Morin hydrate attenuates Staphylococcus aureus virulence by inhibiting the self-assembly of α-hemolysin

A review of chemical signaling mechanisms underlying quorum sensing and its inhibition in Staphylococcus aureus

Staphylococcus aureus is a significant bacterium responsible for multiple infections and is a primary cause of fatalities among patients in hospital environments. The advent of pathogenic bacteria such as methicillin-resistant S. aureus revealed the shortcomings of employing antibiotics to treat bacterial infectious diseases. Quorum sensing enhances S. aureus's survivability through signaling processes. Targeting the key components of quorum sensing has drawn much interest nowadays as a promising strategy for combating infections caused by bacteria. Concentrating on the accessory gene regulator quorum-sensing mechanism is the most commonly suggested anti-virulence approach for S.aureus. Quorum quenching is a common strategy for controlling illnesses triggered by microorganisms since it reduces the pathogenicity of bacteria and improves bacterial biofilm susceptibility to antibiotics, thus providing an intriguing prospect for drug discovery. Quorum sensing inhibition reduces selective stresses and constrains the emergence of antibiotic resistance while limiting bacterial pathogenicity. This review examines the quorum sensing mechanisms involved in S. aureus, quorum sensing targets and gene regulation, environmental factors affecting quorum sensing, quorum sensing inhibition, natural products as quorum sensing inhibitory agents and novel therapeutical strategies to target quorum sensing in S. aureus as drug developing technique to augment conventional antibiotic approaches.

Bacillus velezensis iturins inhibit the hemolytic activity of Staphylococcus aureus

Bovine mastitis caused by S. aureus has a major economic impact on the dairy sector. With the crucial need for new therapies, anti-virulence strategies have gained attention as alternatives to antibiotics. Here we aimed to identify novel compounds that inhibit the production/activity of hemolysins, a virulence factor of S. aureus associated with mastitis severity. We screened Bacillus strains obtained from diverse sources for compounds showing anti-hemolytic activity. Our results demonstrate that lipopeptides produced by Bacillus spp. completely prevented the hemolytic activity of S. aureus at certain concentrations. Following purification, both iturins, fengycins, and surfactins were able to reduce hemolysis caused by S. aureus, with iturins showing the highest anti-hemolytic activity (up to 76% reduction). The lipopeptides showed an effect at the post-translational level. Molecular docking simulations demonstrated that these compounds can bind to hemolysin, possibly interfering with enzyme action. Lastly, molecular dynamics analysis indicated general stability of important residues for hemolysin activity as well as the presence of hydrogen bonds between iturins and these residues, with longevous interactions. Our data reveals, for the first time, an anti-hemolytic activity of lipopeptides and highlights the potential application of iturins as an anti-virulence therapy to control bovine mastitis caused by S. aureus.

Antibacterial and Antihemolytic Activity of New Biomaterial Based on Glycyrrhizic Acid and Quercetin (GAQ) against Staphylococcus aureus

The goal of this study is to obtain and characterize the complex of quercetin with glycyrrhizic acid, which is known to serve as a drug delivery system. Quercetin is a flavonoid with a wide range of biological activities, including an antimicrobial effect. However, quercetin instability and low bioavailability that limits its use in medical practice makes it necessary to look for new nanoformulations of it. The formation of the GAQ complex (2:1) was confirmed by using UV and FT-IR spectroscopies. It was found that the GAQ exhibited antimicrobial and antihemolytical activities against S. aureus bacteria and its main virulent factor-α-hemolysin. The IC₅₀ value for the antihemolytical effect of GAQ was 1.923 ± 0.255 µg/mL. Using a fluorescence method, we also showed that the GAQ bound tightly to the toxin that appears to underlie its antihemolytic activity. In addition, another mechanism of the antihemolytic activity of the GAQ against α-hemolysin was shown, namely, its ability to increase the rigidity of the outer layer of the erythrocyte membrane and thus inhibit the incorporation of α-hemolysin into the target cells, increasing their resistance to the toxin. Both of these effects of GAQ were observed at concentrations below the MIC value for S. aureus growth, indicating the potential of the complex as an antivirulence agent.

A Wall Fragment of Cutibacterium acnes Preserves Junctional Integrity Altered by Staphylococcus aureus in an Ex Vivo Porcine Skin Model

(1) Background alteration of the skin microbiota, dysbiosis, causes skin barrier impairment resulting in disease development. Staphylococcus aureus, the main pathogen associated with dysbiosis, secretes several virulence factors, including α-toxin that damages tight junctions and compromises the integrity of the skin barrier. The use of members of the resident microbiota to restore the skin barrier, bacteriotherapy, represents a safe treatment for skin conditions among innovative options. The aim of this study is the evaluation of a wall fragment derived from a patented strain of Cutibacterium acnes DSM28251 (c40) alone and conjugated to a mucopolysaccharide carrier (HAc40) in counteracting S. aureus pathogenic action on two tight junction proteins (Claudin-1 and ZO-1) in an ex vivo porcine skin infection model. Methods: skin biopsies were infected with live S. aureus strains ATCC29213 and DSM20491. Tissue was pre-incubated or co-incubated with c40 and HAc40. (3) Results: c40 and HAc40 prevent and counteract Claudin-1 and Zo-1 damage (4) Conclusions: c40 and the functional ingredient HAc40 represent a potential non-pharmacological treatment of skin diseases associated with cutaneous dysbiosis of S. aureus. These findings offer numerous avenues for new research.

Electrophysiological and spectroscopic investigation of hydrolysable tannins interaction with α-hemolysin of S. aureus

In this study, using bilayer lipid membrane technique, we report a novel facet of antihemolytic activity of two tannins (1,2,3,4,5-penta-O-galloyl-β-D-glucose (PGG) and 1,2-di-O-galloyl-4,6-valoneoyl-β-D-glucose (dGVG)), which consists in inhibiting the formation of α-hemolysin channels and blocking the conductivity of already formed channels. These effects were observed at tannin concentrations well below minimal inhibitory concentration values for S. aureus growth. Using spectroscopic methods, we show that these two tannins differing in molecular structure but having the same number of -OH groups and aromatic rings form firm complexes with hemolysin in aqueous solutions, which may underlie the disruption of its subsequent interaction with the membrane, thus preventing hemolysis of erythrocytes. In all experimental settings, PGG was the more active compound compared to dGVG, that indicates the important role of the flexibility of the tannin molecule in interaction with the toxin. In addition, we found that PGG, but not dGVG, was able to block the release of the toxin by bacterial cells. This toxin is a strong pathogenic factor causing a number of diseases and therefore is considered as a virulence target for treatment of S. aureus infection, so the data obtained suggest that PGG and possibly other tannins of similar structure have therapeutic potential in fighting the virulence of S. aureus.

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.

Lysine Inhibits Hemolytic Activity of Staphylococcus aureus and Its Application in Food Model Contaminated with Staphylococcus aureus

Alpha-hemolysin (Hla) is one of the important exotoxins of Staphylococcus aureus (S. aureus) and can be used as a target to reduce the virulence of S. aureus. This study explored the inhibitory effect of Lysine (Lys) on Hla and its application in food safety. Lys significantly inhibited the expression of Hla at sub-inhibitory concentrations and directly interacted with Hla to interfere with its oligomerization and thus significantly inhibited its hemolytic activity. Notably, Lys attenuated S. aureus damage to mouse small intestine and Caco-2 cells and delayed mouse mortality. In the food model, Lys inhibited the expression of Hla of S. aureus and had no significant effect on the sensory score. Moreover, Lys had no obvious damage effect on the main organs of mice, which indicated that Lys has good biocompatibility and has the potential to be used in the food industry as an anti-S. aureus preparation.

Liposomes Encapsulating Morin: Investigation of Physicochemical Properties, Dermal Absorption Improvement and Anti-Aging Activity in PM-Induced Keratinocytes

Recently, a global market for anti-aging skin care using botanicals has been noticeably developing. Morin, 3,5,7,2',4'-pentahydroxyflavone, is a polyphenol with many pharmacological properties including antioxidant, anti-inflammation and photoprotection. However, poor aqueous solubility of morin restricts its application in pharmaceuticals. The present study aimed to encapsulate morin into liposomal vesicles to improve its water solubility and skin penetration, and further investigated its ROS inhibition and anti-aging activity in HaCaT keratinocytes induced by particulate matters (PMs). Our data presented that morin was a strong DPPH^• radical scavenger. Morin displayed a remarkable ROS inhibitory ability and protected keratinocytes against PMs by downregulating matrix metalloproteinase-1 (MMP-1) expression via suppressing p-ERK and p-p38 in the MAPK pathway. Moreover, water solubility of liposomal morin (LM) prepared by the thin film hydration method was significantly better than free form of morin due to particle size reduction of LM. Our results also demonstrated that deformable liposomal vesicles were achieved for increasing dermal absorption. Additionally, LM (morin:lecinolws-50:tween-80:PF-68, 1:2.5:2.5:5) was able to effectively reduce generation of ROS, inactivate p-ERK, p-p38 and MMP-1 in HaCaT cells exposed to PM. In conclusion, our findings suggested that LM would be a bright candidate for various topical anti-aging and anti-pollution products.

Inhibitory Effect of Andrographis paniculata Lactone on Staphylococcus aureus α-Hemolysin

We investigated the effect of andrographolide (AP) on the hemolytic capacity of Staphylococcus aureus (S. aureus) isolated from our region. AP is a labdane diterpenoid isolated from the stem and leaves of Andrographis paniculata. The hla gene from 234 S. aureus strains and the quality control standard strain ATCC29213 in dairy cows in some areas of Ningxia was analyzed. Evolutionary analysis, homology modeling, and functional enrichment annotation of α-hemolysin Hla detected from our region were performed through bioinformatics. The hemolytic ability of S. aureus isolates from the region was examined using the hemolysis test, and the effect of AP on S. aureus was quantified. Moreover, the effect of AP on the transcript levels of hla and genes highly related to hla (i.e., clfA and fnbA) was examined through fluorescence quantitative PCR. The mode of action of AP on the detected Hla was analyzed through molecular docking and dynamic simulation. The results showed that S. aureus in our region has a high rate of hla carriage. The hemolytic activity of strains NM98 and XF10 was significant, and ATCC29213 also exhibited some hemolytic activity. AP could inhibit the expression of Hla and its related proteins by downregulating hla, clfA, and fnbA transcript levels, which in turn attenuated the S. aureus hemolytic activity. Meanwhile, the AP molecule can form three hydrogen bonds with residues ASN105, SER106, and THR155 of Hla protein; bind with PRO103 through alkyl intermolecular forces; and form carbon hydrogen bonds with LYS154, reflecting that the AP molecule has a comparatively ideal theoretical binding activity with Hla protein. Among them, PRO103 and LYS154 are highly conserved in Hla protein molecules and play pivotal roles in the biological functions of Hla, and their binding may affect these functions. Their binding may also prevent the conformational transition of Hla from a monomer to an oligomer, thus inhibiting Hla hemolytic activity. This study offers a molecular basis for use of AP as an antivirulence drug and new ideas for developing novel drugs against S. aureus infection.

Mucoadhesive controlled-release formulations containing morin for the control of oral biofilms

This study aimed to evaluate the antimicrobial and anti-biofilm activity of morin on polymicrobial biofilms and its cytotoxicity in controlled-release films and tablets based on gellan gum. Polymicrobial biofilms were formed from saliva for 48 h under an intermittent exposure regime to 1% sucrose and in contact with films or tablets of gellan gum containing 2 mg of morin each. Acidogenicity, bacterial viability, dry weight and insoluble extracellular polysaccharides from biofilms were evaluated. The cytotoxicity of morin was evaluated in oral keratinocytes. Morin released from the systems reduced the viability of all the microbial groups evaluated, as well as the dry weight and insoluble polysaccharide concentration in the matrix and promoted the control of acidogenicity when compared with the control group without the substance. Morin was cytotoxic only at the highest concentration evaluated. In conclusion, morin is an effective agent and shows antimicrobial and anti-biofilm activity against polymicrobial biofilms.

Antimicrobial Effect of Phytochemicals from Edible Plants

Current strategies of combating bacterial infections are limited and involve the use of antibiotics and preservatives. Each of these agents has generally inadequate efficacy and a number of serious adverse effects. Thus, there is an urgent need for new antimicrobial drugs and food preservatives with higher efficacy and lower toxicity. Edible plants have been used in medicine since ancient times and are well known for their successful antimicrobial activity. Often photosensitizers are present in many edible plants; they could be a promising source for a new generation of drugs and food preservatives. The use of photodynamic therapy allows enhancement of antimicrobial properties in plant photosensitizers. The purpose of this review is to present the verified data on the antimicrobial activities of photodynamic phytochemicals in edible species of the world’s flora, including the various mechanisms of their actions.

Morin hydrate inhibits atherosclerosis and LPS-induced endothelial cells inflammatory responses by modulating the NFκB signaling-mediated autophagy

Atherosclerosis (AS) is a chronic inflammatory disease involving blood vessels. Inflammation affects different cells and increases the expression of adhesion molecules. Morin hydrate (MO) is a naturally occurring bioflavonoid with anti-inflammatory and anti-oxidant effects. Although the exact mechanism has not been fully elucidated, MO possibly influences autophagy pathways in immunity and inflammation. In this study, MO showed the potential to inhibit atherosclerotic and promote vascular endothelial autophagy in apolipoprotein E (ApoE)-/- mice with a high-fat diet. Then, we aimed to explore the anti-inflammatory effects of MO in human umbilical vein endothelial cells (HUVECs) and its relationship with autophagy. We found that MO inhibited lipopolysaccharide (LPS)-induced monocyte adhesion and the expression of intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), cyclooxygenase-2 (COX-2), and matrix metallopeptidase 9 (MMP-9) in HUVECs. Moreover, MO reduced the expression of tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) by inhibiting the phosphatidylinositol-3-kinase (PI3K)/protein kinase B (Akt)/nuclear factor kappa B (NFκB) signaling pathway. MO induced autophagy by inhibiting the NFκB signaling pathway in normal HUVECs and LPS-stimulated HUVECs. When autophagy was inhibited by 3-methyladenine (3-MA) or small interfering RNA (siRNA), the anti-inflammatory effect of MO was reduced. In conclusion, MO inhibits atherosclerosis in ApoE-/- mice and LPS-induced inflammatory responses by inhibiting the activation of the PI3K/Akt1/NFκB signaling pathway in a NFκB signaling-mediated autophagy way.

Staphylococcus aureus Toxins: An Update on Their Pathogenic Properties and Potential Treatments

Staphylococcus aureus is a clinically important pathogen that causes a wide range of human infections, from minor skin infections to severe tissue infection and sepsis. S. aureus has a high level of antibiotic resistance and is a common cause of infections in hospitals and the community. The rising prevalence of community-acquired methicillin-resistant S. aureus (CA-MRSA), combined with the important severity of S. aureus infections in general, has resulted in the frequent use of anti-staphylococcal antibiotics, leading to increasing resistance rates. Antibiotic-resistant S. aureus continues to be a major health concern, necessitating the development of novel therapeutic strategies. S. aureus uses a wide range of virulence factors, such as toxins, to develop an infection in the host. Recently, anti-virulence treatments that directly or indirectly neutralize S. aureus toxins have showed promise. In this review, we provide an update on toxin pathogenic characteristics, as well as anti-toxin therapeutical strategies.

Effect and underlying mechanism of morin on the pharmacokinetics of diclofenac sodium in rats

1. Morin, a natural flavonol, is present in many plants. It has anti-inflammatory and immunomodulatory activities and is often used as an adjuvant treatment for arthritis. Diclofenac sodium is the first-choice drug in the treatment of rheumatoid arthritis. However, the herb-drug interaction (HDI) between morin and diclofenac sodium remains unclear.2. The aim of the present research was to investigate whether and how morin affect the pharmacokinetic profile of diclofenac sodium.3. The enzyme kinetic and pharmacokinetic studies showed that morin significantly accelerated the metabolism and reduced systemic exposure of diclofenac sodium. Interestingly, the effect of morin on the pharmacokinetic profile of diclofenac sodium was not in a dose-dependent manner. Therefore, the effect of morin on P-glycoprotein (P-gp) was further investigated.4. The results implied that the influence mechanism of morin on the pharmacokinetic of diclofenac sodium might be related to CYP2C9 and P-gp. Attention should be paid to the risk of HDI between morin and diclofenac sodium in clinical practice.

The anticarcinogenic and anticancer effects of the dietary flavonoid, morin: Current status, challenges, and future perspectives

Flavonoids constitute one of the most important classes of polyphenols, which have been found to have a wide range of biological activities such as anticancer effects. A large body of evidence demonstrates that morin as a pleiotropic dietary flavonoid possesses potent anticarcinogenic and anticancer activities with minimal toxicity against normal cells. The present review comprehensively elaborates the molecular mechanisms underlying antitumorigenic and anticancer effects of morin. Morin exerts its anticarcinogenic effects through multiple cancer preventive mechanisms, including reduction of oxidative stress, activation of phase II enzymes, induction of apoptosis, attenuation of inflammatory mediators, and downregulation of p-Akt and NF-κB expression. A variety of molecular targets and signaling pathways such as apoptosis, cell cycle, reactive oxygen species (ROS), matrix metalloproteinases (MMPs), epithelial-mesenchymal transition (EMT), and microRNAs (miRNAs) as well as signal transducer and activator of transcription 3 (STAT3), NF-κB, phosphatidylinositol 3-kinase (PI3K)/Akt, mitogen-activated protein kinase (MAPK), and Hippo pathways have been found to be involved in the anticancer effects of morin. In the adjuvant therapy, morin has been shown to have synergistic anticancer effects with several chemotherapeutic drugs. The findings of this review indicate that morin can act as a promising chemopreventive and chemotherapeutic agent.

Gram-Positive Pneumonia: Possibilities Offered by Phage Therapy

Pneumonia is an acute pulmonary infection whose high hospitalization and mortality rates can, on occasion, bring healthcare systems to the brink of collapse. Both viral and bacterial pneumonia are uncovering many gaps in our understanding of host–pathogen interactions, and are testing the effectiveness of the currently available antimicrobial strategies. In the case of bacterial pneumonia, the main challenge is antibiotic resistance, which is only expected to increase during the current pandemic due to the widespread use of antibiotics to prevent secondary infections in COVID-19 patients. As a result, alternative therapeutics will be necessary to keep this disease under control. This review evaluates the advantages of phage therapy to treat lung bacterial infections, in particular those caused by the Gram-positive bacteria Streptococcus pneumoniae and Staphylococcus aureus, while also highlighting the regulatory impediments that hamper its clinical use and the difficulties associated with phage research.

Molecular Binding and Simulation Studies of Staphylococcus aureus Superantigens with Flavonoid Compounds

BACKGROUND

Superantigens of Staphylococcus aureus namely enterotoxin A, exfoliative toxin A, and Toxic shock syndrome toxin-1 cause detrimental effects on the cells of the immune system.

METHODS

In this work, the toxins were downloaded from the Protein DataBank database and energies were minimized using KoBaMIN server. Forty flavonoids compounds were identified by pubchem compound database through extensive literature study and their 3D structures were obtained by submitting SMILES to CORINA tool. Based on Lipinski's rule of five, the molecules were filtered that resulted in 27 compounds. Molecular docking was performed for identifying the binding and interaction sites of flavonoids with the toxins using Autodock 4.

RESULTS AND CONCLUSION

The docked complexes were then subjected to molecular dynamics simulation using Gromacs. The analysis revealed the stability of the complexes as indicated by three hydrogen bonds formed during the simulation time period of 20 ns.

Morin hydrate: A comprehensive review on novel natural dietary bioactive compound with versatile biological and pharmacological potential

Flavonoids are natural plant-derived dietary bioactive compounds having a substantial impact on human health. Morin hydrate is a bioflavonoid mainly obtained from fruits, stem, and leaves of Moraceae family members' plants. Plenty of evidences supported that morin hydrate exerts its beneficial effects against various chronic and life-threatening degenerative diseases. Our current article discloses the recent advances that have been studied to explore the biological/pharmacological properties and molecular mechanisms to better understand the beneficial and multiple health benefits of morin hydrate. Indeed, Morin hydrate exerts free radical scavenging, antioxidant, anti-inflammatory, anti-cancerous, anti-microbial, antidiabetic, anti-arthritis, cardioprotective, neuroprotective, nephroprotective, and hepatoprotective effects. Moreover, morin hydrate exhibits its pharmacological activities by modulating various cellular signaling pathways such as Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-қB), Mitogen-activated protein kinase (MAPK), Janus kinases/ Signal transducer and activator of transcription proteins (JAKs/STATs), Kelch-like ECH-associated protein1/Nuclear erythroid-2-related factor (Keap1/Nrf2), Endoplasmic reticulum (ER), Mitochondrial-mediated apoptosis, Wnt/β-catenin, and Mechanistic target of rapamycin (mTOR). Most importantly, morin hydrate has the potential to modulate a variety of biological networks. Therefore, it can be predicted that this therapeutically potent compound could serve as a dietary agent for the expansion of human health and might be helpful for the development of the novel drug in the future. However, due to the lack of clinical trials, special human clinical trials are needed to address the effects of morin hydrate on various life-threatening disparities to recommend morin and/or morin-rich foods with other foods or bioactive dietary components, as well as dose-response interaction and safety profile.

Exploring Virulence Factors and Alternative Therapies against Staphylococcus aureus Pneumonia

Pneumonia is an acute pulmonary infection associated with high mortality and an immense financial burden on healthcare systems. Staphylococcus aureus is an opportunistic pathogen capable of inducing S. aureus pneumonia (SAP), with some lineages also showing multidrug resistance. Given the high level of antibiotic resistance, much research has been focused on targeting S. aureus virulence factors, including toxins and biofilm-associated proteins, in an attempt to develop effective SAP therapeutics. Despite several promising leads, many hurdles still remain for S. aureus vaccine research. Here, we review the state-of-the-art SAP therapeutics, highlight their pitfalls, and discuss alternative approaches of potential significance and future perspectives.

Antimicrobial flavonoids as a potential substitute for overcoming antimicrobial resistance

BACKGROUND

Infectious diseases are the leading cause of death in 21^st century due to antimicrobial resistance and scarcity of new molecules to undertake rising infections. There could be a multiple reasons behind antimicrobial resistance whether it is increased drug metabolism or bacterial endotoxins. The demand of effective medication is increasing day by day to treat microbial infections and combat antimicrobial resistance. In recent years most of the synthetic antimicrobials developed resistance so natural products could provide better options to fulfill this demand. There has been increasing interest in the research on flavonoids because various flavonoids were found to be effective against pathogenic microorganisms.

OBJECTIVE

The objective of this article will be to explore antimicrobial activity of flavonoids with special focus on their possible mechanism of action.

METHODS

The article reviewed recent literature related to flavonoids with antimicrobial activity, which were isolated from various sources and the compounds showing fairly good activity against tested microbial species were discussed.

RESULTS

By throughout literature review it has been found that flavonoids show antimicrobial effect by inhibiting virulence factors, efflux pump, biofilm formation, membrane disruption, cell envelop synthesis, nucleic acid synthesis, and bacterial motility inhibition.

CONCLUSION

Most of the antimicrobial drugs available now a days are ineffective due to development of resistance to them. Flavonoids have the potential to overcome this emerging crisis as this class of natural products showed the antimicrobial activity by different mechanisms than those of conventional drugs, so flavonoid could be an effective treatment of pathogenic infections.

Inhibition of interaction between Staphylococcus aureus α-hemolysin and erythrocytes membrane by hydrolysable tannins: structure-related activity study

The objective of the study was a comparative analysis of the antihemolytic activity against two Staphylococcus aureus strains (8325-4 and NCTC 5655) as well as α-hemolysin and of the membrane modifying action of four hydrolysable tannins with different molecular mass and flexibility: 3,6-bis-O-di-O-galloyl-1,2,4-tri-O-galloyl-β-d-glucose (T1), 1,2,3,4,5-penta-O-galloyl-β-d-glucose (T2), 3-O-galloyl-1,2-valoneoyl-β-d-glucose (T3) and 1,2-di-O-galloyl-4,6-valoneoyl-β-d-glucose (T4). We showed that all the compounds studied manifested antihemolytic effects in the range of 5–50 µM concentrations. However, the degree of the reduction of hemolysis by the investigated tannins was not uniform. A valoneoyl group—containing compounds (T3 and T4) were less active. Inhibition of the hemolysis induced by α-hemolysin was also noticed on preincubated with the tannins and subsequently washed erythrocytes. In this case the efficiency again depended on the tannin structure and could be represented by the following order: T1 > T2 > T4 > T3. We also found a relationship between the degree of antihemolytic activity of the tannins studied and their capacity to increase the ordering parameter of the erythrocyte membrane outer layer and to change zeta potential. Overall, our study showed a potential of the T1 and T2 tannins as anti-virulence agents. The results of this study using tannins with different combinations of molecular mass and flexibility shed additional light on the role of tannin structure in activity manifestation.

Ginkgetin in vitro and in vivo reduces Streptococcus suis virulence by inhibiting suilysin activity

AIMS

Suilysin (SLY), a crucial virulence-related factor, has multiple cytotoxicities that are regarded as playing a key role in several diseases induced by Streptococcus suis. The aim of this study was to identify an effective inhibitor of SLY and to evaluate the potential inhibitory effect of the inhibitor against S. suis virulence.

METHODS AND RESULTS

Antibacterial activity experiments and haemolysis tests were used to identify the SLY inhibitor ginkgetin, and Western blot analysis and oligomerization inhibition tests were employed to determine the potential mechanism for its inhibition effect. The potential inhibitory effect of ginkgetin against S. suis virulence was then assessed through a cytotoxicity test and a mouse infection model. In this study, we demonstrated that the natural ingredient ginkgetin can significantly reduce the haemolytic activity of SLY to protect against S. suis-mediated cell injury in vitro by directly binding to SLY to block the oligomerization of the protein and reducing the bacterial burden in vivo.

CONCLUSIONS

The results suggest that ginkgetin can start being used as a potential lead drug for the treatment of S. suis infections.

SIGNIFICANCE AND IMPACT OF THE STUDY

The prevention and treatment of S. suis infection might be possible through the targeting of SLY by ginkgetin.

Composition, Antivirulence Activity, and Active Property Distribution of the Fruit of Terminalia chebula Retz

The fruit of Terminalia chebula Retz., or Tibet Olive, is widely used as a food supplement in China. It possesses some natural antimicrobial properties; however, its chemical composition and antivirulence effects have not been identified. In this work, 29 compounds were identified from the peel of T. chebula fruit by ultra-high-performance liquid chromatography-tandem quadrupole time-of-flight mass spectrometry. Both the extract of T. chebula and its phenolic acid, corilagin, showed antivirulent activity against Staphylococcus aureus. Specifically, they inhibited biofilm formation. The half maximal inhibitory concentration was 0.13 and 3.18 µg/mL for the extract and corilagin, respectively, whereas for α-hemolysin secretion, the respective concentrations were 30 and 10 µg/mL. Its mechanism of action may be due to reducing the transcription of genes related to quorum sensing. These genes included staphylococcal accessory regulator A, intercellular adhesion accessory gene regulator A, and RNAIII. These findings provide evidence that this food supplement could be an effective antivirulent with corilagin as its active ingredient. PRACTICAL APPLICATION: Corilagin from the fruit of Terminalia chebula Retz. may be used as an antibacterial for its antivirulent activity against Staphylococcus aureus.

Pharmacological Targeting of Pore-Forming Toxins as Adjunctive Therapy for Invasive Bacterial Infection

For many of the most important human bacterial infections, invasive disease severity is fueled by the cell damaging and pro-inflammatory effects of secreted pore-forming toxins (PFTs). Isogenic PFT-knockout mutants, e.g., Staphylococcus aureus lacking α-toxin or Streptococcus pneumoniae deficient in pneumolysin, show attenuation in animal infection models. This knowledge has inspired multi-model investigations of strategies to neutralize PFTs or counteract their toxicity as a novel pharmacological approach to ameliorate disease pathogenesis in clinical disease. Promising examples of small molecule, antibody or nanotherapeutic drug candidates that directly bind and neutralize PFTs, block their oligomerization or membrane receptor interactions, plug establishment membrane pores, or boost host cell resiliency to withstand PFT action have emerged. The present review highlights these new concepts, with a special focus on β-PFTs produced by leading invasive human Gram-positive bacterial pathogens. Such anti-virulence therapies could be applied as an adjunctive therapy to antibiotic-sensitive and -resistant strains alike, and further could be free of deleterious effects that deplete the normal microflora.

Morin Protects Channel Catfish From Aeromonas hydrophila Infection by Blocking Aerolysin Activity

Aeromonas hydrophila (A. hydrophila) is an opportunistic bacterial pathogen widely distributed in the environments, particular aquatic environment. The pathogen can cause a range of infections in both human and animals including fishes. However, the application of antibiotics in treatment of A. hydrophila infections leads to the emergence of resistant strains. Consequently, new approaches need to be developed in fighting this pathogen. Aerolysin, the chief virulence factor produced by pathogenic A. hydrophila strains has been employed as target identifying new drugs. In our present study, we found that morin, a flavonoid without anti-bacterial activity isolated from traditional Chinese medicine, could directly inhibit the hemolytic activity of aerolysin. To determine the binding sites and the action of mechanism of morin against AerA, several assays were performed. Ser36, Pro347, and Arg356 were identified as the main binding sites affecting the conformation of AerA and resulted in block of the heptameric formation. Moreover, morin could protect Vero cells from cell injury mediated by aerolysin. In vivo study showed that morin could provide a protection to channel catfish against A. hydrophila infection. These results demonstrated that morin could be developed as a promising candidate for the treatment of A. hydrophila infections by decreasing the pathogenesis of A. hydrophila.

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.

Erianin against Staphylococcus aureus Infection via Inhibiting Sortase A

With continuous emergence and widespread of multidrug-resistant Staphylococcus aureus infections, common antibiotics have become ineffective in treating these infections in the clinical setting. Anti-virulence strategies could be novel, effective therapeutic strategies against drug-resistant bacterial infections. Sortase A (srtA), a transpeptidase in gram-positive bacteria, can anchor surface proteins that play a vital role in pathogenesis of these bacteria. SrtA is known as a potential antivirulent drug target to treat bacterial infections. In this study, we found that erianin, a natural bibenzyl compound, could inhibit the activity of srtA in vitro (half maximal inhibitory concentration-IC₅₀ = 20.91 ± 2.31 μg/mL, 65.7 ± 7.2 μM) at subminimum inhibitory concentrations (minimum inhibitory concentrations-MIC = 512 μg/mL against S. aureus). The molecular mechanism underlying the inhibition of srtA by erianin was identified using molecular dynamics simulation: erianin binds to srtA residues Ile182, Val193, Trp194, Arg197, and Ile199, forming a stable bond via hydrophobic interactions. In addition, the activities of S. aureus binding to fibronectin and biofilm formation were inhibited by erianin, when co-culture with S. aureus. In vivo, erianin could improve the survival in mice that infected with S. aureus by tail vein injection. Experimental results showed that erianin is a potential novel therapeutic compound against S. aureus infections via affecting srtA.

P2X-Receptor Antagonists Inhibit the Interaction of S. aureus Hemolysin A with Membranes

The pore forming hemolysin A, Hla, is a major virulence factor of Staphylococcus aureus. Apparently, 1–2 pore(s) per cell suffice(s) to cause cell death. Accumulated experimental evidence points towards a major role of ATP-gated purinergic receptors (P2XR) for hemolysis caused by Hla, complement and other pore forming proteins, presumably by increasing membrane permeability. Indeed, in experiments employing rabbit erythrocytes, inhibitory concentrations of frequently employed P2XR-antagonists were in a similar range as previously reported for erythrocytes of other species and other toxins. However, Hla-dependent hemolysis was not enhanced by extracellular ATP, and oxidized adenosinetriphosphate (oxATP) had only a minor inhibitory effect. Unexpectedly, P2XR-inhibitors also prevented Hla-induced lysis of pure lipid membranes, demonstrating that the inhibition did not even depend on the presence of P2XR. Fluorescence microscopy and gel-electrophoresis clearly revealed that P2XR-inhibitors interfere with binding and subsequent oligomerisation of Hla with membranes. Similar results were obtained employing HaCaT-cells. Furthermore, calorimetric data and hemolysis experiments with Hla pre-treated with pyridoxal phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) showed that this compound directly binds to Hla. Our results call for a critical re-assessment of the appealing concept, which suggests that P2XR are general amplifiers of damage by pore-forming proteins.

Morin Attenuates Streptococcus suis Pathogenicity in Mice by Neutralizing Suilysin Activity

Streptococcus suis, a Gram-positive pathogen, is widely recognized as an important agent of swine infection, and it is also known to cause a variety of zoonoses, such as meningitis, polyarthritis and pneumonia. Suilysin (SLY), an extracellular pore-forming toxin that belongs to the cholesterol-dependent cytolysin family, is an essential virulence factor of S. suis capsular type 2 (SS2). Here, we found that morin hydrate (morin), a natural flavonoid that lacks anti-SS2 activity, inhibits the hemolytic activity of SLY, protects J774 cells from SS2-induced injury and protects mice from SS2 infection. Further, by molecular modeling and mutational analysis, we found that morin binds to the "stem" domain 2 in SLY and hinders its transformation from the monomer form to the oligomer form, which causes the loss of SLY activity. Our study demonstrates that morin hinders the cell lysis activity of SLY through a novel mechanism of interrupting the heptamer formation. These findings may lead to the development of promising therapeutic candidates for the treatment of SS2 infections.

One-Pot Hydrothermal Synthesis of Carbon Dots with Efficient Up- and Down-Converted Photoluminescence for the Sensitive Detection of Morin in a Dual-Readout Assay

Blue luminescent carbon dots (CDs) with a high photoluminescence (PL) quantum yield (48.3 ± 5.3%) were prepared by the one-pot hydrothermal reaction of citric acid with poly(ethylenimine) (PEI). The CDs display bright PL, narrow emission spectra, pH-dependent PL intensity, high photostability, and up-converted luminescence. The CDs exhibit a quenching of both down- and up-conversion PL in the presence of morin and thus serve as useful probes for morin detection. Both down- and up-conversion measurements allow the quantification of concentrations from 0 to 300 μmol/L with a detection limit of 0.6 μmol/L, and this dual-mode detection increases the reliability of the measurement. The proposed method of determination is simple, sensitive, and cost-effective, with potential applications in clinical and biochemical assays.

A highly sensitive morin sensor based on PEDT–Au/rGO nanocomposites modified glassy carbon electrode

A facilely prepared PEDT–Au/rGO/GC sensor shows high sensitivity for electrochemical determination of morin.

The combination of osthole with baicalin protects mice from Staphylococcus aureus pneumonia

We reported the inhibition of α-Hemolysin (Hla) production in methicillin-resistant Staphylococcus aureus USA300 by osthole and further investigated the combination of osthole and baicalin in the treatment of staphylococcal pneumonia. Using cytotoxicity assays and a mouse model of intranasal lung infection, we evaluated the effect of combined therapy. Our results suggest that the combination of osthole and baicalin alleviated S. aureus-mediated A549 cell injury and protected mice from S. aureus pneumonia.

Molecular Mechanism of the Flavonoid Natural Product Dryocrassin ABBA against Staphylococcus aureus Sortase A

The intractability of bacterial resistance presents a dilemma for therapies against Staphylococcus aureus (S. aureus) infection. Effective anti-virulence strategies are urgently needed, reflecting the proliferation of resistant strains. Inhibitors of sortase A (SrtA), enzymes that anchor virulence-related surface proteins, are regarded as promising candidates for countermeasures against bacterial infections. In the present study, the inhibitory effect of dryocrassin ABBA (ABBA) against SrtA and its molecular basis has been examined. Fluorescence resonance energy transfer (FRET) assays were used to determine the inhibitory activity of ABBA against SrtA. To identify the mechanism underlying this activity, molecular dynamics simulations and mutagenesis assays were applied, and the results revealed that the direct engagement of SrtA via ABBA through binding to V166 and V168 significantly attenuated the catalytic activity of SrtA. Taken together, these findings indicated that ABBA is a potential novel antimicrobial agent for S. aureus infection via targeting SrtA.

Indolo[3,2-b]quinoline Derivatives Suppressed the Hemolytic Activity of Beta-Pore Forming Toxins, Aerolysin-Like Hemolysin Produced by Aeromonas sobria and Alpha-Hemolysin Produced by Staphylococcus aureus

In an attempt to discover inhibitory compounds against pore-forming toxins, some of the major toxins produced by bacteria, we herein examined the effects of four kinds of indolo[3,2-b]quinoline derivatives on hemolysis induced by the aerolysin-like hemolysin (ALH) of Aeromonas sobria and also by the alpha-hemolysin of Staphylococcus aureus. The results showed that hemolysis induced by ALH was significantly reduced by every derivative, while that induced by alpha-hemolysis was significantly reduced by three out of the four derivatives. However, the degrees of reduction induced by these derivatives were not uniform. Each derivative exhibited its own activity to inhibit the respective hemolysin. Compounds 1 and 2, which possessed the amino group bonding the naphthalene moiety at the C-11 position of indolo[3,2-b]quinoline, had strong inhibitory effects on the activity of ALH. Compound 4 which consisted of benzofuran and quinoline had strong inhibitory effects on the activity of alpha-hemolysin. These results indicated that the amino group bonding the naphthalene moiety of compounds 1 and 2 assisted in their ability to inhibit ALH activity, while the oxygen atom at the 10 position of compound 4 strengthened its interaction with alpha-hemolysin. These compounds also suppressed the hemolytic activity of the supernatant of A. sobria or A. hydrophila, suggesting that these compounds were effective at the site of infection of these bacteria.

Morin inhibits biofilm production and reduces the virulence of Listeria monocytogenes - An in vitro and in vivo approach

The current study explores the in vitro and in vivo antibiofilm efficacy of morin against a leading foodborne pathogen-Listeria monocytogenes (LM). Minimum inhibitory concentration (MIC) of morin against LM strains was found to be 100μg/ml. The non-antibacterial effect of morin at its sub-MICs (6.25, 12.5 and 25μg/ml) was determined through growth curve and XTT assay. Morin at its sub-MICs demonstrated a significant dose dependent inhibitory efficacy against LM biofilm formation which was also evidenced through light, confocal and scanning electron microscopic analyses. However, morin failed to disperse the mature biofilm of LM even at its MIC. Our data also revealed the anti-virulence efficacy of morin, as it significantly inhibited the production of hemolysin and motility of LM. Concentration-dependent susceptibility of morin treated LM cells to normal human serum was observed. In vivo studies revealed that morin extended the lifespan of LM infected Caenorhabditis elegans by about 85%. Furthermore, the non-toxic nature and in vivo anti-adherence efficacy of morin were also ascertained through C. elegans-LM infection model. Overall, the data of the current study identifies morin as a promising antibiofilm agent and its suitability to formulate protective strategies against biofilm associated infections caused by LM.

Plant Natural Products Targeting Bacterial Virulence Factors

Decreased antimicrobial efficiency has become a global public health issue. The paucity of new antibacterial drugs is evident, and the arsenal against infectious diseases needs to be improved urgently. The selection of plants as a source of prototype compounds is appropriate, since plant species naturally produce a wide range of secondary metabolites that act as a chemical line of defense against microorganisms in the environment. Although traditional approaches to combat microbial infections remain effective, targeting microbial virulence rather than survival seems to be an exciting strategy, since the modulation of virulence factors might lead to a milder evolutionary pressure for the development of resistance. Additionally, anti-infective chemotherapies may be successfully achieved by combining antivirulence and conventional antimicrobials, extending the lifespan of these drugs. This review presents an updated discussion of natural compounds isolated from plants with chemically characterized structures and activity against the major bacterial virulence factors: quorum sensing, bacterial biofilms, bacterial motility, bacterial toxins, bacterial pigments, bacterial enzymes, and bacterial surfactants. Moreover, a critical analysis of the most promising virulence factors is presented, highlighting their potential as targets to attenuate bacterial virulence. The ongoing progress in the field of antivirulence therapy may therefore help to translate this promising concept into real intervention strategies in clinical areas.

Considerations and caveats in anti-virulence drug development

As antibiotic resistance remains a major public health threat, anti-virulence therapy research is gaining interest. Hundreds of potential anti-virulence compounds have been examined, but very few have made it to clinical trials and none have been approved. This review surveys the current anti-virulence research field with a focus on the highly resistant and deadly ESKAPE pathogens, especially Pseudomonas aeruginosa. We discuss timely considerations and caveats in anti-virulence drug development, including target identification, administration, preclinical development, and metrics for success in clinical trials. Development of a defined pipeline for anti-virulence agents, which differs in important ways from conventional antibiotics, is imperative for the future success of these critically needed drugs.

Imperatorin inhibits the expression of alpha-hemolysin in Staphylococcus aureus strain BAA-1717 (USA300)

Both community-associated and hospital-acquired infections with methicillin-resistant Staphylococcus aureus (MRSA) have been increasingly reported around the world in the past 20 years. In 2006, the Centers for Disease Control and Prevention reported that 64 % of MRSA isolates were of the USA300 clonal type in infected patients in USA. The aim of our study was to estimate the in vitro effect of imperatorin on MRSA strain BAA-1717 (USA300). The effects of imperatorin on alpha-hemolysin (Hla) production, when strain BAA-1717 was co-cultured with sub-inhibitory concentrations of imperatorin, were analysed using susceptibility testing, hemolysis assays, western blotting and real-time PCR. Live/Dead analysis and cytotoxicity assays were employed to examine the protective effect of imperatorin against the strain BAA-1717-mediated injury of human alveolar epithelial cells (A549). The results showed that imperatorin has no anti-S. aureus activity at the tested concentrations in vitro. However, imperatorin can observably inhibit the production of Hla in culture supernatants and reduce the transcriptional levels of hla (the gene encoding Hla) and arg (the accessory gene regulator). Imperatorin prevented Hla-mediated A549 epithelial cell injury in a co-culture system. In conclusion, our results suggested that imperatorin has the potential to be developed as a new anti-virulence drug candidate for managing S. aureus infection.

Targeting Staphylococcus aureus Toxins: A Potential form of Anti-Virulence Therapy

Staphylococcus aureus is an opportunistic pathogen and the leading cause of a wide range of severe clinical infections. The range of diseases reflects the diversity of virulence factors produced by this pathogen. To establish an infection in the host, S. aureus expresses an inclusive set of virulence factors such as toxins, enzymes, adhesins, and other surface proteins that allow the pathogen to survive under extreme conditions and are essential for the bacteria’s ability to spread through tissues. Expression and secretion of this array of toxins and enzymes are tightly controlled by a number of regulatory systems. S. aureus is also notorious for its ability to resist the arsenal of currently available antibiotics and dissemination of various multidrug-resistant S. aureus clones limits therapeutic options for a S. aureus infection. Recently, the development of anti-virulence therapeutics that neutralize S. aureus toxins or block the pathways that regulate toxin production has shown potential in thwarting the bacteria’s acquisition of antibiotic resistance. In this review, we provide insights into the regulation of S. aureus toxin production and potential anti-virulence strategies that target S. aureus toxins.

Phloretin derived from apple can reduce alpha-hemolysin expression in methicillin-resistant Staphylococcus aureus USA300

Methicillin-resistant Staphylococcus aureus (MRSA) has become increasingly important because it is the most common cause of hospital-acquired infections, which have become globally epidemic. Our study specifically focused on the MRSA strain USA300, which was shown in 2014 to be responsible for the most current pandemic of highly virulent MRSA in the United States. We aimed to evaluate the in vitro effect of phloretin on USA300. Susceptibility testing, western blotting assays, hemolysis assays and real-time RT-PCR were employed to examine the in vitro effects of phloretin on alpha-hemolysin (Hla) production when the bacterium was co-cultured with phloretin. The protective effect of phloretin against the USA300-mediated injury of human alveolar epithelial cells (A549) was tested using the live/dead analysis and cytotoxicity assays. We showed that sub-inhibitory concentrations of phloretin have no effect on bacterial viability; however, they can markedly inhibit the production of Hla in culture supernatants and the transcriptional levels of hla (the gene encoding Hla) and agrA (the accessory gene regulator). Phloretin, at a final concentration of 16 µg/ml, could protect A549 cells from injury caused by USA300 in the co-culture system. Our study suggests that phloretin might have a potential application in the development of treatment for MRSA infections.