Quercetin impairs Streptococcus pneumoniae biofilm formation by inhibiting sortase A activity

Are secondary bacterial pneumonia mortalities increased because of insufficient pro-resolving mediators?

Respiratory viral infections, including respiratory syncytial virus (RSV), parainfluenza viruses and type A and B influenza viruses, can have severe outcomes. Bacterial infections frequently follow viral infections, and influenza or other viral epidemics periodically have higher mortalities from secondary bacterial pneumonias. Most secondary bacterial infections can cause lung immunosuppression by fatty acid mediators which activate cellular receptors to manipulate neutrophils, macrophages, natural killer cells, dendritic cells and other lung immune cells. Bacterial infections induce synthesis of inflammatory mediators including prostaglandins and leukotrienes, then eventually also special pro-resolving mediators, including lipoxins, resolvins, protectins and maresins, which normally resolve inflammation and immunosuppression. Concurrent viral and secondary bacterial infections are more dangerous, because viral infections can cause inflammation and immunosuppression before the secondary bacterial infections worsen inflammation and immunosuppression. Plausibly, the higher mortalities of secondary bacterial pneumonias are caused by the overwhelming inflammation and immunosuppression, which the special pro-resolving mediators might not resolve.

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

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

Current View on Major Natural Compounds Endowed with Antibacterial and Antiviral Effects

Nowadays, infectious diseases of bacterial and viral origins represent a serious medical problem worldwide. In fact, the development of antibiotic resistance is responsible for the emergence of bacterial strains that are refractory even to new classes of antibiotics. Furthermore, the recent COVID-19 pandemic suggests that new viruses can emerge and spread all over the world. The increase in infectious diseases depends on multiple factors, including malnutrition, massive migration of population from developing to industrialized areas, and alteration of the human microbiota. Alternative treatments to conventional antibiotics and antiviral drugs have intensively been explored. In this regard, plants and marine organisms represent an immense source of products, such as polyphenols, alkaloids, lanthipeptides, and terpenoids, which possess antibacterial and antiviral activities. Their main mechanisms of action involve modifications of bacterial cell membranes, with the formation of pores, the release of cellular content, and the inhibition of bacterial adherence to host cells, as well as of the efflux pump. Natural antivirals can interfere with viral replication and spreading, protecting the host with the enhanced production of interferon. Of note, these antivirals are not free of side effects, and their administration to humans needs more research in terms of safety. Preclinical research with natural antibacterial and antiviral compounds confirms their effects against bacteria and viruses, but there are still only a few clinical trials. Therefore, their full exploitation and more intensive clinical studies represent the next steps to be pursued in this area of medicine.

Therapeutic potential activity of quercetin complexes against Streptococcus pneumoniae

This study investigates quercetin complexes as potential synergistic agents against the important respiratory pathogen Streptococcus pneumoniae. Six quercetin complexes (QCX1-6) were synthesized by reacting quercetin with various metal salts and boronic acids and characterized using FTIR spectroscopy. Their antibacterial activity alone and in synergism with antibiotics was evaluated against S. pneumoniae ATCC 49619 using disc diffusion screening, broth microdilution MIC determination, and checkerboard assays. Complexes QCX-3 and QCX-4 demonstrated synergy when combined with levofloxacin via fractional inhibitory concentration indices ≤ 0.5 as confirmed by time-kill kinetics. Molecular docking elucidated interactions of these combinations with virulence enzymes sortase A and sialidase. A biofilm inhibition assay found the synergistic combinations more potently reduced biofilm formation versus monotherapy. Additionally, gene-gene interaction networks, biological activity predictions and in-silico toxicity profiling provided insights into potential mechanisms of action and safety.

Plant-derived bioactive compounds for the inhibition of biofilm formation: a comprehensive review

Biofilm formation is a widespread phenomenon that impacts different fields, including the food industry, agriculture, health care and the environment. Accordingly, there is a serious need for new methods of managing the problem of biofilm formation. Natural products have historically been a rich source of varied compounds with a wide variety of biological functions, including antibiofilm agents. In this review, we critically highlight and discuss the recent progress in understanding the antibiofilm effects of several bioactive compounds isolated from different plants, and in elucidating the underlying mechanisms of action and the factors influencing their adhesion. The literature shows that bioactive compounds have promising antibiofilm potential against both Gram-negative and Gram-positive bacterial and fungal strains, via several mechanisms of action, such as suppressing the formation of the polymer matrix, limiting O2 consumption, inhibiting microbial DNA replication, decreasing hydrophobicity of cell surfaces and blocking the quorum sensing network. This antibiofilm activity is influenced by several environmental factors, such as nutritional cues, pH values, O2 availability and temperature. This review demonstrates that several bioactive compounds could mitigate the problem of biofilm production. However, toxicological assessment and pharmacokinetic investigations of these molecules are strongly required to validate their safety.

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

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

Enhancement of Streptomyces thinghirensis WAE1 for production of bioactive metabolites under different optimization strategies

Isolation of novel bioactive metabolites from Streptomyces strains is a promising source for drug discovery. However, conventional screening approaches have limitations in identifying new leads due to redundant discoveries. Optimization of culture conditions is important but traditionally optimized one factor at a time, failing to consider interactions. This study addressed these gaps by enhancing metabolite production from Streptomyces thinghirensis WAE1 through statistical optimization. Various chemical and physical factors impacting metabolite production were identified. Response surface methodology with a central composite design was applied to optimize significant factors like carbon source, nitrogen source, inoculum size, pH, temperature and incubation period. This optimized production against Streptococcus pneumoniae, increasing antibacterial activity by 74.92%. Gas chromatography-mass spectrometry revealed 19 bioactive compounds, including 1,25-dihydroxyvitamin D3 inhibiting cell wall development. This highlights S. thinghirensis WAE1's potential as a bioresource and emphasizes studying metabolite production from novel Streptomyces strains to discover new antibacterial drugs.

Bavachin Rejuvenates Sensitivity of Colistin against Colistin-Resistant Gram-Negative Bacteria

The emergence of plasmid-mediated colistin resistance threatens the efficacy of colistin as a last-resort antibiotic used to treat infection caused by Gram-negative bacteria (GNB). Given the shortage of new antibiotics, the discovery of adjuvants to existing antibiotics is a promising strategy to combat infections caused by multidrug-resistant (MDR) GNB. This study was designed to investigate the potential synergistic antibacterial activity of bavachin, a bioactive compound extracted from the Psoralea Fructus, combined with colistin against MDR GNB. Herein, the synergistic efficacy in vitro and the therapeutic efficacy of colistin combined with bavachin in vivo were evaluated. The synergistic mechanism was detected by fluorescent probe and the transcript levels of mcr-1. Bavachin combined with colistin showed an excellent synergistic activity against GNB, as the FICI ≤ 0.5. In contrast to colistin alone, combination therapy dramatically increased the survival rate of Galleria mellonella and mice in vivo. Moreover, the combination of bavachin and colistin significantly reduced the amount of bacterial biofilm formation, improved the membrane disruption of colistin and inhibited mcr-1 transcription. These findings show that bavachin is a potential adjuvant of colistin, which may provide a new strategy to combat colistin-resistant bacteria infection with lower doses of colistin.

Inhibition of pathogenic microbes in oral infectious diseases by natural products: Sources, mechanisms, and challenges

The maintenance of oral health is of utmost importance for an individual's holistic well-being and standard of living. Within the oral cavity, symbiotic microorganisms actively safeguard themselves against potential foreign diseases by upholding a multifaceted equilibrium. Nevertheless, the occurrence of an imbalance can give rise to a range of oral infectious ailments, such as dental caries, periodontitis, and oral candidiasis. Presently, clinical interventions encompass the physical elimination of pathogens and the administration of antibiotics to regulate bacterial and fungal infections. Given the limitations of various antimicrobial drugs frequently employed in dental practice, the rising incidence of oral inflammation, and the escalating bacterial resistance to antibiotics, it is imperative to explore alternative remedies that are dependable, efficacious, and affordable for the prevention and management of oral infectious ailments. There is an increasing interest in the creation of novel antimicrobial agents derived from natural sources, which possess attributes such as safety, cost-effectiveness, and minimal adverse effects. This review provides a comprehensive overview of the impact of natural products on the development and progression of oral infectious diseases. Specifically, these products exert their influences by mitigating dental biofilm formation, impeding the proliferation of oral pathogens, and hindering bacterial adhesion to tooth surfaces. The review also encompasses an examination of the various classes of natural products, their antimicrobial mechanisms, and their potential therapeutic applications and limitations in the context of oral infections. The insights garnered from this review can support the promising application of natural products as viable therapeutic options for managing oral infectious diseases.

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.

Deadly interactions: Synergistic manipulations of concurrent pathogen infections potentially enabling future pandemics

Certain mono-infections of influenza viruses and novel coronaviruses, including severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) are significant threats to human health. Concurrent infections by influenza viruses and coronaviruses increases their danger. Influenza viruses have eight manipulations capable of assisting SARS-CoV-2 and other coronaviruses, and several of these manipulations, which are not specific to viruses, can also directly or indirectly boost dangerous secondary bacterial pneumonias. The influenza virus manipulations include: inhibiting transcription factors and cytokine expression; impairing defensive protein expression; increasing RNA viral replication; inhibiting defenses by manipulating cellular sensors and signaling pathways; inhibiting defenses by secreting exosomes; stimulating cholesterol production to increase synthesized virion infectivities; increasing cellular autophagy to assist viral replication; and stimulating glucocorticoid synthesis to suppress innate and adaptive immune defenses by inhibiting cytokine, chemokine, and adhesion molecule production. Teaser: Rapidly spreading multidrug-resistant respiratory bacteria, combined with influenza virus's far-reaching cellular defense manipulations benefiting evolving SARS-CoV-2 or other coronaviruses and/or respiratory bacteria, can enable more severe pandemics or co-pandemics.

Antimicrobial effect of quercetin against Streptococcus pneumoniae

Streptococcus pneumoniae is a bacterium that causes serious infections, including pneumonia. The limited range of available vaccines and the rise of antibiotic-resistant bacteria mean that new treatments are needed. This study looked at the potential of quercetin as an antimicrobial agent against S. pneumoniae in both isolation and in biofilms. The researchers used microdilution tests, checkerboard assays, and death curve assays, as well as in silico and in vitro cytotoxicity evaluations. They found that quercetin at a concentration of 125.0 μg/mL had both inhibitory and bactericidal effects against S. pneumoniae, and these effects were increased when quercetin was combined with ampicillin. Quercetin also reduced the growth of pneumococcal biofilms. In addition, quercetin (absence or in combination with ampicillin) reduced the death time of Tenebrio molitor larvae compared to the infection control. The study also demonstrated that quercetin had low toxicity in both in silico and in vivo assays, suggesting that it could be a promising treatment for infections caused by S. pneumoniae.

Impact of Senolytic Treatment on Gene Expression in Aged Lung

Cellular senescence plays a key role in mediating tissue remodeling and modulation of host responses to pathogenic stimuli. Our current study was designed to gain a better understanding of the impact of short-term senolytic treatment or inflammatory stimulation on lung senescence. The results of our study demonstrate that short term treatment of aged adult mice (20 months of age) with senolytics, quercetin, and dasatinib decreases p16 and p21 expression in lung tissue. Short-term treatment with senolytics also significantly improved the expression of genes associated with genomic instability, telomere attrition, mitochondrial dysfunction, DNA binding, and the inflammatory response. In contrast, in response to low-dose LPS administration, there was increased expression of genes associated with genomic instability, mitochondrial dysfunction, and heightened inflammatory responses in young adult murine lung (3 months of age). Taken together, the results of our current study illustrate the efficacy of senolytic treatment on modulating responses in aged lung and the potential role of chronic low dose inflammation on senescence induction in the lung.

The action of phytochemicals in biofilm control

Covering: 2009 to 2021Antimicrobial resistance is now rising to dangerously high levels in all parts of the world, threatening the treatment of an ever-increasing range of infectious diseases. This has becoming a serious public health problem, especially due to the emergence of multidrug-resistance among clinically important bacterial species and their ability to form biofilms. In addition, current anti-infective therapies have low efficacy in the treatment of biofilm-related infections, leading to recurrence, chronicity, and increased morbidity and mortality. Therefore, it is necessary to search for innovative strategies/antibacterial agents capable of overcoming the limitations of conventional antibiotics. Natural compounds, in particular those obtained from plants, have been exhibiting promising properties in this field. Plant secondary metabolites (phytochemicals) can act as antibiofilm agents through different mechanisms of action from the available antibiotics (inhibition of quorum-sensing, motility, adhesion, and reactive oxygen species production, among others). The combination of different phytochemicals and antibiotics have revealed synergistic or additive effects in biofilm control. This review aims to bring together the most relevant reports on the antibiofilm properties of phytochemicals, as well as insights into their structure and mechanistic action against bacterial pathogens, spanning December 2008 to December 2021.

Wogonin attenuates the pathogenicity of Streptococcus pneumoniae by double-target inhibition of Pneumolysin and Sortase A

Streptococcus pneumoniae (S. pneumoniae) is a major causative agent of respiratory disease in patients and can cause respiratory distress and other symptoms in severe cases. Pneumolysin (PLY) is a pore-forming toxin that induces host tissue injury and inflammatory responses. Sortase A (SrtA), a catalytic enzyme that anchors surface-associated virulence factors, is critical for S. pneumoniae virulence. Here, we found that the active ingredient of the Chinese herb Scutellaria baicalensis, wogonin, simultaneously inhibited the haemolytic activity of PLY and SrtA activity. Consequently, wogonin decreased PLY-mediated cell damage and reduced SrtA-mediated biofilm formation by S. pneumoniae. Furthermore, our data indicated that wogonin did not affect PLY expression but directly altered its oligomerization, leading to reduced activity. Furthermore, the analysis of a mouse pneumonia model further revealed that wogonin reduced mortality in mice infected with S. pneumoniae laboratory strain D39 and S. pneumoniae clinical isolate E1, reduced the number of colony-forming units in infected mice and decreased the W/D ratio and levels of the inflammatory factors TNF-α, IL-6 and IL-1β in the lungs of infected mice. Thus, wogonin reduces S. pneumoniae pathogenicity by inhibiting the dual targets PLY and SrtA, providing a treatment option for S. pneumoniae infection.

Natural Medicine a Promising Candidate in Combating Microbial Biofilm

Studies on biofilm-related infections are gaining prominence owing to their involvement in most clinical infections and seriously threatening global public health. A biofilm is a natural form of bacterial growth ubiquitous in ecological niches, considered to be a generic survival mechanism adopted by both pathogenic and non-pathogenic microorganisms and entailing heterogeneous cell development within the matrix. In the ecological niche, quorum sensing is a communication channel that is crucial to developing biofilms. Biofilm formation leads to increased resistance to unfavourable ecological effects, comprising resistance to antibiotics and antimicrobial agents. Biofilms are frequently combated with modern conventional medicines such as antibiotics, but at present, they are considered inadequate for the treatment of multi-drug resistance; therefore, it is vital to discover some new antimicrobial agents that can prevent the production and growth of biofilm, in addition to minimizing the side effects of such therapies. In the search for some alternative and safe therapies, natural plant-derived phytomedicines are gaining popularity among the research community. Phytomedicines are natural agents derived from natural plants. These plant-derived agents may include flavonoids, terpenoids, lectins, alkaloids, polypeptides, polyacetylenes, phenolics, and essential oils. Since they are natural agents, they cause minimal side effects, so could be administered with dose flexibility. It is vital to discover some new antimicrobial agents that can control the production and growth of biofilms. This review summarizes and analyzes the efficacy characteristics and corresponding mechanisms of natural-product-based antibiofilm agents, i.e., phytochemicals, biosurfactants, antimicrobial peptides, and their sources, along with their mechanism, quorum sensing signalling pathways, disrupting extracellular matrix adhesion. The review also provides some other strategies to inhibit biofilm-related illness. The prepared list of newly discovered natural antibiofilm agents could help in devising novel strategies for biofilm-associated infections.

Bioactive compounds: a goldmine for defining new strategies against pathogenic bacterial biofilms?

Most human infectious diseases are caused by microorganisms growing as biofilms. These three-dimensional self-organized communities are embedded in a dense matrix allowing microorganisms to persistently inhabit abiotic and biotic surfaces due to increased resistance to both antibiotics and effectors of the immune system. Consequently, there is an urgent need for novel strategies to control biofilm-associated infections. Natural products offer a vast array of chemical structures and possess a wide variety of biological properties; therefore, they have been and continue to be exploited in the search for potential biofilm inhibitors with a specific or multi-locus mechanism of action. This review provides an updated discussion of the major bioactive compounds isolated from several natural sources - such as plants, lichens, algae, microorganisms, animals, and humans - with the potential to inhibit biofilm formation and/or to disperse established biofilms by bacterial pathogens. Despite the very large number of bioactive products, their exact mechanism of action often remains to be clarified and, in some cases, the identity of the active molecule is still unknown. This knowledge gap should be filled thus allowing development of these products not only as novel drugs to combat bacterial biofilms, but also as antibiotic adjuvants to restore the therapeutic efficacy of current antibiotics.

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

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

The therapeutic potential of quercetin for cigarette smoking-induced chronic obstructive pulmonary disease: a narrative review

Quercetin is a flavonoid with antioxidant and anti-inflammatory properties. Quercetin has potentially beneficial therapeutic effects for several diseases, including cigarette smoking-induced chronic obstructive pulmonary disease (CS-COPD). Many studies have shown that quercetin's antioxidant and anti-inflammatory properties have positive therapeutic potential for CS-COPD. In addition, quercetin's immunomodulatory, anti-cellular senescence, mitochondrial autophagy-modulating, and gut microbiota-modulating effects may also have therapeutic value for CS-COPD. However, there appears to be no review of the possible mechanisms of quercetin for treating CS-COPD. Moreover, the combination of quercetin with common therapeutic drugs for CS-COPD needs further refinement. Therefore, in this article, after introducing the definition and metabolism of quercetin, and its safety, we comprehensively presented the pathogenesis of CS-COPD related to oxidative stress, inflammation, immunity, cellular senescence, mitochondrial autophagy, and gut microbiota. We then reviewed quercetin's anti-CS-COPD effects, performed by influencing these mechanisms. Finally, we explored the possibility of using quercetin with commonly used drugs for treating CS-COPD, providing a basis for future screening of excellent drug combinations for treating CS-COPD. This review has provided meaningful information on quercetin's mechanisms and clinical use in treating CS-COPD.

Therapeutic potential of kaempferol on Streptococcus pneumoniae infection

Co-infections with pathogens and secondary bacterial infections play significant roles during the pandemic coronavirus disease 2019 (COVID-19) pathogenetic process, caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Notably, co-infections with Streptococcus pneumoniae (S. pneumoniae), as a major Gram-positive pathogen causing pneumonia or meningitis, severely threaten the diagnosis, therapy, and prognosis of COVID-19 worldwide. Accumulating evidences have emerged indicating that S. pneumoniae evolves multiple virulence factors, including pneumolysin (PLY) and sortase A (SrtA), which have been extensively explored as alternative anti-infection targets. In our study, natural flavonoid kaempferol was identified as a potential candidate drug for infection therapeutics via anti-virulence mechanisms. We found that kaempferol could interfere with the pore-forming activity of PLY by engaging with catalytic active sites and consequently inhibit PLY-mediated cytotoxicity. Additionally, exposed to kaempferol significantly reduced the SrtA peptidase activity by occupying the active sites of SrtA. Further, the biofilms formation and bacterial adhesion to the host cells could be significantly thwarted by kaempferol incubation. In vivo infection model by S. pneumoniae highlighted that kaempferol oral administration exhibited notable treatment benefits, as evidenced by decreased bacterial burden, suggesting that kaempferol has tremendous potential to attenuate S. pneumoniae pathogenicity. Scientifically, our study implies that kaempferol is a promising therapeutic option by targeting bacterial virulence factors.

Cinnamaldehyde Resist Salmonella Typhimurium Adhesion by Inhibiting Type I Fimbriae

Salmonella Typhimurium (S. Typhimurium), a common foodborne pathogen, severely harms the public and food security. Type I fimbriae (T1F) of S. Typhimurium, plays a crucial role in the pathogenic processes; it mediates the adhesion of bacteria to the mannose receptor on the host cell, assists the bacteria to invade the host cell, and triggers an inflammatory response. Cinnamaldehyde is the main ingredient in cinnamon essential oil. In this study, cinnamaldehyde was demonstrated to inhibit the expression of T1F by hemagglutination inhibition test, transmission electron microscopy, and biofilms. The mechanism of cinnamaldehyde action was studied by proteomics technology, PCR and Western blotting. The results showed that cinnamaldehyde can inhibit T1F in S. typhimurium without the growth of bacteria, by regulating the level of expression and transcription of fimA, fimZ, fimY, fimH and fimW. Proteomics results showed that cinnamaldehyde downregulated the subunits and regulators of T1F. In addition, the invasion assays proved that cinnamaldehyde can indeed reduce the ability of S. typhimurium to adhere to cells. The results of animal experiments showed that the colonization in the intestinal tract and the expression levels of inflammatory cytokine were significantly decreased, and the intestinal mucosal immune factors MUC1 and MUC2 were increased under cinnamaldehyde treatment. Therefore, cinnamaldehyde may be a potential drug to target T1F to treat Salmonella infections.

Anti-inflammatory, antioxidant and anti-virulence roles of atractylodin in attenuating Listeria monocytogenes infection

Background

Listeria monocytogenes (L. monocytogenes), as a pandemic foodborne pathogen, severely threatens food security and public health care worldwide, which evolves multiple bacterial virulence factors (such as listeriolysin O, LLO) for manipulating the immune response of L. monocytogenes-host interactions.

Methods

Hemolysis assay was employed to screen a potential LLO inhibitor and the underlying mechanisms were investigated using molecular dynamics (MD) simulation and oligomerization assay. The effects of candidates on immune response were examined by qRT-PCR and immunoblotting analysis. Histological analysis, ELISA assay and biochemistry detection were conducted to assess in vivo efficacy of candidates.

Results

In the present study, natural terpenoid atractylodin was characterized as an alternative drug candidate for the treatment of L. monocytogenes by the regulation of LLO function and host Nrf2/NLRP3 signaling pathway. Notably, in vivo infection model by L. monocytogenes also highlighted that atractylodin treatment provided effective therapeutic benefits, as evidenced by decreased bacterial burden and diminished inflammation. Congruently, the survival rate of L. monocytogenes-infection mice increased significantly from 10.0% to 40.0% by atractylodin treatment.

Conclusion

Collectively, our study showed for the first time that atractylodin has tremendous potential to attenuate L. monocytogenes pathogenicity by blocking LLO pore formation and mediating the suppression of inflammation and oxidative stress, providing a promising therapeutic strategy and broadening the applications of atractylodin against L. monocytogenes infection.

The Current Antimicrobial and Antibiofilm Activities of Synthetic/Herbal/Biomaterials in Dental Application

Herbal and chemical products are used for oral care and biofilm treatment and also have been reported to be controversial in the massive trials conducted in this regard. The present review is aimed at evaluating the potential of relevant herbal and chemical products and comparing their outcomes to conventional oral care products and summarizing the current state of evidence of the antibiofilm properties of different products by evaluating studies from the past eleven years. Chlorhexidine gluconate (CHX), essential oils (EOs), and acetylpyridinium chloride were, respectively, the most commonly studied agents in the included studies. As confirmed by all systematic reviews, CHX and EO significantly control the plaque formation and gingival indices. Fluoride is another interesting reagent in oral care products that has shown promising results of oral health improvement, but the evidence quality needs to be refined. The synergy between natural plants and chemical products should be targeted in the future to accede to the formation of new, efficient, and healthy anticaries strategies. Moreover, to discover their biofilm-interfering or biofilm-inhibiting activities, effective clinical trials are needed. In this review article, therapeutic applications of herbal/chemical materials in oral biofilm infections are discussed in recent years (2010-2022).

An Overview of Biofilm Formation-Combating Strategies and Mechanisms of Action of Antibiofilm Agents

Biofilm formation on surfaces via microbial colonization causes infections and has become a major health issue globally. The biofilm lifestyle provides resistance to environmental stresses and antimicrobial therapies. Biofilms can cause several chronic conditions, and effective treatment has become a challenge due to increased antimicrobial resistance. Antibiotics available for treating biofilm-associated infections are generally not very effective and require high doses that may cause toxicity in the host. Therefore, it is essential to study and develop efficient anti-biofilm strategies that can significantly reduce the rate of biofilm-associated healthcare problems. In this context, some effective combating strategies with potential anti-biofilm agents, including plant extracts, peptides, enzymes, lantibiotics, chelating agents, biosurfactants, polysaccharides, organic, inorganic, and metal nanoparticles, etc., have been reviewed to overcome biofilm-associated healthcare problems. From their extensive literature survey, it can be concluded that these molecules with considerable structural alterations might be applied to the treatment of biofilm-associated infections, by evaluating their significant delivery to the target site of the host. To design effective anti-biofilm molecules, it must be assured that the minimum inhibitory concentrations of these anti-biofilm compounds can eradicate biofilm-associated infections without causing toxic effects at a significant rate.

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.

Antimicrobial Activity of Quercetin: An Approach to Its Mechanistic Principle

Quercetin, an essential plant flavonoid, possesses a variety of pharmacological activities. Extensive literature investigates its antimicrobial activity and possible mechanism of action. Quercetin has been shown to inhibit the growth of different Gram-positive and Gram-negative bacteria as well as fungi and viruses. The mechanism of its antimicrobial action includes cell membrane damage, change of membrane permeability, inhibition of synthesis of nucleic acids and proteins, reduction of expression of virulence factors, mitochondrial dysfunction, and preventing biofilm formation. Quercetin has also been shown to inhibit the growth of various drug-resistant microorganisms, thereby suggesting its use as a potent antimicrobial agent against drug-resistant strains. Furthermore, certain structural modifications of quercetin have sometimes been shown to enhance its antimicrobial activity compared to that of the parent molecule. In this review, we have summarized the antimicrobial activity of quercetin with a special focus on its mechanistic principle. Therefore, this review will provide further insights into the scientific understanding of quercetin’s mechanism of action, and the implications for its use as a clinically relevant antimicrobial agent.

Synergistic Effect of Lithocholic Acid with Gentamicin against Gram-Positive Bacteria but Not against Gram-Negative Bacteria

Listeria monocytogenes (L. monocytogenes) is an important Gram-positive food-borne pathogen that severely threatens public health. A checkerboard microdilution method was performed to evaluate the synergistic effect of lithocholic acid (LCA) with Gentamicin (Genta) against L. monocytogenes. BacLight LIVE/DEAD staining, scanning electron microscopy and biofilm inhibition assays were further used to explore the bactericidal effect and antibiofilm effect of this combination on L. monocytogenes. Additionally, the synergistic effects of LCA derivatives with Genta were also evaluated against L. monocytogenes, S.aureus and S. suis. The results indicated that a synergistic bactericidal effect was observed for the combined therapy of LCA at the concentration without affecting bacteria viability, with Genta. Additionally, LCA in combination with Genta had a synergistic effect against Gram-positive bacteria (L. monocytogenes, S. aureus and S. suis) but not against Gram-negative bacteria (E. coli, A. baumannii and Salmonella). BacLight LIVE/DEAD staining and scanning electron microscopy analysis revealed that the combination of LCA with Genta caused L. monocytogenes membrane injury, leading to bacteria death. We found that 8 μg/mL LCA treatment effectively improved the ability of Genta to eradicate L. monocytogenes biofilms. In addition, we found that chenodeoxycholic acid, as a cholic acid derivative, also improved the bactericidal effect of Genta against Gram-positive bacteria. Our results indicate that LCA represents a broad-spectrum adjuvant with Genta for infection caused by L. monocytogenes and other Gram-positive pathogens.

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.

Sources, Properties and Pharmacological Effects of Quercetin

Abstract:

The present review aims to describe an overview of quercetin with its various pharmacological effects. Quercetin is used as antioxidant, anticancer, antibacterial and antimicrobial, anti-inflammatory, antidiabetic, antihypertensive, antifungal, anti-allergic and antiproliferative agents, which are described in the manuscript. It is mainly obtained from plant resources, which is also described in the manuscript. The manuscript also focuses on describing the various studies related to quercetin which shows various pharmacological activities. It is concluded from the study that quercetin has shown an efficacious effect on various diseases.

UPLC-ESI-MS/MS profiling of the underground parts of common Iris species in relation to their anti-virulence activities against Staphylococcusaureus

ETHNOPHARMACOLOGICAL RELEVANCE

The use of plant extracts and their phytochemicals as candidates for targeting the microbial resistance inhibition is increasingly focused in last decades. In Mongolian traditional medicine, Irises were long used for the treatment of bacterial infections. Irises have been used since the Ancient Egyptians.

AIM OF THE STUDY

Chemical composition and virulence inhibition potential of both polar (PF) and non-polar fractions (NPF) of three common Iris species (I. confusa, I. pseudacorus and I. germanica) were explored.

MATERIAL AND METHODS

Secondary metabolites profiling was characterized by the UPLC-HRMS/MS technique. Multi-variate data analysis was performed using Metaboanalyst 3.0. Anti-virulence inhibitory activity was evaluated via anti-haemolytic assay and Quantitative biofilm inhibition assay.

RESULTS

I. pseudacorus PF exhibited the most potent effect against S. aureus haemolytic activity. All the tested fractions from all species, except I. pseudacorus NPF, have no significant inhibition on the biofilm formation of methicillin resistant and sensitive (MRSA and MSSA) S. aureus. I. pseudacorus NPF showed potent biofilm inhibitory potential of 71.4 and 85.8% against biofilm formation of MRSA and MSSA, respectively. Metabolite profiling of the investigated species revealed ninety and forty-five metabolites detected in the PFs and NPFs, respectively. Nigricin-type, tectorigenin-type isoflavonids and xanthones allowed the discrimination of I. pseudacorus PF from the other species, highlighting the importance of those metabolites in exerting its promising activity. On the other hand, triterpene acids, iridals, triacylglycerols and ceramides represented the metabolites detected in highest abundance in I. pseudacorus NPF.

CONCLUSIONS

This is the sole map represents the secondary metabolites profiling of the PFs and NPFs of common Iris species correlating them with the potent explored Staphylococcus aureus anti-virulence activity.

Optimized protocols for assessing libraries of poorly soluble sortase A inhibitors for antibacterial activity against medically-relevant bacteria, toxicity and enzyme inhibition

Increasing antimicrobial resistance is a major global health concern. Conventional antibiotics apply selection pressures, which promote the accumulation of resistant microbes. Anti-virulence strategies, in contrast, are less potent antimicrobials, but are less likely to select for resistance, can be combined with existing antibiotics to improve their activity, and in some cases can overcome antimicrobial resistance towards other antimicrobials. Sortase A inhibitors (SrtAIs) represent an exciting example of this class; however, many reported examples demonstrate poor water solubility, which complicates their biological assessment and activity. This includes reports that use antimicrobial concentrations of organic solvents or conditions that fail to solubilise these compounds for minimal inhibitory concentration (MIC) assessments. Herein, we report the first study to optimise screening processes for a library of prospective SrtAIs (trans-chalcone (TC), berberine (BR), curcumin (CUR), and quercetin (QC)), including comparative assessment of the effects of various co-solvent concentrations, along with comparative assessment of their antimicrobial activities against multiple disease relevant bacterial strains (methicillin-sensitive and resistant S. aureus, E. coli, and P. aeruginosa), inhibition of the sortase A enzyme, and toxicity towards mammalian cells (HEK-293), using these optimised conditions. Optimal solubility with minimal effect on bacterial viability was observed in the presence of 5% (v/v) dimethyl sulfoxide (DMSO)-Mueller-Hinton Broth. Three antimicrobial susceptibility tests (broth microdilution, agar dilution, and disk diffusion) were assessed for their ability to accurately determine minimal inhibitory concentration (MIC) data for each SrtAI. Broth microdilution and agar dilution were both effective; however, the broth microdilution assay required the addition of a colorimetric metabolic indicator (resazurin) to enable simple and reliable MIC determination due to the development of precipitants over time. In contrast, disk diffusion did not provide reliable zone of inhibition data. Identical MIC data was observed with methicillin-sensitive and -resistant S. aureus (MRSA; ATCC43300), with lower potency activity against E. coli and P. aeruginosa. Under these conditions, TC and CUR demonstrated significant toxicity towards human embryonic kidney (HEK-293) cells, with QC showing less toxicity and BR limited-to-no toxicity at its MIC. Overall, the findings of this work provide optimised processes, which will prove useful for the study of other poorly soluble antimicrobial agents and SrtAIs. The obtained data suggests that BR should be considered in preference to the other SrtAIs for the development of new antimicrobial formulations, based on its superior antimicrobial and SrtA inhibition potency, and greatly reduced toxicity.

Natural Product Rottlerin Derivatives Targeting Quorum Sensing

Rottlerin is a natural product consisting of chalcone and flavonoid scaffolds, both of which have previously shown quorum sensing (QS) inhibition in various bacteria. Therefore, the unique rottlerin scaffold highlights great potential in inhibiting the QS system of Pseudomonas aeruginosa. Rottlerin analogues were synthesised by modifications at its chalcone- and methylene-bridged acetophenone moieties. The synthesis of analogues was achieved using an established five-step synthetic strategy for chalcone derivatives and utilising the Mannich reaction at C6 of the chromene to construct morpholine analogues. Several pyranochromene chalcone derivatives were also generated using aldol conditions. All the synthetic rottlerin derivatives were screened for QS inhibition and growth inhibition against the related LasR QS system. The pyranochromene chalcone structures displayed high QS inhibitory activity with the most potent compounds, 8b and 8d, achieving QS inhibition of 49.4% and 40.6% and no effect on bacterial growth inhibition at 31 µM, respectively. Both compounds also displayed moderate biofilm inhibitory activity and reduced the production of pyocyanin.

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

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

Inhibitory Effect of the Traditional Chinese Medicine Ephedra sinica granules on Streptococcus pneumoniae Pneumolysin

Streptococcus pneumoniae (S. pneumoniae) is an opportunistic pathogen that causes pneumonia, meningitis and bacteremia in humans and animals. Pneumolysin (PLY), a major pore-forming toxin that is important for S. pneumoniae pathogenicity, is a promising target for the development of anti-infective agents. Ephedra sinica granules (ESG) is one of the oldest medical preparation with multiple biological activities (such as a divergent wind and cold effect); however, the detailed mechanism remains unknown. In this study, we found that ESG treatment significantly inhibited the oligomerization of PLY and then reduced the activity of PLY without affecting S. pneumoniae growth and PLY production. In a PLY and A549 cell co-incubation system, the addition of ESG resulted in significant protection against PLY-mediated cell injury. Furthermore, S. pneumoniae-infected mice showed decreased mortality, and alleviated tissue damage and inflammatory reactions following treatment with ESG. Our results indicate that ESG is a potential candidate treatment for S. pneumoniae infection that targets PLY. This finding partially elucidates the mechanism of the Chinese herbal formula ESG in the treatment of pneumococcal disease.

Quercetin: Its Main Pharmacological Activity and Potential Application in Clinical Medicine

Quercetin is a flavonoid compound widely present in plants and exhibits a variety of biological activities. Research on quercetin has shown its potential for medical application. In this research, we elucidate its antioxidant mechanism and the broad-spectrum antibacterial and antiparasite properties; summarise its potential application in antioncology and cardiovascular protection and anti-immunosuppression treatment; and demonstrate its ability to alleviate the toxicity of mycotoxins. This research is expected to offer some insights and inspirations for the further study of quercetin, its properties, and the scientific basis for its better application in clinical practice.

Catalytic and lectin domains in neuraminidase A from Streptococcus pneumoniae are capable of an intermolecular assembly: Implications for biofilm formation

Neuraminidase A from Streptococcus pneumoniae (NanA) is a cell wall-bound modular enzyme containing one lectin and one catalytic domain. Unlike homologous NanB and NanC expressed by the same bacterium, the two domains within one NanA molecule do not form a stable interaction and are spatially separated by a 16-amino acid-long flexible linker. In this work, the ability of NanA to form intermolecular assemblies was characterized using the methods of molecular modeling and bioinformatic analysis based on crystallographic data and by bringing together previously published experimental data. It was concluded that two catalytic domains, as well as one catalytic and one lectin domain, originating from two cell wall-bound NanA molecules, can interact through a previously uncharacterized interdomain interface to form complexes stabilized by a network of intermolecular hydrogen bonds and salt bridges. Supercomputer modeling strongly indicated that artocarpin, an earlier experimentally discovered inhibitor of the pneumococcal biofilm formation, is able to bind to a site located in the catalytic domain of one NanA entity and prevent its interaction with the lectin or catalytic domain of another NanA entity, thus directly precluding the generation of intermolecular assemblies. The revealed structural adaptation is discussed as one plausible mechanism of noncatalytic participation of this potentially key pathogenicity enzyme in pneumococcal biofilm formation.

Patent evaluation of US2019338018 (A1) 2019-11-07 (antibody fragments for the treatment of biofilm-related disorders)

INTRODUCTION

To date, microbial infections are also difficult to eradicate due to the increasing capability of bacteria to form a biofilm. In the era of antibiotic resistance, antibody-based approaches represent great promise in curing infective pathogens. The authors of US2019338018 patent propose a method for the treatment of biofilm-related disorders by using specific antibody fragments.

AREAS COVERED

The US2019338018 patent reports antibody fragments, pharmaceutical composition that contains it, and their application for the treatment of biofilm-linked disorders. Proof concept and preclinical results show that mAb mIhfB4_NTHI Fab caused robust eradication of the biofilm in the middle ear lumen of chinchillas affected by Hemophilus influenzae infection.

EXPERT OPINION

Fab fragments of the US2019338018 patent are new in a general concept to treat bacterial biofilms and biofilm-linked disorders. However, pre-clinical data are only shown for the treatment with Fab fragments of infections caused by H. influenzae in the middle ear of chinchillas. There are no clinical trials that demonstrate that the treatment with Fab fragments may induce a disruption of biofilm produced by H. influenzae or other pathogens and an anti-inflammatory response in infected patients.

Common plant flavonoids prevent the assembly of amyloid curli fibres and can interfere with bacterial biofilm formation

Like all macroorganisms, plants have to control bacterial biofilm formation on their surfaces. On the other hand, biofilms are highly tolerant against antimicrobial agents and other stresses. Consequently, biofilms are also involved in human chronic infectious diseases, which generates a strong demand for anti-biofilm agents. Therefore, we systematically explored major plant flavonoids as putative anti-biofilm agents using different types of biofilms produced by Gram-negative and Gram-positive bacteria. In Escherichia coli macrocolony biofilms, the flavone luteolin and the flavonols myricetin, morin and quercetin were found to strongly reduce the extracellular matrix. These agents directly inhibit the assembly of amyloid curli fibres by driving CsgA subunits into an off-pathway leading to SDS-insoluble oligomers. In addition, they can interfere with cellulose production by still unknown mechanisms. Submerged biofilm formation, however, is hardly affected. Moreover, the same flavonoids tend to stimulate macrocolony and submerged biofilm formation by Pseudomonas aeruginosa. For Bacillus subtilis, the flavonone naringenin and the chalcone phloretin were found to inhibit growth. Thus, plant flavonoids are not general anti-biofilm compounds but show species-specific effects. However, based on their strong and direct anti-amyloidogenic activities, distinct plant flavonoids may provide an attractive strategy to specifically combat amyloid-based biofilms of some relevant pathogens.

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.

Phytochemical Composition and In Vitro Biological Activity of Iris spp. (Iridaceae): A New Source of Bioactive Constituents for the Inhibition of Oral Bacterial Biofilms

The inhibition and eradication of oral biofilms is increasingly focused on the use of plant extracts as mouthwashes and toothpastes adjuvants. Here, we report on the chemical composition and the antibiofilm activity of 15 methanolic extracts of Iris species against both mono-(Pseudomonas aeruginosa, Staphylococcus aureus) and multi-species oral biofilms (Streptococcus gordonii, Veillonella parvula, Fusobacterium nucleatum subsp. nucleatum, and Actinomyces naeslundii). The phytochemical profiles of Iris pallida s.l., Iris versicolor L., Iris lactea Pall., Iris carthaliniae Fomin, and Iris germanica were determined by ultra-high performance liquid chromatography-high-resolution tandem mass spectroscopy (UHPLC-HRMS/MS) analysis, and a total of 180 compounds were identified among Iris species with (iso)flavonoid dominancy. I. pallida, I. versicolor, and I. germanica inhibited both the quorum sensing and adhesion during biofilm formation in a concentration-dependent manner. However, the extracts were less active against maturated biofilms. Of the five tested species, Iris pallida s.l. was the most effective at both inhibiting biofilm formation and disrupting existing biofilms, and the leaf extract exhibited the strongest inhibitory effect compared to the root and rhizome extracts. The cytotoxicity of the extracts was excluded in human fibroblasts. The inhibition of bacterial adhesion significantly correlated with myristic acid content, and quorum sensing inhibition correlated with the 7-β-hydroxystigmast-4-en-3-one content. These findings could be useful for establishing an effective tool for the control of oral biofilms and thus dental diseases.

Defeating Antibiotic-Resistant Bacteria: Exploring Alternative Therapies for a Post-Antibiotic Era

Antibiotics are one of the greatest medical advances of the 20th century, however, they are quickly becoming useless due to antibiotic resistance that has been augmented by poor antibiotic stewardship and a void in novel antibiotic discovery. Few novel classes of antibiotics have been discovered since 1960, and the pipeline of antibiotics under development is limited. We therefore are heading for a post-antibiotic era in which common infections become untreatable and once again deadly. There is thus an emergent need for both novel classes of antibiotics and novel approaches to treatment, including the repurposing of existing drugs or preclinical compounds and expanded implementation of combination therapies. In this review, we highlight to utilize alternative drug targets/therapies such as combinational therapy, anti-regulator, anti-signal transduction, anti-virulence, anti-toxin, engineered bacteriophages, and microbiome, to defeat antibiotic-resistant bacteria.

An overview on anti-biofilm properties of quercetin against bacterial pathogens

Bacterial biofilms are multicellular aggregates enclosed in a self-created biopolymer matrix. Biofilm-producing bacteria have become a great public health problem worldwide because biofilms enable these microorganisms to evade several clearance mechanisms produced by host and synthetic sources. Over the past years, different flavonoids including quercetin have engrossed considerable interest among researchers owing to their potential anti-biofilm properties. To our knowledge, there is no review regarding effects of quercetin towards bacterial biofilms, prompting us to summarize experimental evidence on its anti-biofilm properties. Quercetin inhibits biofilm development by a diverse array of bacterial pathogens such as Enterococcus faecalis, Staphylococcus aureus, Streptococcus mutans, Escherichia coli, and Pseudomonas aeruginosa. Prevention of bacterial adhesion, suppression of quorum-sensing pathways, disruption or alteration of plasma membrane, inhibition of efflux pumps, and blocking nucleic acid synthesis have been documented as major anti-biofilm mechanisms of quercetin. Overall, anti-biofilm activity of quercetin can open up new horizons in a wide range of biomedical areas, from food industry to medicine.

Developing natural products as potential anti-biofilm agents

Biofilm is a natural form of bacterial growth ubiquitously in environmental niches. The biofilm formation results in increased resistance to negative environmental influences including resistance to antibiotics and antimicrobial agents. Quorum sensing (QS) is cell-to-cell communication mechanism, which plays an important role in biofilm development and balances the environment when the bacteria density becomes high. Due to the prominent points of biofilms implicated in infectious disease and the spread of multi-drug resistance, it is urgent to discover new antibacterial agents that can regulate biofilm formation and development. Accumulated evidences demonstrated that natural products from plants had antimicrobial and chemo-preventive properties in modulation of biofilm formation in the last two decades. This review will summarize recent studies on the discovery of natural anti-biofilm agents from plants with clear-cut mechanisms or identified molecular addresses, as well as some herbs with unknown mechanisms or unidentified bioactive ingredients. We also focus on the progression of techniques on the extraction and identification of natural anti-biofilm substances. Besides, anti-biofilm therapeutics undergoing clinical trials are discussed. These newly discovered natural anti-biofilm agents are promising candidates which could provide novel strategies for biofilm-associated infections.

Quercetin impairs Streptococcus pneumoniae biofilm formation by inhibiting sortase A activity

Biofilm formation mediated by sortase A (srtA) is important for bacterial colonisation and resistance to antibiotics. Thus, the inhibitor of SrtA may represent a promising agent for bacterial infection. The structure of Streptococcus pneumoniae D39 srtA has been characterised by crystallisation. Site‐directed mutagenesis was used for the determination of the key residues for the activity of S. pneumoniae D39 srtA. An effective srtA inhibitor, quercetin, and its mechanism was further identified using srtA activity inhibition assay and molecular modelling. In this study, the crystal structure of S. pneumoniae D39 srtA has been solved and shown to contain a unique domain B. Additionally, its transpeptidase activity was evaluated in vitro. Based on the structure, we identified Cys207 as the catalytic residue, with His141 and Arg215 serving as binding sites for the peptide substrate. We found that quercetin can specifically compete with the natural substrate, leading to a significant decrease in the catalytic activity of this enzyme. In cells co‐cultured with this small molecule inhibitor, NanA cannot anchor to the cell wall effectively, and biofilm formation and biomass decrease significantly. Interestingly, when we supplemented cultures with sialic acid, a crucial signal for pneumococcal coloniation and the invasion of the host in the co‐culture system, biofilm loss did not occur. This result indicates that quercetin inhibits biofilm formation by affecting sialic acid production. In conclusion, the inhibition of pneumococcal srtA by the small molecule quercetin offers a novel strategy for pneumococcal preventative therapy.