Molecular mechanisms of N-acyl homoserine lactone signals perception by plants

N-acyl homoserine lactones (AHLs) belong to the class of bacterial quorum sensing signal molecules involved in distance signal transduction between Gram-negative bacteria colonizers of the rhizosphere, as well as bacteria and plants. AHLs synchronize the activity of genes from individual cells, allowing the bacterial population to act as a multicellular organism, and establish a symbiotic or antagonistic relationship with the host plant. Although the effect of AHLs on plants has been studied for more than ten years, the mechanisms of plant perception of AHL signals are not fully understood. The specificity of the reactions caused by AHL indicates the existence of appropriate mechanisms for their perception by plants. In the current review, we summarize available data on the molecular mechanisms of AHL-signal perception in plants, its effect on plant growth, development, and stress resistance. We describe the latest research demonstrating direct (on plants) and indirect (on rhizosphere microflora) effects of AHLs, as well as the prospects of using these compounds in biotechnology to increase plant resistance to biotic and abiotic stresses.

Chitosan/cyclodextrin surface-adsorbed naringenin-loaded nanocapsules enhance bacterial quorum quenching and anti-biofilm activities

Pathogenic bacteria use quorum sensing (QS), a cell-to-cell communication process that relies on small signaling molecules, to regulate the genetic expression that leads to several essential virulence factors such as bioluminescence, biofilm formation, bacterial motility, among other. Naringenin (NAR), a bitter and colorless flavanone ubiquitous in herbs and fruits, has been shown to inhibit QS activity in P. aeruginosa by decreasing the production of pyocyanin and elastase. In this study, to evaluate the anti-QS activity of naringenin against an E. coli Top 10 biosensor, we developed a novel core-corona nanocapsule formulation comprising surface co-adsorbed β-cyclodextrin (Captisol®) and chitosan loaded with NAR. The results showed that both the nanocapsule (NC) and nanoemulsion (NE) formulations, NAR payload associated with high efficiency , namely ~ 92.88 and ~ 67.98%, respectively. These formulations remained stable for 24 h and showed a biphasic controlled release profile in bacterial M9 medium. Captisol® was effectively immobilized on the NC's surface, resulting in a surface charge inversion from positive (~ + 42 mV) to negative (~ -32 mV) ζ-potential. The biosensor assay revealed that NC outperformed NE in quenching the QS response and the incorporation of naringenin at the NC's multifunctional surface enhanced this bioactivity. Cytotoxicity assays showed that when NAR was associated in NC (188 µM) it was not cytotoxic to Caco2 cells, by contrast with its free form, thus highlighting the cytoprotective effect of the developed formulation. Biofilm formation was inhibited up to ~ 60% in NAR-loaded NC (188 μM), indicating the synergistic effect of positively charged chitosan with the bioactivity of NAR while harnessing the NC's high surface area-to-volume ratio.

Quorum Sensing Regulation as a Target for Antimicrobial Therapy

Some bacterial species use a cell-to-cell communication mechanism called Quorum Sensing (QS). Bacteria release small diffusible molecules, usually termed signals which allow the activation of beneficial phenotypes that guarantee bacterial survival and the expression of a diversity of virulence genes in response to an increase in population density. The study of the molecular mechanisms that relate signal molecules with bacterial pathogenesis is an area of growing interest due to its use as a possible therapeutic alternative through the development of synthetic analogues of autoinducers as a strategy to regulate bacterial communication as well as the study of bacterial resistance phenomena, the study of these relationships is based on the structural diversity of natural or synthetic autoinducers and their ability to inhibit bacterial QS, which can be approached with a molecular perspective from the following topics: i) Molecular signals and their role in QS regulation; ii) Strategies in the modulation of Quorum Sensing; iii) Analysis of Bacterial QS circuit regulation strategies; iv) Structural evolution of natural and synthetic autoinducers as QS regulators. This mini-review allows a molecular view of the QS systems, showing a perspective on the importance of the molecular diversity of autoinducer analogs as a strategy for the design of new antimicrobial agents.

Repurposing phytochemicals as anti-virulent agents to attenuate quorum sensing-regulated virulence factors and biofilm formation in Pseudomonas aeruginosa

Unregulated consumption and overexploitation of antibiotics have paved the way for emergence of antibiotic‐resistant strains and ‘superbugs’. Pseudomonas aeruginosa is among the opportunistic nosocomial pathogens causing devastating infections in clinical set‐ups globally. Its artillery equipped with diversified virulence elements, extensive antibiotic resistance and biofilms has made it a ‘hard‐to‐treat’ pathogen. The pathogenicity of P. aeruginosa is modulated by an intricate cell density‐dependent mechanism called quorum sensing (QS). The virulence artillery of P. aeruginosa is firmly controlled by QS genes, and their expression drives the aggressiveness of the infection. Attempts to identify and develop novel antimicrobials have seen a sharp rise in the past decade. Among different proposed mechanisms, a novel anti‐virulence approach to target pseudomonal infections by virtue of anti‐QS and anti‐biofilm drugs appears to occupy the centre stage. In this respect, bioactive phytochemicals have gained prominence among the scientific community owing to their significant quorum quenching (QQ) properties. Recent studies have shed light on the QQ activities of various phytochemicals and other drugs in perturbing the QS‐dependent virulence in P. aeruginosa. This review highlights the recent evidences that reinforce the application of plant bioactives for combating pseudomonal infections, their advantages and shortcomings in anti‐virulence therapy.

Arabidopsis assemble distinct root-associated microbiomes through the synthesis of an array of defense metabolites

Plant associated microbiomes are known to confer fitness advantages to the host. Understanding how plant factors including biochemical traits influence host associated microbiome assembly could facilitate the development of microbiome-mediated solutions for sustainable plant production. Here, we examined microbial community structures of a set of well-characterized Arabidopsis thaliana mutants disrupted in metabolic pathways for the production of glucosinolates, flavonoids, or a number of defense signalling molecules. A. thaliana lines were grown in a natural soil and maintained under greenhouse conditions for 4 weeks before collection of roots for bacterial and fungal community profiling. We found distinct relative abundances and diversities of bacterial and fungal communities assembled in the individual A. thaliana mutants compared to their parental lines. Bacterial and fungal genera were mostly enriched than depleted in secondary metabolite and defense signaling mutants, except for flavonoid mutations on fungi communities. Bacterial genera Azospirillum and Flavobacterium were significantly enriched in most of the glucosinolate, flavonoid and signalling mutants while the fungal taxa Sporobolomyces and Emericellopsis were enriched in several glucosinolates and signalling mutants. Whilst the present study revealed marked differences in microbiomes of Arabidopsis mutants and their parental lines, it is suggestive that unknown enzymatic and pleiotropic activities of the mutated genes could contribute to the identified host-associated microbiomes. Notwithstanding, this study revealed interesting gene-microbiota links, and thus represents valuable resource data for selecting candidate A. thaliana mutants for analyzing the links between host genetics and the associated microbiome.

Quorum Sensing Inhibitors: Curbing Pathogenic Infections through Inhibition of Bacterial Communication

Currently, most of the developed and developing countries are facing the problem of infectious diseases. The genius way of an exaggerated application of antibiotics led the infectious agents to respond by bringing a regime of persisters to resist antibiotics attacks prolonging their survival. Persisters have the dexterity to communicate among themself using signal molecules via the process of Quorum Sensing (QS), which regulates virulence gene expression and biofilms formation, making them more vulnerable to antibiotic attack. Our review aims at the different approaches applied in the ordeal to solve the riddle for QS inhibitors. QS inhibitors, their origin, structures and key interactions for QS inhibitory activity have been summarized. Solicitation of a potent QS inhibitor molecule would be beneficial, giving new life to the simplest antibiotics in adjuvant therapy.

Potential of MurA Enzyme and GBAP in Fsr Quorum Sensing System as Antibacterial Drugs Target: In vitro and In silico Study of Antibacterial Compounds from Myrmecodia pendans

Background

Increasing the resistance issue has become the reason for the development of new antibacterial in crucial condition. Many ways are tracked to determine the most effective antibacterial agent. Some proteins that are a key role in bacteria metabolism are targeted, including MurA in cell wall biosynthesis and gelatinase biosynthesis-activating pheromone (GBAP) in Fsr Quorum Sensing (QS) system.

Objective

The objective of this research is the analysis of compounds 1-4 from M. pendans as antibacterial and anti-QS activity trough protein inhibition by in silico study; focus on the structure-activity relationships, to appraise their role as an antibacterial and anti-QS agent in the molecular level.

Methods

Both activities of M. pendans compounds (1-4) were analyzed by in silico, compared to Fosfomycin, Ambuic acid, Quercetin, and Taxifolin as a standard. Chemical structures of M. pendans compounds were converted using an online program molview. The compounds were docked to MurA, GBAP, gelatinase and serine protease using Autodock Vina in Pyrx 0.8 followed PYMOL to visualization and proteis.plus program to analyze of the complex.

Results

All compounds from M. pendans bound on MurA, GBAP, gelatinase and serine protease except compound 2. This biflavonoid did not attach to MurA and serine protease yet is the favorable ligand for GBAP and gelatinase with the binding affinity of -6.9 and -9.4 Kcal/mol respectively. Meanwhile, for MurA and serine protease, compound 4 is the highest of bonding energy with values of -8.7 and -6.4 Kcal/mol before quercetin (MurA, -8.9 Kcal/mol) and taxifolin (serine protease, -6.6 Kcal/mol).

Conclusion

Based on the data, biflavonoid acts better as anti-QS than an inhibitor of MurA enzyme while the others can be acted into both of them either the therapeutic agent of anti-QS or antibacterial agent of MurA inhibitor.

Effect of chlorogenic acid on the quorum-sensing system of clinically isolated multidrug-resistant Pseudomonas aeruginosa

AIMS

Quorum sensing (QS) is the intercellular communication used by bacteria to regulate collective behaviour. QS regulates the production of virulence factors in many bacterial species and is considered to be an attractive target for reducing bacterial pathogenicity. Chlorogenic acid (CA) is abundant in vegetables, fruits, and traditional Chinese medicine, and has multiple activities. This study aimed to investigate the QS quenching activity of CA against clinically isolated multidrug-resistant Pseudomonas aeruginosa.

METHODS AND RESULTS

The results showed that CA inhibited the mobility of bacteria, reduced the production of pyocyanin, and inhibited the activity of elastase. Furthermore, crystal violet staining and scanning electron microscope experiments showed that CA inhibited the formation of multidrug-resistant P. aeruginosa biofilm. CA at or below the concentration of 2560 µg/mL exerted negligible cytotoxicity to RAW264.7 cells. The study also examined the expression of QS-related genes, including lasI, lasR, rhlI, rhlR, pqsA, and pqsR in P. aeruginosa and found that the expression of these genes was down-regulated under CA treatment.

CONCLUSIONS

The study showed that CA could be used as an anti-virulence factor for treating clinical P. aeruginosa infection.

SIGNIFICANCE AND IMPACT OF STUDY

For the first time, this study took clinically isolated multidrug-resistant P. aeruginosa as the experimental object, and suggested that CA might be an effective antimicrobial compound targeting QS in treating P. aeruginosa infection, thus providing a new therapeutic direction for treating bacterial infection and effectively alleviating bacterial resistance.

Interkingdom Signaling Interference: The Effect of Plant-Derived Small Molecules on Quorum Sensing in Plant-Pathogenic Bacteria

In the battle between bacteria and plants, bacteria often use a population density-dependent regulatory system known as quorum sensing (QS) to coordinate virulence gene expression. In response, plants use innate and induced defense mechanisms that include low-molecular-weight compounds, some of which serve as antivirulence agents by interfering with the QS machinery. The best-characterized QS system is driven by the autoinducer N-acyl-homoserine lactone (AHL), which is produced by AHL synthases (LuxI homologs) and perceived by response regulators (LuxR homologs). Several plant compounds have been shown to directly inhibit LuxI or LuxR. Gaining atomic-level insight into their mode of action and how they interfere with QS enzymes supports the identification and design of novel QS inhibitors.Such information can be gained by combining experimental work with molecular modeling and docking simulations. The summary of these findings shows that plant-derived compounds act as interkingdom cues and that these allomones specifically target bacterial communication systems.

Metabolic interactions in beneficial microbe recruitment by plants

During millions of years of evolution, land plants and microorganisms have established elaborate partnerships. Microbes play essential roles in plant fitness and help plants cope with environmental challenges. Vice versa, plants provide the microbes with a niche and food. In the soil, a complex network of interactions mediated by metabolic signals drives the relationship between plants and microbes. Here, we review the roles of metabolic signaling in the plant-microbiome interaction. We discuss how plant-produced small molecules are involved in the recruitment of the microbiome. Also the microbial partners in this relationship use small molecules, such as quorum sensing molecules and volatiles for intra-species and inter-species communication. We give an overview of the regulation of the biosynthesis, secretion and perception of both plant and microbial small molecules and discuss the examples of biotechnological approaches to engineer the plant-microbiome interaction by targeting these metabolic dialogues. Ultimately, an improved understanding of the plant-microbiome interaction and engineering possibilities will pave the way to a more sustainable agriculture.

Plant growth promoting bacteria induce anti-quorum-sensing substances in chickpea legume seedling bioassay

Microorganisms and their hosts communicate through an array of signals. Many physiological processes regulated in quorum sensing (QS) are dependent on auto-inducers, like N-acyl-homoserine lactones (AHLs) as in numerous groups of both gram-positive and gram-negative bacteria. In vitro grown seven-day old chickpea seedlings treated with plant growth promoting bacteria (PGPRs) were used to screen the AHL mimicking and for phytochemical substances like phytohormones and secondary metabolites such as phenolics and flavonoids. Potential anti-quorum sensing (anti-QS) activity surrounding the roots on semi-solid agar lawn of Chromobacterium violaceum (ATCC12742) was observed. Crude protein (4.46-8.30 μg/mL) and methanolic extracts (100 μg/mL) of seedling gave moderate anti-QS activity against CV12742 anti QS bioassay, respectively. Crude protein and methanolic extract of Bacillus amyloliquefaciens (34.00 ± 2.23; 34.00 ± 4.33 mm) and B. subtilis A (27.00 ± 2.10; 3.29 ± 2.16 mm) treated samples showed higher zone of inhibition due to anti-QS activity. Phytohormone analysis using LC-MS for zeatin, auxin and methyl jasmonate (MeJA) indicated that phytohormones were significantly upregulated by 1909.80 ng/g FW, 669.67 ng/g FW and 244.55 ng/g FW, respectively in Pseudomonas brassicacearum treated seedlings compared to control. UHPLC of PGPR treated seedlings showed overly expressed gallic acid, protocatechuic acid, catechin, p-hydroxybenzoic acid, caffeic acid, catechol, vanillin, and ferulic acid in B. amyloliquefaciens treated seedlings compared to others. Enrichment analysis identified significant pathways related to metabolism, biosynthesis of secondary metabolites. The present study indicates that chickpea neutralizes an extensive range of functional responses to AHLs that may play important role in legume host-microbe interactions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01034-x.

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.

Phytochemicals as modifiers of gut microbial communities

A healthy gut microbiota (GM) is paramount for a healthy lifestyle. Alterations of the GM have been involved in the aetiology of several chronic diseases, including obesity and type 2 diabetes, as well as cardiovascular and neurodegenerative diseases. In pathological conditions, the diversity of the GM is commonly reduced or altered, often toward an increased Firmicutes/Bacteroidetes ratio. The colonic fermentation of dietary fiber has shown to stimulate the fraction of bacteria purported to have beneficial health effects, acting as prebiotics, and to increase the production of short chain fatty acids, e.g. propionate and butyrate, while also improving gut epithelium integrity such as tight junction functionality. However, a variety of phytochemicals, often associated with dietary fiber, have also been proposed to modulate the GM. Many phytochemicals possess antioxidant and anti-inflammatory properties that may positively affect the GM, including polyphenols, carotenoids, phytosterols/phytostanols, lignans, alkaloids, glucosinolates and terpenes. Some polyphenols may act as prebiotics, while carotenoids have been shown to alter immunoglobulin A expression, an important factor for bacteria colonization. Other phytochemicals may interact with the mucosa, another important factor for colonization, and prevent its degradation. Certain polyphenols have shown to influence bacterial communication, interacting with quorum sensing. Finally, phytochemicals can be metabolized in the gut into bioactive constituents, e.g. equol from daidzein and enterolactone from secoisolariciresinol, while bacteria can use glycosides for energy. In this review, we strive to highlight the potential interactions between prominent phytochemicals and health benefits related to the GM, emphasizing their potential as adjuvant strategies for GM-related diseases.

Anti-quorum sensing and antibiofilm potential of 1,8-cineole derived from Musa paradisiaca against Pseudomonas aeruginosa strain PAO1

Pseudomonas aeruginosa is one of the vulnerable opportunistic pathogens associated with nosocomial infections, cystic fibrosis, burn wounds and surgical site infections. Several studies have reported that quorum sensing (QS) systems are controlled the P. aeruginosa pathogenicity. Hence, the targeting of QS considered as an alternative approach to control P. aeruginosa infections. This study aimed to evaluate the anti-quorum sensing and antibiofilm inhibitory potential of Musa paradisiaca against Chromobacterium violaceum (ATCC 12472) and Pseudomonas aeruginosa. The methanol extract of M. paradisiacsa exhibits that better antibiofilm potential against P. aeruginosa. Then, the crude methanol extract was subjected to purify by column chromatography and collected the fractions. The mass-spectrometric analysis of a methanol extract of M. paradisiaca revealed that 1,8-cineole is the major compounds. 1, 8-cineole significantly inhibited the QS regulated violacein production in C. violaceum. Moreover, 1,8-cineole significantly inhibited the QS mediated virulence production and biofilm formation of P. aeruginosa without affecting their growth. The real-time PCR analysis showed the downregulation of autoinducer synthase and transcriptional regulator genes upon 1,8-cineole treatment. The findings of the present study strongly suggested that metabolite of M. paradisiaca impedes P. aeruginosa QS system and associated virulence productions.

Biological nitrification inhibition in the rhizosphere: determining interactions and impact on microbially mediated processes and potential applications

Nitrification is the microbial conversion of reduced forms of nitrogen (N) to nitrate (NO3-), and in fertilized soils it can lead to substantial N losses via NO3- leaching or nitrous oxide (N2O) production. To limit such problems, synthetic nitrification inhibitors have been applied but their performance differs between soils. In recent years, there has been an increasing interest in the occurrence of biological nitrification inhibition (BNI), a natural phenomenon according to which certain plants can inhibit nitrification through the release of active compounds in root exudates. Here, we synthesize the current state of research but also unravel knowledge gaps in the field. The nitrification process is discussed considering recent discoveries in genomics, biochemistry and ecology of nitrifiers. Secondly, we focus on the 'where' and 'how' of BNI. The N transformations and their interconnections as they occur in, and are affected by, the rhizosphere, are also discussed. The NH4+ and NO3- retention pathways alternative to BNI are reviewed as well. We also provide hypotheses on how plant compounds with putative BNI ability can reach their targets inside the cell and inhibit ammonia oxidation. Finally, we discuss a set of techniques that can be successfully applied to solve unresearched questions in BNI studies.

Inhibition of biofilm formation, quorum sensing and other virulence factors in Pseudomonas aeruginosa by polyphenols of Gynura procumbens leaves

Quorum sensing (QS) enables virulence factors in bacteria for biofilm formation and pathogenic invasion. Therefore, quorum quenching (QQ), disruption of QS circuit, becomes an alternative antimicrobial therapy. In this study, leaf extract of Gynura procumbens (GP) was used to inhibit biofilm and virulent factors in Pseudomonas aeruginosa. The extract inhibited the biofilm production (p ≤ 0.05) in P. aeruginosa strains MZ2F and MZ4A. The minimum biofilm eradication concentration (MBEC) was recorded at 250 and 500 μg/ml while total activity was found at 288 and 144 ml/g, respectively. Moreover, a significant reduction of virulence factors (p ≤ 0.05) at sub-MBEC without affecting the growth implies the QQ action of the extract. The bioactive fractions were rich in polyphenols and tentatively identified as quercetin and myricetin (R_f=0.53-0.60). Furthermore, we employed computational methods to validate our findings and their interactions with QS receptors (LasR and RhlR). Interestingly, docking studies have also shown that quercetin and myricetin are the promising anti-QS agents out of 31 GP compounds. Notably, their binding affinity ranged between -9.77 and -10.52 kcal/mol for both QS receptors, with controls ranging from -5.40 to -8.97 kcal/mol. Besides, ΔG of quercetin and myricetin with LasR was -71.56 and -74.88 kcal/mol, respectively. Moreover, compounds were suitable drug candidates with stable binding interactions. Therefore, the anti-QS activity of GP leaves and the identified polyphenols can be used in developing QQ-based therapeutics. Communicated by Ramaswamy H. Sarma.

Next generation quorum sensing inhibitors: Accounts on structure activity relationship studies and biological activities

Bacterial resistance is a growing threat which represents major scourge throughout the world. The suitable way to control the present critical situation of antimicrobial resistance would be to develop entirely novel strategies to fight antibiotic resistant pathogens such as quorum sensing (QS) inhibitors or its combination with antibiotics. Anti QS agents can eliminate the QS signals and put the barrier in bio-film formation, consequently, bacterial virulence will be reduced without causing drug-resistance to the pathogens. Among the various anti QS agents identified, especially those of natural origin, furanones or acylatedhomoserine lactones (AHLs) are most popular. Semi-synthetic and synthetic inhibitors have shown greatest potential and have inspired chemists to design synthetically modified QS inhibitors with lactone moiety. This review focuses on anti QS agents (bio-film inhibitors) of both natural and synthetic origins. Further, the synthesis, structure activity relationship and anti QS activity covering literature from 2015 till March 2020 has been discussed.

Plant Derived Natural Products against Pseudomonas aeruginosa and Staphylococcus aureus: Antibiofilm Activity and Molecular Mechanisms

Bacteria are social organisms able to build complex structures, such as biofilms, that are highly organized surface-associated communities of microorganisms, encased within a self- produced extracellular matrix. Biofilm is commonly associated with many health problems since its formation increases resistance to antibiotics and antimicrobial agents, as in the case of Pseudomonas aeruginosa and Staphylococcus aureus, two human pathogens causing major concern. P. aeruginosa is responsible for severe nosocomial infections, the most frequent of which is ventilator-associated pneumonia, while S. aureus causes several problems, like skin infections, septic arthritis, and endocarditis, to name just a few. Literature data suggest that natural products from plants, bacteria, fungi, and marine organisms have proven to be effective as anti-biofilm agents, inhibiting the formation of the polymer matrix, suppressing cell adhesion and attachment, and decreasing the virulence factors' production, thereby blocking the quorum sensing network. Here, we focus on plant derived chemicals, and provide an updated literature review on the anti-biofilm properties of terpenes, flavonoids, alkaloids, and phenolic compounds. Moreover, whenever information is available, we also report the mechanisms of action.

Tackling Antibiotic Resistance with Compounds of Natural Origin: A Comprehensive Review

Drug-resistant bacteria pose a serious threat to human health worldwide. Current antibiotics are losing efficacy and new antimicrobial agents are urgently needed. Living organisms are an invaluable source of antimicrobial compounds. The antimicrobial activity of the most representative natural products of animal, bacterial, fungal and plant origin are reviewed in this paper. Their activity against drug-resistant bacteria, their mechanisms of action, the possible development of resistance against them, their role in current medicine and their future perspectives are discussed. Electronic databases such as PubMed, Scopus and ScienceDirect were used to search scientific contributions until September 2020, using relevant keywords. Natural compounds of heterogeneous origins have been shown to possess antimicrobial capabilities, including against antibiotic-resistant bacteria. The most commonly found mechanisms of antimicrobial action are related to protein biosynthesis and alteration of cell walls and membranes. Various natural compounds, especially phytochemicals, have shown synergistic capacity with antibiotics. There is little literature on the development of specific resistance mechanisms against natural antimicrobial compounds. New technologies such as -omics, network pharmacology and informatics have the potential to identify and characterize new natural antimicrobial compounds in the future. This knowledge may be useful for the development of future therapeutic strategies.

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.

Role of plant compounds in the modulation of the conjugative transfer of pRet42a

One of the most studied mechanisms involved in bacterial evolution and diversification is conjugative transfer (CT) of plasmids. Plasmids able to transfer by CT often encode beneficial traits for bacterial survival under specific environmental conditions. Rhizobium etli CFN42 is a Gram-negative bacterium of agricultural relevance due to its symbiotic association with Phaseolus vulgaris through the formation of Nitrogen-fixing nodules. The genome of R. etli CFN42 consists of one chromosome and six large plasmids. Among these, pRet42a has been identified as a conjugative plasmid. The expression of the transfer genes is regulated by a quorum sensing (QS) system that includes a traI gene, which encodes an acyl-homoserine lactone (AHL) synthase and two transcriptional regulators (TraR and CinR). Recently, we have shown that pRet42a can perform CT on the root surface and inside nodules. The aim of this work was to determine the role of plant-related compounds in the CT of pRet42a. We found that bean root exudates or root and nodule extracts induce the CT of pRet42a in the plant rhizosphere. One possibility is that these compounds are used as nutrients, allowing the bacteria to increase their growth rate and reach the population density leading to the activation of the QS system in a shorter time. We tested if P. vulgaris compounds could substitute the bacterial AHL synthesized by TraI, to activate the conjugation machinery. The results showed that the transfer of pRet42a in the presence of the plant is dependent on the bacterial QS system, which cannot be substituted by plant compounds. Additionally, individual compounds of the plant exudates were evaluated; among these, some increased and others decreased the CT. With these results, we suggest that the plant could participate at different levels to modulate the CT, and that some compounds could be activating genes in the conjugation machinery.

Anti-biofilm activity of plant derived extracts against infectious pathogen-Pseudomonas aeruginosa PAO1

BACKGROUND

Biofilm forming ability of Pseudomonas aeruginosa make them vulnerable, because it makes them recalcitrant against various antibiotics. Quorum sensing (QS) is cell density based signaling that helps in bacterial cell-cell communication, which regulated various virulence factors such as pigment and biofilm formation that contribute in the establishment of chronic infections. The interruption of QS is one of the effective approach to control various virulence factors. Present study was intended with the aim to authenticate antibiofilm potential in different solvents based extracts of selected medicinal plant species viz. Berginia ciliata, Clematis grata and Clematis viticella traditionally used by the inhabitants of Himalayan region of Pakistan to treat various pathogenic diseases. P. aeruginosa PAO1, an opportunistic pathogen and involves in various life-threatening infections specifically in immune deficient patients was used as a model pathogen.

METHODS

Plants were extracted in various organic (ethanol, methanol, acetone, ethyl acetate, hexane, chloroform) as well as in aqueous solvents and their ability to inhibit biofilm was measured. Biofilm of PAO1 was grown in Jensen's medium while growing at 30°C and crystal violet assay was performed to assess the biofilm inhibiting activity of plant extracts.

RESULTS

Solvents play a vital role in extraction of plant components and it was found that the plants in various solvents exhibit different activity against the PAO1 biofilm. Comparatively, 1% methanolic extract of B. ciliata (rhizome with skin), showed more than 80% inhibition of biofilm formation without effecting on the growth of the bacterium. Significant correlation between flavonoids content and antibiofilm activity in methanolic extract revealed the contribution of secondary metabolites in P. aeruginosa (PAO1) biofilm inhibition.

CONCLUSION

Our study revealed that plants under investigation more specifically B. ciliata could be a potential candidate for drug discovery to treat P. aeruginosa PAO1, induced infectious diseases especially for its biofilm treatment.

Seagrass Halodule pinifolia active constituent 4-methoxybenzioic acid (4-MBA) inhibits quorum sensing mediated virulence production of Pseudomonas aeruginosa

Biofilm associated, multidrug resistant Pseudomonas aeruginosa infection remain a challenging problem in the clinical field since the conventional antibiotic therapy are largely inefficient and new approaches are needed. Inactivating the QS virulence mechanism with anti-infective agent is an attractive approach to prevent bacterial infections without resistance development. Seagrass Halodule pinifolia (Miki) Hartog has been shown to exhibit potential antimicrobial activities against harmful pathogens. Our study investigated the effects of seagrass H. pinifolia leaf extract and its bioactive constituents on QS-mediated virulence factors and biofilm formation in P. aerugonasa PAO1. Preliminary screening on antibiofilm activity showed that the methanolic extract of H. pinifolia exhibited potential inhibition of biofilm formation (96%) as compared to the control respectively. Further, the potential extract was column fractionated and the active fraction was characterized by GC-MS. In total eight active compounds (protocatacheuic acid (69.3%), rosmarinic acid (63.5%), caffeic acid (59.18%), p-coumaric acid (59.08%), 4-methoxybenzoic acid (53.19%), naringenin (52.9%), vanillic acid (49.19%), 4-hydroxybenzoic acid (41.73%)) were profiled from fraction 2 and were purified by HPLC, structurally confirmed by NMR. Among the eight compounds studied, 4-methoxybenzoic acid (4-MBA) showed an effective inhibition of bacterial growth and was considered as a lead molecule with minimum inhibitory concentration (MIC) of 62.5 μg/mL. Further the effect of 4-MBA on QS mediated virulence factors demonstrated that the compound at MIC concentration reduced the virulence factor production such as elastase (87.5%), protease (79.38%), pyocyanin (91.46%), rhamnolipid (86%), alginate (86%), chitinase (55%), exopolysaccharide production (83.72%) and CSH (78.39%) over the control respectively. Moreover, 4-MBA down regulated the QS-mediated virulence transcript levels upon treatment with 4-MBA. The present findings suggests that seagrasses may act as a newer source for the marine based drug discovery and the lead compound 4-MBA derived from H. pinifolia may act as anti-infective agent against P. aeruginosa as it controls the QS-mediated virulence production.

Possible drugs for the treatment of bacterial infections in the future: anti-virulence drugs

Antibiotic resistance is a global threat that should be urgently resolved. Finding a new antibiotic is one way, whereas the repression of the dissemination of virulent pathogenic bacteria is another. From this point of view, this paper summarizes first the mechanisms of conjugation and transformation, two important processes of horizontal gene transfer, and then discusses the approaches for disarming virulent pathogenic bacteria, that is, virulence factor inhibitors. In contrast to antibiotics, anti-virulence drugs do not impose a high selective pressure on a bacterial population, and repress the dissemination of antibiotic resistance and virulence genes. Disarmed virulence factors make virulent pathogens avirulent bacteria or pathobionts, so that we human will be able to coexist with these disarmed bacteria peacefully.

Hypertonic glucose inhibits growth and attenuates virulence factors of multidrug-resistant Pseudomonas aeruginosa

BACKGROUND

Pseudomonas aeruginosa is the most common Gram-negative pathogen responsible for chronic wound infections, such as diabetic foot infections, and further exacerbates the treatment options and cost of such conditions. Hypertonic glucose, a commonly used prolotherapy solution, can accelerate the proliferation of granulation tissue and improve microcirculation in wounds. However, the action of hypertonic glucose on bacterial pathogens that infect wounds is unclear. In this study, we investigated the inhibitory effects of hypertonic glucose on multidrug-resistant P. aeruginosa strains isolated from diabetic foot infections. Hypertonic glucose represents a novel approach to control chronic wound infections caused by P. aeruginosa.

RESULTS

Four multidrug-resistant P. aeruginosa clinical strains isolated from diabetic foot ulcers from a tertiary hospital in China and the reference P. aeruginosa PAO1 strain were studied. Hypertonic glucose significantly inhibited the growth, biofilm formation, and swimming motility of P. aeruginosa clinical strains and PAO1. Furthermore, hypertonic glucose significantly reduced the production of pyocyanin and elastase virulence factors in P. aeruginosa. The expression of major quorum sensing genes (lasI, lasR, rhlI, and rhlR) in P. aeruginosa were all downregulated in response to hypertonic glucose treatment. In a Galleria mellonella larvae infection model, the administration of hypertonic glucose was shown to increase the survival rates of larvae infected by P. aeruginosa strains (3/5).

CONCLUSIONS

Hypertonic glucose inhibited the growth, biofilm formation, and swimming motility of P. aeruginosa, as well as reduced the production of virulence factors and quorum sensing gene expression. Further studies that investigate hypertonic glucose therapy should be considered in treating chronic wound infections.

Inhibition of quorum sensing-associated virulence factors and biofilm formation in Pseudomonas aeruginosa PAO1 by Mycoleptodiscus indicus PUTY1

Pseudomonas aeruginosa is the second most emerging multidrug-resistant, opportunistic pathogen after Acinetobacter baumannii that poses a threat in nursing homes, hospitals, and patients who need devices such as ventilators and blood catheters. Its ability to form quorum sensing-regulated virulence factors and biofilm makes it more resistant to top most therapeutic agents such as carbapenems and next-generation antibiotics. In the current study, we studied the quorum quenching potential of secondary metabolites of Mycoleptodiscus indicus PUTY1 strain. In vitro observation showed a mitigation in virulence factors such as rhamnolipids, protease, elastase pyocyanin, exopolysaccharides, and hydrogen cyanide gas. Furthermore, a significant reduction in the motility such as swimming, swarming, twitching, and inhibition in biofilm formation by Pseudomonas aeruginosa PAO1 was observed. Results of in vitro studies were further confirmed by in silico studies through docking and molecular dynamic simulation of GC-MS-detected compounds of Mycoleptodiscus indicus employing LasR and RhlR proteins. Both in vitro and in silico observations indicate a new alternative approach for combating virulence of Pseudomonas aeruginosa by targeting its protein receptors LasR and RhlR. Graphical abstract.

Bioactive Properties of Syzygium cumini (L.) Skeels Pulp and Seed Phenolic Extracts

The emergence of bacterial strains resistant to different antibiotics has prompted the search for new sources of antimicrobial compounds. Studies have shown that jambolan [Syzygium cumini (L.) Skeels], a tropical fruit from the Mirtaceae family, contains a great variety of phytochemical compounds with high antioxidant and antimicrobial activity. This study aimed to determine the centesimal composition and physicochemical characteristics of the pulp and seed of S. cumini (L.) Skeels, as well as the content of total phenolic compounds and the antioxidant, antibacterial, antibiofilm and anti-quorum sensing (QS) activities of the phenolic extracts obtained from the pulp and the seeds of this fruit. The in vitro antibacterial and anti-QS activities of active films incorporating phenolic extracts were also evaluated. Additionally, we performed molecular docking of phenolic compounds present in jambolan with the CviR QS regulator of Chromobacterium violaceum. The composition and physicochemical characteristics of the samples presented similar values to those found for the species. However, the seed phenolic extract had a higher content of phenolic compounds and antioxidant activity than the pulp. Both phenolic extracts presented antibacterial activity against Aeromonas hydrophila, C. violaceum, Escherichia coli, Pseudomonas aeruginosa, Salmonella enterica serovar Typhimurium, Serratia marcescens, Listeria monocytogenes, and Staphylococcus aureus. The seed phenolic extract was particularly inhibitory against S. aureus. The pulp phenolic extract inhibited swarming motility and biofilm formation of A. hydrophila, E. coli, and S. marcescens in sub-MIC concentrations. The pulp and seed phenolic extracts inhibited violacein production in C. violaceum. Films incorporating both phenolic extracts inhibited the growth of bacteria, particularly Pseudomonas fluorescens, L. monocytogenes, and S. aureus, as well as QS in C. violaceum. Molecular docking showed that a variety of compounds found in pulp and seed extracts of jambolan, particularly chlorogenic acid and dihydroquercetin, potentially bind CviR protein and may interfere with QS. Our results indicate that pulp and seed of jambolan are good sources of antibacterial, antibiofilm, and anti-QS compounds that can be used in the development of natural preservatives and for application in antibacterial active films.

Effect of Adiantum philippense Extract on Biofilm Formation, Adhesion With Its Antibacterial Activities Against Foodborne Pathogens, and Characterization of Bioactive Metabolites: An in vitro-in silico Approach

Adiantum philippense (A. philippense), an ethnomedicinally important fern, has become an interesting herb in the search for novel bioactive metabolites, which can also be used as therapeutic agents. Primarily, in this study, A. philippense crude extract was screened for its phytochemical constituents, antagonistic potential, and effect on bacterial adhesion and biofilm formation against common food pathogens. Phytochemical profiling of A. philippense was carried out by using High Resolution-Liquid Chromatography and Mass Spectroscopy (HR-LCMS) followed by antibacterial activity via agar cup/well diffusion, broth microdilution susceptibility methods, and growth curve analysis. Antibiofilm potency and efficacy were assessed on the development, formation, and texture of biofilms through light microscopy, fluorescent microscopy, scanning electron microscopy, and the assessment of exopolysaccharide production. Correspondingly, a checkerboard test was performed to evaluate the combinatorial effect of A. philippense and chloramphenicol. Lastly, molecular docking studies of identified phytochemicals with adhesin proteins of tested food pathogens, which helps the bacteria in surface attachment and leads to biofilm formation, were assessed. A. philippense crude extract was found to be active against all tested food pathogens, displaying the rapid time-dependent kinetics of bacterial killing. A. philippense crude extract also impedes the biofilm matrix by reducing the total content of exopolysaccharide, and, likewise, the microscopic images revealed a great extent of disruption in the architecture of biofilms. A synergy was observed between A. philippense crude extract and chloramphenicol for E. coli, S. aureus, and P. aeruginosa, whereas an additive effect was observed for S. flexneri. Various bioactive phytochemicals were categorized from A. philippense crude extract using HR-LCMS. The molecular docking of these identified phytochemicals was interrelated with the active site residues of adhesin proteins, IcsA, Sortase A, OprD, EspA, and FimH from S. flexneri, S. aureus, P. aeruginosa, and E. coli, respectively. Thus, our findings represent the bioactivity and potency of A. philippense crude extract against food pathogens not only in their planktonic forms but also against/in biofilms for the first time. We have also correlated these findings with the possible mechanism of biofilm inhibition via targeting adhesin proteins, which could be explored further to design new bioactive compounds against biofilm producing foodborne bacterial pathogens.

Plant Secondary Metabolites in the Battle of Drugs and Drug-Resistant Bacteria: New Heroes or Worse Clones of Antibiotics?

Infectious diseases that are caused by bacteria are an important cause of mortality and morbidity in all regions of the world. Bacterial drug resistance has grown in the last decades, but the rate of discovery of new antibiotics has steadily decreased. Therefore, the search for new effective antibacterial agents has become a top priority. The plant kingdom seems to be a deep well for searching for novel antimicrobial agents. This is due to the many attractive features of plants: they are readily available and cheap, extracts or compounds from plant sources often demonstrate high-level activity against pathogens, and they rarely have severe side effects. The huge variety of plant-derived compounds provides very diverse chemical structures that may supply both the novel mechanisms of antimicrobial action and provide us with new targets within the bacterial cell. In addition, the rapid development of modern biotechnologies opens up the way for obtaining bioactive compounds in environmentally friendly and low-toxic conditions. In this short review, we ask the question: do antibacterial agents derived from plants have a chance to become a panacea against infectious diseases in the "post-antibiotics era".

Attenuation of Pseudomonas aeruginosa Quorum Sensing by Natural Products: Virtual Screening, Evaluation and Biomolecular Interactions

Natural products play vital roles against infectious diseases since ancient times and most drugs in use today are derived from natural sources. Worldwide, multi-drug resistance becomes a massive threat to the society with increasing mortality. Hence, it is very crucial to identify alternate strategies to control these 'super bugs'. Pseudomonas aeruginosa is an opportunistic pathogen reported to be resistant to a large number of critically important antibiotics. Quorum sensing (QS) is a cell-cell communication mechanism, regulates the biofilm formation and virulence factors that endow pathogenesis in various bacteria including P. aeruginosa. In this study, we identified and evaluated quorum sensing inhibitors (QSIs) from plant-based natural products against P. aeruginosa. In silico studies revealed that catechin-7-xyloside (C7X), sappanol and butein were capable of interacting with LasR, a LuxR-type quorum sensing regulator of P. aeruginosa. In vitro assays suggested that these QSIs significantly reduced the biofilm formation, pyocyanin, elastase, and rhamnolipid without influencing the growth. Especially, butein reduced the biofilm formation up to 72.45% at 100 µM concentration while C7X and sappanol inhibited the biofilm up to 66% and 54.26% respectively. Microscale thermophoresis analysis revealed that C7X had potential interaction with LasR (KD = 933±369 nM) and thermal shift assay further confirmed the biomolecular interactions. These results suggested that QSIs are able to substantially obstruct the P. aeruginosa QS. Since LuxR-type transcriptional regulator homologues are present in numerous bacterial species, these QSIs may be developed as broad spectrum anti-infectives in the future.

Polyphenols from Salix tetrasperma Impair Virulence and Inhibit Quorum Sensing of Pseudomonas aeruginosa

Bacterial resistance represents one of the emerging obstacles in plants, animals, and humans that impairs treatment with antibacterial agents. Targeting of the bacterial quorum sensing system is one of the strategies to overcome this problem. Recently, research has been focused on natural and food components which can function as quorum sensing inhibitors. In this study, a methanol extract from Salix tetrasperma stem bark was phytochemically profiled by LC-MS analysis. This resulted in the identification of 38 secondary metabolites with (epi)catechin-(epi)catechin, epicatechin, tremulacin, salicortin, and trichocarposide as the major constituents. The extracts of both stem bark and the previously profiled flower of S. tetrasperma were tested for anti-quorum sensing activity in a common and widely distributed pathogen Pseudomonas aeruginosa. The natural products inhibited swimming and swarming motilities, as well as proteolytic and hemolytic activities in a dose-dependent manner. Molecular docking of the constituents from both extracts against the quorum sensing controlling systems Lasl/LasR, rhll/rhlR, and PQS/MvfR showed that epicatechin, (epi)catechin-(epi)catechin, p-hydroxy benzoyl galloyl glucose, p-hydroxy benzoyl protocatechuic acid glucose, and caffeoylmalic acid could be the main active components. This study supports the importance of secondary metabolites, especially polyphenols, as quorum sensing inhibitors.

Calycopterin, a major flavonoid from Marcetia latifolia, modulates virulence-related traits in Pseudomonas aeruginosa

Although bacterial resistance is a worldwide growing concern, the development of bacteriostatic and bactericidal drugs has been decreasing in the last decade. Compounds that modulate the microorganism virulence, without killing it, have been considered promising alternatives to combat bacterial infections. However, most signaling pathways that regulate virulence are complex and not completely understood. The rich chemical diversity of natural products offers a good starting point to identify key compounds that shed some light on this matter. Therefore, we investigated the role of Marcetia latifolia ethanolic extract, as well as its major constituent, calycopterin (5,4'-dihydroxy-3,6,7,8-tetramethoxylflavone), in the regulation of virulence-related phenotypes of Pseudomonas aeruginosa. Our results show that calycopterin inhibits pyocyanin production (EC50 = 32 μM), reduces motility and increases biofilm formation in a dose-dependent manner. Such biological profile suggests that calycopterin modulates targets that may act upstream the quorum sensing regulators and points to its utility as a chemical probe to further investigate P. aeruginosa transition from planktonic to sessile lifestyle.

Naringenin Inhibition of the Pseudomonas aeruginosa Quorum Sensing Response Is Based on Its Time-Dependent Competition With N-(3-Oxo-dodecanoyl)-L-homoserine Lactone for LasR Binding

Bacterial quorum sensing (QS) is a cell-to-cell communication system that governs the expression of a large set of genes involved in bacterial-host interactions, including the production of virulence factors. Conversely, the hosts can produce anti-QS compounds to impair virulence of bacterial pathogens. One of these inhibitors is the plant flavonoid naringenin, which impairs the production of QS-regulated Pseudomonas aeruginosa virulence factors. In the present work, we analyze the molecular basis for such inhibition. Our data indicate that naringenin produces its effect by directly binding the QS regulator LasR, hence competing with its physiological activator, N-(3-oxo-dodecanoyl)-L-homoserine lactone (3OC12-HSL). The in vitro analysis of LasR binding to its cognate target DNA showed that the capacity of naringenin to outcompete 3OC12-HSL, when the latter is previously bound to LasR, is low. By using an E. coli LasR-based biosensor strain, which does not produce 3OC12-HSL, we determined that the inhibition of LasR is more efficient when naringenin binds to nascent LasR than when this regulator is already activated through 3OC12-HSL binding. According to these findings, at early exponential growth phase, when the amount of 3OC12-HSL is low, naringenin should proficiently inhibit the P. aeruginosa QS response, whereas at later stages of growth, once 3OC12-HSL concentration reaches a threshold enough for binding LasR, naringenin would not efficiently inhibit the QS response. To test this hypothesis, we analyze the potential effect of naringenin over the QS response by adding naringenin to P. aeruginosa cultures at either time zero (early inhibition) or at stationary growth phase (late inhibition). In early inhibitory conditions, naringenin inhibited the expression of QS-regulated genes, as well as the production of the QS-regulated virulence factors, pyocyanin and elastase. Nevertheless, in late inhibitory conditions, the P. aeruginosa QS response was not inhibited by naringenin. Therefore, this time-dependent inhibition may compromise the efficiency of this flavonoid, which will be effective just when used against bacterial populations presenting low cellular densities, and highlight the importance of searching for QS inhibitors whose mechanism of action does not depend on the QS status of the population.

Plant Phenolics and Phenolic-Enriched Extracts as Antimicrobial Agents against Food-Contaminating Microorganisms

Phenolic compounds and extracts with bioactive properties can be obtained from many kinds of plant materials. These natural substances have gained attention in the food research as possible growth inhibitors of foodborne pathogenic and spoilage bacteria. Many phenolic-enriched plant extracts and individual phenolics have promising anti-quorum sensing potential as well and can suppress the biofilm formation and toxin production of food-related pathogens. Various studies have shown that plant phenolics can substitute or support the activity of synthetic food preservatives and disinfectants, which, by the way, can provoke serious concerns in consumers. In this review, we will provide a brief insight into the bioactive properties, i.e., the antimicrobial, anti-quorum sensing, anti-biofilm and anti-enterotoxin activities, of plant phenolic extracts and compounds, with special attention to pathogen microorganisms that have food relation. Carbohydrase aided applications to improve the antimicrobial properties of phenolic extracts are also discussed.

Prevention of biofilm formation by quorum quenching

Quorum sensing (QS) is a mechanism that enables microbial communication. It is based on the constant secretion of signaling molecules to the environment. The main role of QS is the regulation of vital processes in the cell such as virulence factor production or biofilm formation. Due to still growing bacterial resistance to antibiotics that have been overused, it is necessary to search for alternative antimicrobial therapies. One of them is quorum quenching (QQ) that disrupts microbial communication. QQ-driving molecules can decrease or even completely inhibit the production of virulence factors (including biofilm formation). There are few QQ strategies that comprise the use of the structural analogues of QS receptor autoinductors (AI). They may be found in nature or be designed and synthesized via chemical engineering. Many of the characterized QQ molecules are enzymes with the ability to degrade signaling molecules. They can also impede cellular signaling cascades. There are different techniques used for testing QS/QQ, including chromatography-mass spectroscopy, bioluminescence, chemiluminescence, fluorescence, electrochemistry, and colorimetry. They all enable qualitative and quantitative measurements of QS/QQ molecules. This article gathers the information about the mechanisms of QS and QQ, and their effect on microbial biofilm formation. Basic methods used to study QS/QQ, as well as the medical and biotechnological applications of QQ, are also described. Basis research methods are also described as well as medical and biotechnological application.

Musa acuminata and its bioactive metabolite 5-Hydroxymethylfurfural mitigates quorum sensing (las and rhl) mediated biofilm and virulence production of nosocomial pathogen Pseudomonas aeruginosa in vitro

ETHNOPHARMACOLOGICAL RELEVANCE

Musa acuminata, a tropical plant belongs to the family Musaceae. The fruit peels of this plant have been well documented for their therapeutic value in Asia and Africa. It has also been previously reported for numerous biological applications such as antimicrobial, antioxidant, itching, psoriasis and anti-diarrheal activities. Moreover, M. acuminata peels have been well known for its anti-healing and antiseptic properties and most commonly used for healing wounds and heat burns in South Asian and African traditional medicines.

AIM OF THE STUDY

To evaluate the QS-mediated antibiofilm and antivirulence potential of M. acuminata, and its bioactive metabolites 5-Hydroxymethylfurfural (5HMF) against Pseudomonas aeruginosa.

MATERIALS AND METHODS

The M. acuminata peel methanol extract (MAM) was evaluated for its antibiofilm potential against P. aeruginosa with increasing concentration. Besides, biofilm related phenomenon's such as total biofilm proteins, microcolony formation exopolysaccharides (EPS) and cell surface hydrophobicity (CSH) productions were also examined to support the antibiofilm potential of MAM. Further, MAM was evaluated for its antivirulence efficacy against P. aeruginosa by assessing the protease, LasA protease, LasB elastase, pyocyanin, alginate and rhamnolipid productions at 400 μg ml^-1 concentration. Transcriptional analysis of QS regulated virulence genes expression level was also done by real-time PCR analysis. Then, the MAM was subjected to column chromatography for further fractions and the bioactive compounds present in MAM were identified by gas chromatograph-mass spectrometry analysis. Further, the major compounds such as 5-hydroxymethylfurfural, vaccenic acid and pentanoic acid identified from active fraction of MAM were evaluated for their antibiofilm and antivirulence potential against P. aeruginosa.

RESULTS

MAM significantly inhibited the biofilm formation in P. aeruginosa at 400 μg ml^-1 concentration which also inhibited the production of biofilm proteins, biofilm adherence, EPS and CSH productions to the level of 79%, 82% and 77% respectively. Further, the antivirulence potential was confirmed through numerous virulence inhibition assays. The MAM at 400 μg ml^-1 concentration inhibited the QS-mediated virulence production such as protease, LasA protease, LasB elastase, pyocyanin, alginate and rhamnolipid productions to the level of 77%, 75%, 68%, 80%, 78% and 69% respectively. Moreover, the results of qPCR analysis confirmed the downregulation of QS regulated virulence genes expression upon treatment with MAM. The chromatographic analysis revealed the presence of 5-Hydroxymethylfurfural (5HMF), vaccenic acid and pentanoic acid in MAM and the potential bioactive compounds with antibiofilm and antivirulence was identified as 5-hydroxymethylfurfural, without exerting any growth inhibition in P. aeruginosa.

CONCLUSION

This study investigated the ideal antibiofilm and antivirulence potential of MAM and its bioactive compound 5HMF, and confirms the ethnopharmacological value of these peels against P. aeruginosa infections.

Soil organic matter attenuates the efficacy of flavonoid-based plant-microbe communication

Plant-microbe interactions are mediated by signaling compounds that control vital plant functions, such as nodulation, defense, and allelopathy. While interruption of signaling is typically attributed to biological processes, potential abiotic controls remain less studied. Here, we show that higher organic carbon (OC) contents in soils repress flavonoid signals by up to 70%. Furthermore, the magnitude of repression is differentially dependent on the chemical structure of the signaling molecule, the availability of metal ions, and the source of the plant-derived OC. Up to 63% of the signaling repression occurs between dissolved OC and flavonoids rather than through flavonoid sorption to particulate OC. In plant experiments, OC interrupts the signaling between a legume and a nitrogen-fixing microbial symbiont, resulting in a 75% decrease in nodule formation. Our results suggest that soil OC decreases the lifetime of flavonoids underlying plant-microbe interactions.

Comprehensive multivariate correlations between climatic effect, metabolite-profile, antioxidant capacity and antibacterial activity of Brazilian red propolis metabolites during seasonal study

The standardization of apiceutical products like as propolis extracts has been widely debated worldwide and variations in the propolis chemical composition are still very relevant topics for use-standardized of different propolis-type as medication by much of the world’s population. The present manuscript discuss important issues related to the climate effect and variations in propolis metabolite-profiling changes, antioxidant capacity and variations of the antibacterial activity of the Brazilian red propolis metabolites using comprehensive multivariate correlations. It was observed the increasing of guttiferones concentrations during the intense drought period and drastic decreasing in rainy period. The climate variation induced the high concentration of flavonoids in rainy period with pronounced dropped in some rainy months. The Pearson´s analysis demonstrated correlation between IC₅₀ from DPPH and guttiferones and flavonoids concentrations. The PCA-X and Hotelling T2 test showed outliers during the months with lowest concentrations of formononetin and isoliquiritigenin was observed in antibacterial tests. The PLS-DA, OPLS-DA and VIP analysis demonstrate guttiferone E, guttiferone B, liquiritigenin, naringenin are considered important substances responsible by anti-staphylococcal activity in red propolis composition during the rainy season and drought period, but a synergistic effect with other flavonoids and isoflavonoids are not ruled out.

Anti-Virulence Strategy against the Multidrug-Resistant Bacterial Pathogen Pseudomonas aeruginosa: Pseudolysin (Elastase B) as a Potential Druggable Target

Pseudomonas aeruginosa is a non-fermentative, gram-negative bacterium that is one of the most common pathogens responsible for hospital-acquired infections worldwide. The management of the infections caused by P. aeruginosa represents a huge challenge in the healthcare settings due to the increased emergence of resistant isolates, some of them resistant to all the currently available antimicrobials, which results in elevated morbimortality rates. Consequently, the development of new therapeutic strategies against multidrug-resistant P. aeruginosa is urgent and needful. P. aeruginosa is wellrecognized for its extreme genetic versatility and its ability to produce a lush variety of virulence factors. In this context, pseudolysin (or elastase B) outstands as a pivotal virulence attribute during the infectious process, playing multifunctional roles in different aspects of the pathogen-host interaction. This protein is a 33-kDa neutral zinc-dependent metallopeptidase that is the most abundant peptidase found in pseudomonal secretions, which contributes to the invasiveness of P. aeruginosa due to its ability to cleave several extracellular matrix proteins and to disrupt the basolateral intercellular junctions present in the host tissues. Moreover, pseudolysin makes P. aeruginosa able to overcome host defenses by the hydrolysis of many immunologically relevant molecules, including antibodies and complement components. The attenuation of this striking peptidase therefore emerges as an alternative and promising antivirulence strategy to combat antibiotic-refractory infections caused by P. aeruginosa. The anti-virulence approach aims to disarm the P. aeruginosa infective arsenal by inhibiting the expression/activity of bacterial virulence factors in order to reduce the invasiveness of P. aeruginosa, avoiding the emergence of resistance since the proliferation is not affected. This review summarizes the most relevant features of pseudolysin and highlights this enzyme as a promising target for the development of new anti-virulence compounds.

Swarming Inhibitory Potential of Cinnamtannin B1 from Cinnamomum tamala T. Nees and Eberm on Pseudomonas aeruginosa

In a preliminary screening, the methanol extract of Cinnamomum tamala leaves was found to inhibit the swarming motility of Pseudomonas aeruginosa. Bioassay-guided fractionation by silica gel column chromatography led to the identification of cinnamtannin B1 (1) as one of the active components of the extract. It inhibited the swarming motility (at 12.5 μg/mL) and biofilm formation (at 25 μg/mL) ofP. aeruginosa. Comparative gene expression analysis revealed downregulation of rhlA and fliC genes upon treatment with the tannin. The tannin may be affecting rhamnolipid and flagellin production. Thus, cinnamtannin B1 is an active component of C. tamala responsible for inhibiting the swarming motility of P. aeruginosa.

Leveraging phytochemicals: the plant phylogeny predicts sources of novel antibacterial compounds

AIM

The goal of this study was to use phylogenetic evidence to determine plant families with high representation of antibacterial activity and identify potential sources to focus on for antibacterial drug discovery.

MATERIALS & METHODS

We reconstructed the molecular phylogeny of plant taxa with antibacterial activity and mapped antibacterial mechanisms of action on the phylogeny.

RESULTS

The phylogeny highlighted seven plant families (Combretaceae, Cupressaceae, Fabaceae, Lamiaceae, Lauraceae, Myrtaceae and Zingiberaceae) with disproportionately represented antibacterial activity. Phytochemicals produced were primarily involved in the disruption of the bacterial cell wall/membrane and inhibition of quorum sensing/biofilm production.

CONCLUSION

The study provides phylogenetic evidence of seven plant families that should be examined as promising leads for novel antibacterial development.

Effect of Quercetin Rich Onion Extracts on Bacterial Quorum Sensing

Quorum sensing (QS) regulates bacterial gene expression and studies suggest quercetin, a flavonol found in onion, as a QS inhibitor. There are no studies showing the anti-QS activity of plants containing quercetin in its native glycosylated forms. This study aimed to evaluate the antimicrobial and anti-QS potential of organic extracts of onion varieties and its representative phenolic compounds quercetin aglycone and quercetin 3-β-D-glucoside in the QS model bacteria Chromobacterium violaceum ATCC 12472, Pseudomonas aeruginosa PAO1, and Serratia marcescens MG1. Three phenolic extracts were obtained: red onion extract in methanol acidified with 2.5% acetic acid (RO-1), white onion extract in methanol (WO-1) and white onion extract in methanol ammonium (WO-2). Quercetin 4-O-glucoside and quercetin 3,4-O-diglucoside were identified as the predominant compounds in both onion varieties using HPLC-DAD and LC-ESI-MS/MS. However, quercetin aglycone, cyanidin 3-O-glucoside and quercetin glycoside were identified only in RO-1. The three extracts showed minimum inhibitory concentration (MIC) values equal to or above 125 μg/ml of dried extract. Violacein production was significantly reduced by RO-1 and quercetin aglycone, but not by quercetin 3-β-D-glucoside. Motility in P. aeruginosa PAO1 was inhibited by RO-1, while WO-2 inhibited S. marcescens MG1 motility only in high concentration. Quercetin aglycone and quercetin 3-β-D-glucoside were effective at inhibiting motility in P. aeruginosa PAO1 and S. marcescens MG1. Surprisingly, biofilm formation was not affected by any extracts or the quercetins tested at sub-MIC concentrations. In silico studies suggested a better interaction and placement of quercetin aglycone in the structures of the CviR protein of C. violaceum ATCC 12472 than the glycosylated compound which corroborates the better inhibitory effect of the former over violacein production. On the other hand, the two quercetins were well placed in the AHLs binding pockets of the LasR protein of P. aeruginosa PAO1. Overall onion extracts and quercetin presented antimicrobial activity, and interference on QS regulated production of violacein and swarming motility.

Antimicrobial activity of novel chalcones and modulation of virulence factors in hospital strains of Acinetobacter baumannii and Pseudomonas aeruginosa

Acinetobacter baumannii and Pseudomonas aeruginosa are frequent multiresistant nosocomial pathogens that cause wound and pulmonary infections in hospitalized patients. As being increasingly resistant to most clinically available antibiotics, there is a constant need for exploration of new substances that could kill them or inhibit their growth, or alternatively inhibit some of their essential virulence factors. Chalcones are chemical compounds with well-documented antimicrobial potential. The aim of this study was to examine effectiveness of four newly-synthesized chalcones against the multiresistant clinical strains of A. baumannii and P. aeruginosa. Antibacterial activity of chalcones was investigated with broth-microdilution test and time-dependent killing assay. Synergistic effects of tested compounds with antibiotics (meropenem, amikacin and ciprofloxacin) were determined by checkerboard assay. The effects of chalcones on expression of virulence factors in P. aeruginosa (pyocyanin production, swimming and swarming motility) and A. baumannii (twitching and surface-associated motility), along with their biofilm production, were also examined. The obtained results indicate substantial antimicrobial activity of the tested chalcones (MICs = 100-175 μg/mL) and several synergistic interactions with antibiotics, as well as notable reduction in expression of all investigated virulence factors. These promising results may constitute a good basis for further research.

β-Sitosterol derived compound from onion husks non-polar fraction reduces quorum sensing controlled virulence and biofilm production

Quorum sensing is an important regulatory factor of P. aeruginosa virulence induction such as BF, motility, formations of proteases, pyocyanin, and some toxins. The aim of the current study is to detect the effect of the pet.ether extract from onion husk and compound drive from it on quorum sensing and virulence formations of P. aeruginosa. Quorum sensing inhibiting effect of the pet.ether extract of onion husk and a compound drive from it, was evaluated by C. violaceum reporter using dilution method as well as an antioxidant by using DPPH. The efficacious of: Quorum sensing inhibiting on pet.ether fraction and compound derived from it, were investigated for their activities toward biofilm and pyocyanin synthesis as well as motility from P. aeruginosa. The pet.ether fraction and compound derived from it of onion husk exhibited potent antimicrobial, antioxidant and Quorum sensing inhibiting effects. The pet.ether fraction and compound derived from it possesses significant reduction on pyocyanin and biofilm induction of P. aeruginosa. Moreover, they significantly inhibited swimming motilities of P. aeruginosa. For the first time, our study showed the medical importance of Allium cepa L. as antimicrobial, antioxidant as well as Quorum sensing inhibiting and virulence suppressors of P. aeruginosa. Thus, these might emphasized on Allium cepa L as a natural source for attenuating toxins of the Pseudomonas.

Novel O-alkyl Derivatives of Naringenin and Their Oximes with Antimicrobial and Anticancer Activity

In our investigation, we concentrated on naringenin (NG)—a widely studied flavanone that occurs in citrus fruits. As a result of a reaction with a range of alkyl iodides, 7 novel O-alkyl derivatives of naringenin (7a–11a, 13a, 17a) were obtained. Another chemical modification led to 9 oximes of O-alkyl naringenin derivatives (7b–13b, 16b–17b) that were never described before. The obtained compounds were evaluated for their potential antibacterial activity against Escherichia coli, Staphylococcus aureus, and Bacillus subtilis. The results were reported as the standard minimal inhibitory concentration (MIC) values and compared with naringenin and its known O-alkyl derivatives. Compounds 4a, 10a, 12a, 14a, 4b, 10b, 11b, and 14b were described with MIC of 25 µg/mL or lower. The strongest bacteriostatic activity was observed for 7-O-butylnaringenin (12a) against S. aureus (MIC = 6.25 µg/mL). Moreover, the antitumor effect of flavonoids was examined on human colon cancer cell line HT-29. Twenty-six compounds were characterized as possessing an antiproliferative activity stronger than that of naringenin. The replacement of the carbonyl group with an oxime moiety significantly increased the anticancer properties. The IC₅₀ values below 5 µg/mL were demonstrated for four oxime derivatives (8b, 11b, 13b and 16b).