate processes like bioluminescence, virulence and biofilm formation. Autoinducers are detected by receptors, some of which are part of two-component signal transduction systems (TCS), which comprise of a (usually membrane-bound) sensor histidine kinase (HK) and a cognate response regulator (RR). Different QS systems are used by different bacterial taxa, and their relative evolutionary relationships have not been extensively studied. To address this, we used the Kyoto Encyclopedia of Genes and Genomes (KEGG) database to identify all the QS HKs and RRs that are part of TCSs and examined their conservation across microbial taxa. We compared the combinations of the highly conserved domains in the different families of receptors and response regulators using the Simple Modular Architecture Research Tool (SMART) and KEGG databases, and we also carried out phylogenetic analyses for each family, and all families together. The distribution of the different QS systems across taxa, indicates flexibility in HK–RR pairing and highlights the need for further study of the most abundant systems. For both the QS receptors and the response regulators, our results indicate close evolutionary relationships between certain families, highlighting a common evolutionary history which can inform future applications, such as the design of novel inhibitors for pathogenic QS systems.

A quorum-based fluorescent probe for imaging pathogenic bacteria

Imaging of bacterial infections can be used for a wide range of investigations, including diagnosis and pathogenesis of infections, and molecular probes targeting biological processes during infection have been used...

Identification of small molecules that strongly inhibit bacterial quorum sensing using a high-throughput lipid vesicle lysis assay

Strategies to both monitor and block bacterial quorum sensing (QS), and thus associated infections, are of significant interest. We developed a straightforward assay to monitor biosurfactants and lytic agents produced by bacteria under the control of QS. The method is based on the lysis of synthetic lipid vesicles containing the environmentally sensitive fluorescent dye calcein. This assay allows for the in situ screening of compounds capable of altering biosurfactant production by bacteria, and thereby the identification of molecules that could potentially modulate QS pathways, and avoids the constraints of many of the cell-based assays in use today. Application of this assay in a high-throughput format revealed five molecules capable of blocking vesicle lysis by S. aureus. Two of these compounds were found to almost completely inhibit agr-based QS in S. aureus and represent the most potent small-molecule-derived QS inhibitors reported in this formidable pathogen.

Pathogenicity and virulence of Staphylococcus aureus

Staphylococcus aureus is one of the most frequent worldwide causes of morbidity and mortality due to an infectious agent. This pathogen can cause a wide variety of diseases, ranging from moderately severe skin infections to fatal pneumonia and sepsis. Treatment of S. aureus infections is complicated by antibiotic resistance and a working vaccine is not available. There has been ongoing and increasing interest in the extraordinarily high number of toxins and other virulence determinants that S. aureus produces and how they impact disease. In this review, we will give an overview of how S. aureus initiates and maintains infection and discuss the main determinants involved. A more in-depth understanding of the function and contribution of S. aureus virulence determinants to S. aureus infection will enable us to develop anti-virulence strategies to counteract the lack of an anti-S. aureus vaccine and the ever-increasing shortage of working antibiotics against this important pathogen.