Anti-biofilm activity of marine algae-derived bioactive compounds

A large number of microbial species tend to communicate and produce biofilm which causes numerous microbial infections, antibiotic resistance, and economic problems across different industries. Therefore, advanced anti-biofilms are required with novel attributes and targets, such as quorum sensing communication system. Meanwhile, quorum sensing inhibitors as promising anti-biofilm molecules result in the inhibition of particular phenotype expression blocking of cell-to-cell communication, which would be more acceptable than conventional strategies. Many natural products are identified as anti-biofilm agents from different plants, microorganisms, and marine extracts. Marine algae are promising sources of broadly novel compounds with anti-biofilm activity. Algae extracts and their metabolites such as sulfated polysaccharides (fucoidan), carotenoids (zeaxanthin and lutein), lipid and fatty acids (γ-linolenic acid and linoleic acid), and phlorotannins can inhibit the cell attachment, reduce the cell growth, interfere in quorum sensing pathway by blocking related enzymes, and disrupt extracellular polymeric substances. In this review, the mechanisms of biofilm formation, quorum sensing pathway, and recently identified marine algae natural products as anti-biofilm agents will be discussed.

Targeting the Pseudomonas aeruginosa quorum sensing system to inhibit virulence factors and eradicate biofilm formation using AHL-analogue phytochemicals

Quorum sensing plays a major role in the expression of virulence and development of biofilm in the human pathogen Pseudomonas aeruginosa. Natural compounds are well-known for their antibacterial characteristics by blocking various metabolic pathways. The goal of this study is to find natural compounds that mimic AHL (Acyl homoserine lactone) and suppress virulence in P. aeruginosa, which is triggered by quorum sensing-dependent pathways as an alternative drug development strategy. To support this rationale, functional network analysis and in silico investigations were carried out to find natural AHL analogues, followed by molecular docking studies. Out of the 16 top-hit AHL analogues derived from phytochemicals, seven ligands were found to bind to the quorum sensing activator proteins. Cassialactone, an AHL analogue, exhibited the highest binding affinity for RhlI, RhlR, and PqsE of P. aeruginosa, with a docking score of -9.4, -8.9, and -8.7 kcal/mol, respectively. 2(5H)-Furanone, a well-known inhibitor, was also docked to compare the docking score and intermolecular interactions between the ligand and the target protein. Furthermore, molecular dynamics simulations and binding free energy calculations were performed to determine the stability of the docked complexes. Additionally, the ADME properties of the analogues were also analyzed to evaluate the pharmacological parameters. Functional network analysis further showed that the interconnectedness of proteins such as RhlI, RhlR, LasI, and PqsE with the virulence and biofilm phenotype of the pathogen could offer potential as a therapeutic target.Communicated by Ramaswamy H. Sarma.

Methyl gallate from Camellia nitidissima Chi flowers reduces quorum sensing related virulence and biofilm formation against Aeromonas hydrophila

Aeromonas hydrophila, a Gram-negative zoonotic bacterium, causes high mortality in fish farming and immunocompromised patients. This study aimed to extract methyl gallate (MG) from the flowers of Camellia nitidissima Chi and evaluate its potential as a quorum sensing inhibitor (QSI) against Aeromonas hydrophila SHAe 115. MG reduced QS-associated virulence factors, including hemolysis, protease, and lipase, while impairing swimming motility and biofilm formation. Additionally, MG down-regulated positive regulatory genes (ahyR, fleQ) and up-regulated negative regulators (litR, fleN). This highlights MG's promise as a potent QSI for A. hydrophila SHAe 115, advancing strategies against infections in aquaculture and human health.

A bibliometric analysis on discovering anti-quorum sensing agents against clinically relevant pathogens: current status, development, and future directions

Background

Quorum sensing is bacteria's ability to communicate and regulate their behavior based on population density. Anti-quorum sensing agents (anti-QSA) is promising strategy to treat resistant infections, as well as reduce selective pressure that leads to antibiotic resistance of clinically relevant pathogens. This study analyzes the output, hotspots, and trends of research in the field of anti-QSA against clinically relevant pathogens.

Methods

The literature on anti-QSA from the Web of Science Core Collection database was retrieved and analyzed. Tools such as CiteSpace and Alluvial Generator were used to visualize and interpret the data.

Results

From 1998 to 2023, the number of publications related to anti-QAS research increased rapidly, with a total of 1,743 articles and reviews published in 558 journals. The United States was the largest contributor and the most influential country, with an H-index of 88, higher than other countries. Williams was the most productive author, and Hoiby N was the most cited author. Frontiers in Microbiology was the most prolific and the most cited journal. Burst detection indicated that the main frontier disciplines shifted from MICROBIOLOGY, CLINICAL, MOLECULAR BIOLOGY, and other biomedicine-related fields to FOOD, MATERIALS, NATURAL PRODUCTS, and MULTIDISCIPLINARY. In the whole research history, the strongest burst keyword was cystic-fibrosis patients, and the strongest burst reference was Lee and Zhang (2015). In the latest period (burst until 2023), the strongest burst keyword was silver nanoparticle, and the strongest burst reference was Whiteley et al. (2017). The co-citation network revealed that the most important interest and research direction was anti-biofilm/anti-virulence drug development, and timeline analysis suggested that this direction is also the most active. The key concepts alluvial flow visualization revealed seven terms with the longest time span and lasting until now, namely Escherichia coli, virulence, Pseudomonas aeruginosa, virulence factor, bacterial biofilm, gene expression, quorum sensing. Comprehensive analysis shows that nanomaterials, marine natural products, and artificial intelligence (AI) may become hotspots in the future.

Conclusion

This bibliometric study reveals the current status and trends of anti-QSA research and may assist researchers in identifying hot topics and exploring new research directions.

Light-Based Anti-Biofilm and Antibacterial Strategies

Biofilm formation and antimicrobial resistance pose significant challenges not only in clinical settings (i.e., implant-associated infections, endocarditis, and urinary tract infections) but also in industrial settings and in the environment, where the spreading of antibiotic-resistant bacteria is on the rise. Indeed, developing effective strategies to prevent biofilm formation and treat infections will be one of the major global challenges in the next few years. As traditional pharmacological treatments are becoming inadequate to curb this problem, a constant commitment to the exploration of novel therapeutic strategies is necessary. Light-triggered therapies have emerged as promising alternatives to traditional approaches due to their non-invasive nature, precise spatial and temporal control, and potential multifunctional properties. Here, we provide a comprehensive overview of the different biofilm formation stages and the molecular mechanism of biofilm disruption, with a major focus on the quorum sensing machinery. Moreover, we highlight the principal guidelines for the development of light-responsive materials and photosensitive compounds. The synergistic effects of combining light-triggered therapies with conventional treatments are also discussed. Through elegant molecular and material design solutions, remarkable results have been achieved in the fight against biofilm formation and antibacterial resistance. However, further research and development in this field are essential to optimize therapeutic strategies and translate them into clinical and industrial applications, ultimately addressing the global challenges posed by biofilm and antimicrobial resistance.

Synthesis of Novel Quinazolinone Analogues for Quorum Sensing Inhibition

As bacteria continue to develop resistance mechanisms against antimicrobials, an alternative method to tackle this global concern must be developed. As the pqs system is the most well-known and responsible for biofilm and pyocyanin production, quinazolinone inhibitors of the pqs system in P. aeruginosa were developed. Molecular docking following a rationalised medicinal chemistry approach was adopted to design these analogues. An analysis of docking data suggested that compound 6b could bind with the key residues in the ligand binding domain of PqsR in a similar fashion to the known antagonist M64. The modification of cyclic groups at the 3-position of the quinazolinone core, the introduction of a halogen at the aromatic core and the modification of the terminal group with aromatic and aliphatic chains were investigated to guide the synthesis of a library of 16 quinazolinone analogues. All quinazolinone analogues were tested in vitro for pqs inhibition, with the most active compounds 6b and 6e being tested for biofilm and growth inhibition in P. aeruginosa (PAO1). Compound 6b displayed the highest pqs inhibitory activity (73.4%, 72.1% and 53.7% at 100, 50 and 25 µM, respectively) with no bacterial growth inhibition. However, compounds 6b and 6e only inhibited biofilm formation by 10% and 5%, respectively.

Use of marine microorganisms in designing anti-infective strategies for sustainable aquaculture production

Aquaculture, a noteworthy food production sector, is confronted with disease occurrences. Treatment of aquaculture pathogens with antibiotics is often rendered ineffective due to biofilm formation and the development of resistant strains. Marine ecosystems encompass unusual microorganisms that produce novel bioactive compounds, including agents that could be used as alternatives to antibiotics. Moreover, biomass and/or biomolecules associated with these microorganisms could act as feed supplements to enhance overall health of aquaculture species' and improve water quality parameters. The present review summarizes contents of studies on such marine microorganisms with the potential to be developed as agents for tackling bacterial diseases in the aquaculture segment. Bioactive compounds produced by marine bacteria are known to inhibit biofilm associated infections mediated by their bactericidal properties (produced by Bacillus, Vibrio, Photobacterium and Pseudoalteromonas species), surfactant activity (obtained from different species of Bacillus and Staphylococcus lentus), anti-adhesive activity (derived from Bacillus sp. and Brevibacterium sp.) and quorum sensing inhibition. Several marine fungal isolates capable of producing antibacterial agents have also been effective in inhibiting aquaculture associated pathogens. Another strategy followed by investigators to reduce severity of infections is the use of bacterial, yeast and microalgae biomass as feed supplements, probiotics and immunostimulants. In some cases, marine microalgae have been employed as sustainable alternatives to fish oil and fish meal without compromising on nutritional quality. Their inclusion in aquaculture feed have enhanced growth, favoured better survival of cultured species and improved water quality parameters. Marine microorganisms (by providing effective bioactive compounds and being used as feed supplements) could enable aquaculture practices to be more sustainable in the future.

Screening of the Medicines for Malaria Venture Pandemic Response Box for Discovery of Antivirulent Drug against Pseudomonas aeruginosa

Resistance development and exhaustion of the arsenal of existing antibacterial agents urgently require an alternative approach toward drug discovery. Herein, we report the screening of Medicines for Malaria Venture (MMV) Pandemic Response Box (PRB) through a cascade developed to streamline the potential compounds with antivirulent properties to combat an opportunistic pathogen, Pseudomonas aeruginosa. To find an agent suppressing the production of P. aeruginosa virulence factors, we assessed the potential of the compounds in PRB with quorum sensing inhibitory activity. Our approach led us to identify four compounds with significant inhibition of extracellular virulence factor production and biofilm formation. This provides an opportunity to expand and redirect the application of these data sets toward the development of a drug with unexplored target-based activity. IMPORTANCE The rise of drug-resistant pathogens as well as overuse and misuse of antibiotics threatens modern medicine as the number of effective antimicrobial drugs steadily decreases. Given the nature of antimicrobial resistance development under intense selective pressure such as the one posed by pathogen-eliminating antibiotics, new treatment options which could slow down the emergence of resistance are urgently needed. Antivirulence therapy aims at suppressing a pathogen's ability to cause disease rather than eliminating it, generating significantly lower selective pressure. Quorum sensing inhibitors are thought to be able to downregulate the production of virulence factors, allowing for smaller amounts of antimicrobials to be used and thus preventing the emergence of resistance. The PRB constitutes an unprecedented opportunity to repurpose new as well as known compounds with cytotoxicity and in vitro absorption, distribution, metabolism and excretion (ADME) profile available, thus shortening the time between compound discovery and medicinal use.

Quorum sensing in human gut and food microbiomes: Significance and potential for therapeutic targeting

Human gut and food microbiomes interact during digestion. The outcome of these interactions influences the taxonomical composition and functional capacity of the resident human gut microbiome, with potential consequential impacts on health and disease. Microbe-microbe interactions between the resident and introduced microbiomes, which likely influence host colonisation, are orchestrated by environmental conditions, elements of the food matrix, host-associated factors as well as social cues from other microorganisms. Quorum sensing is one example of a social cue that allows bacterial communities to regulate genetic expression based on their respective population density and has emerged as an attractive target for therapeutic intervention. By interfering with bacterial quorum sensing, for instance, enzymatic degradation of signalling molecules (quorum quenching) or the application of quorum sensing inhibitory compounds, it may be possible to modulate the microbial composition of communities of interest without incurring negative effects associated with traditional antimicrobial approaches. In this review, we summarise and critically discuss the literature relating to quorum sensing from the perspective of the interactions between the food and human gut microbiome, providing a general overview of the current understanding of the prevalence and influence of quorum sensing in this context, and assessing the potential for therapeutic targeting of quorum sensing mechanisms.

Genome analysis and phenotypic characterization of Halomonas hibernica isolated from a traditional food process with novel quorum quenching and catalase activities

Traditional food processes can utilize bacteria to promote positive organoleptic qualities and increase shelf life. Wiltshire curing has a vital bacterial component that has not been fully investigated from a microbial perspective. During the investigation of a Wiltshire brine, a culturable novel bacterium of the genus Halomonas was identified by 16S rRNA gene (MN822133) sequencing and analysis. The isolate was confirmed as representing a novel species (Halomonas hibernica B1.N12) using a housekeeping (HK) gene phylogenetic tree reconstruction with the selected genes 16S rRNA, 23S rRNA, atpA, gyrB, rpoD and secA. The genome of the new isolate was sequenced and annotated and comparative genome analysis was conducted. Functional analysis revealed that the isolate has a unique phenotypic signature including high salt tolerance, a wide temperature growth range and substrate metabolism. Phenotypic and biochemical profiling demonstrated that H. hibernica B1.N12 possesses strong catalase activity which is an important feature for an industrial food processing bacterium, as it can promote an increased product shelf life and improve organoleptic qualities. Moreover, H. hibernica exhibits biocontrol properties based on its quorum quenching capabilities. Our work on this novel isolate advances knowledge on potential mechanistic interplays operating in complex microbial communities that mediate traditional food processes.

Characterization of an Insoluble and Soluble Form of Melanin Produced by Streptomyces cavourensis SV 21, a Sea Cucumber Associated Bacterium

Melanin is a widely distributed and striking dark-colored pigment produced by countless living organisms. Although a wide range of bioactivities have been recognized, there are still major constraints in using melanin for biotechnological applications such as its fragmentary known chemical structure and its insolubility in inorganic and organic solvents. In this study, a bacterial culture of Streptomyces cavourensis SV 21 produced two distinct forms of melanin: (1) a particulate, insoluble form as well as (2) a rarely observed water-soluble form. The here presented novel, acid-free purification protocol of purified particulate melanin (PPM) and purified dissolved melanin (PDM) represents the basis for an in-depth comparison of their physicochemical and biological properties, which were compared to the traditional acid-based precipitation of melanin (AM) and to a synthetic melanin standard (SM). Our data show that the differences in solubility between PDM and PPM in aqueous solutions may be a result of different adjoining cation species, since the soluble PDM polymer is largely composed of Mg²⁺ ions and the insoluble PPM is dominated by Ca²⁺ ions. Furthermore, AM shared most properties with SM, which is likely attributed to a similar, acid-based production protocol. The here presented gentler approach of purifying melanin facilitates a new perspective of an intact form of soluble and insoluble melanin that is less chemical altered and thus closer to its original biological form.

Inhibition of Quorum Sensing and Virulence Factors of Pseudomonas aeruginosa by Biologically Synthesized Gold and Selenium Nanoparticles

The development of microbial resistance requires a novel approach to control microbial infection. This study implies the microbial synthesis of nanometals and assessment of their antivirulent activity against Pseudomonas aeruginosa. Streptomyces isolate S91 was isolated from soil with substantial ability for growth at high salts concentrations. The cell-free supernatant of S91was utilized for the synthesis of Au-NPs and Se-NPs. The 16S rRNA sequence analysis of Streptomyces S91 revealed that S91 had a high similarity (98.82%) to Streptomyces olivaceous. The biosynthesized Au-NPs and Se-NPs were characterized using a UV-Vis spectrophotometer, dynamic light scattering, transmission electron microscopy, energy dispersive X-ray diffraction and Fourier-transform infrared spectroscopy. The quorum sensing inhibitory (QSI) potential of Au-NPs and Se-NPs and the antivirulence activity was examined against P. aeruginosa. The QSI potential was confirmed using RT-PCR. The synthesized Au-NPs and Se-NPs were monodispersed spherical shapes with particle size of 12.2 and 67.98 nm, respectively. Au-NPs and Se-NPs eliminated QS in P. aeruginosa at a concentration range of 2.3-18.5 µg/mL for Au-NPs and 2.3-592 µg/mL for Se-NPs. In addition, Au-NPs and Se-NPs significantly inhibited QS-related virulence factors, such as pyocyanin, protease and, elastase in P. aeruginosa. At the molecular level, Au-NPs and Se-NPs significantly suppressed the relative expression of QS genes and toxins. Hence, the biosynthesized Au-NPS and Se-NPS could be substantial inhibitors of QS and virulence traits of P. aeruginosa.

Quorum Sensing Inhibiting Activity of Cefoperazone and Its Metallic Derivatives on Pseudomonas aeruginosa

The last decade has witnessed a massive increase in the rate of mortalities caused by multidrug-resistant Pseudomonas aeruginosa. Therefore, developing new strategies to control virulence factors and pathogenicity has received much attention. One of these strategies is quorum sensing inhibition (QSI) which was developed to control Pseudomonas infection. This study aims to validate the effect of one of the most used β-lactam antibiotics; cefoperazone (CFP) and its metallic-derivatives on quorum sensing (QS) and virulence factors of P. aeruginosa. Assessment of quorum sensing inhibitory activity of CFP, cefoperazone Iron complex (CFPF) and cefoperazone Cobalt complex (CFPC) was performed by using reporter strain Chromobacterium violaceum ATCC 12472. Minimal inhibitory concentration (MIC) was carried out by the microbroth dilution method. The influence of sub-MICs (1/4 and 1/2 MICs) of CFP, CFPF and CFPC on virulence factors of P. aeruginosa was evaluated. Data was confirmed on the molecular level by RT-PCR. Also, molecular docking analysis was conducted to figure out the possible mechanisms of QSI. CFP, CFPF, and CFPC inhibited violacein pigment production of C. violaceum ATCC 12472. Sub-MICs of CFP (128- 256 μg/mL), and significantly low concentrations of CFPC (0.5- 16 μg/mL) and CFPF (0.5- 64 μg/mL) reduced the production of QS related virulence factors such as pyocyanin, protease, hemolysin and eliminated biofilm assembly by P. aeruginosa standard strains PAO1 and PA14, and P. aeruginosa clinical isolates Ps1, Ps2, and Ps3, without affecting bacterial viability. In addition, CFP, CFPF, and CFPC significantly reduced the expression of lasI and rhlI genes. The molecular docking analysis elucidated that the QS inhibitory effect was possibly caused by the interaction with QS receptors. Both CFPF and CFPC interacted strongly with LasI, LasR and PqsR receptors with a much high ICM scores compared to CFP that could be the cause of elimination of natural ligand binding. Therefore, CFPC and CFPF are potent inhibitors of quorum sensing signaling and virulence factors of P. aeruginosa.

Design and Evaluation of New Quinazolin-4(3H)-one Derived PqsR Antagonists as Quorum Sensing Quenchers in Pseudomonas aeruginosa

P. aeruginosa (PA) continues to pose a threat to global public health due to its high levels of antimicrobial resistance (AMR). The ongoing AMR crisis has led to an alarming shortage of effective treatments for resistant microbes, and hence there is a pressing demand for the development of novel antimicrobial interventions. The potential use of antivirulence therapeutics to tackle bacterial infections has attracted considerable attention over the past decades as they hamper the pathogenicity of target microbes with reduced selective pressure, minimizing the emergence of resistance. One such approach is to interfere with the PA pqs quorum sensing system which upon the interaction of PqsR, a Lys-R type transcriptional regulator, with its cognate signal molecules 4-hydroxy-2-heptylquinoline (HHQ) and 2-heptyl-3-hydroxy-4-quinolone (PQS), governs multiple virulence traits and host-microbe interactions. In this study, we report the hit identification and optimization of PqsR antagonists using virtual screening coupled with whole cell assay validation. The optimized hit compound 61 ((R)-2-(4-(3-(6-chloro-4-oxoquinazolin-3(4H)-yl)-2-hydroxypropoxy)phenyl)acetonitrile) was found to inhibit the expression of the PA P_pqsA promoter controlled by PqsR with an IC₅₀ of 1 μM. Using isothermal titration calorimetry, a K_d of 10 nM for the PqsR ligand binding domain (PqsR^LBD) was determined for 61. Furthermore, the crystal structure of 61 with PqsR^LBD was attained with a resolution of 2.65 Å. Compound 61 significantly reduced levels of pyocyanin, PQS, and HHQ in PAO1-L, PA14 lab strains and PAK6085 clinical isolate. Furthermore, this compound potentiated the effect of ciprofloxacin in early stages of biofilm treatment and in Galleria mellonella infected with PA. Altogether, this data shows 61 as a potent PqsR inhibitor with potential for hit to lead optimization toward the identification of a PA QS inhibitor which can be advanced into preclinical development.

Characterization of Pseudomonas aeruginosa Quorum Sensing Inhibitors from the Endophyte Lasiodiplodia venezuelensis and Evaluation of Their Antivirulence Effects by Metabolomics

The opportunistic pathogen Pseudomonas aeruginosa is one of the "critical priority pathogens" due to its multidrug resistance to a wide range of antibiotics. Its ability to invade and damage host tissues is due to the use of quorum sensing (QS) to collectively produce a plethora of virulence factors. Inhibition of QS is an attractive strategy for new antimicrobial agents because it disrupts the initial events of infection without killing the pathogen. Highly diverse microorganisms as endophytes represent an under-explored source of bioactive natural products, offering opportunities for the discovery of novel QS inhibitors (QSI). In the present work, the objective was to explore selective QSIs within a unique collection of fungal endophytes isolated from the tropical palm Astrocaryum sciophilum. The fungi were cultured, extracted, and screened for their antibacterial and specific anti-QS activities against P. aeruginosa. The endophytic strain Lasiodiplodia venezuelensis was prioritized for scaled-up fractionation for its selective activity, leading to the isolation of eight compounds in a single step. Among them, two pyran-derivatives were found to be responsible for the QSI activity, with an effect on some QS-regulated virulence factors. Additional non-targeted metabolomic studies on P. aeruginosa documented their effects on the production of various virulence-related metabolites.

Natural and Enantiopure Alkylglycerols as Antibiofilms Against Clinical Bacterial Isolates and Quorum Sensing Inhibitors of Chromobacterium violaceum ATCC 12472

Resistance mechanisms occur in almost all clinical bacterial isolates and represent one of the most worrisome health problems worldwide. Bacteria can form biofilms and communicate through quorum sensing (QS), which allow them to develop resistance against conventional antibiotics. Thus, new therapeutic candidates are sought. We focus on alkylglycerols (AKGs) because of their recently discovered quorum sensing inhibition (QSI) ability and antibiofilm potential. Fifteen natural enantiopure AKGs were tested to determine their effect on the biofilm formation of other clinical bacterial isolates, two reference strains and their QSI was determined using Chromobacterium violaceum ATCC 12472. The highest biofilm inhibition rates (%) and minimum QS inhibitory concentration were determined by a microtiter plate assay and ciprofloxacin was used as the standard antibiotic. At subinhibitory concentrations, each AKG reduced biofilm formation in a concentration-dependent manner against seven bacterial isolates, with values up to 97.2%. Each AKG displayed QSI at different levels of ability without affecting the growth of C. violaceum. AKG (2S)-3-O-(cis-13’-docosenyl)-1,2-propanediol was the best QS inhibitor (20 μM), while (2S)-3-O-(cis-9’-hexadecenyl)-1,2-propanediol was the least effective (795 μM). The results showed for the first time the QSI activity of this natural AKG series and suggest that AKGs could be promising candidates for further studies on preventing antimicrobial resistance.

Quorum Sensing Inhibition or Quenching in Acinetobacter baumannii: The Novel Therapeutic Strategies for New Drug Development

Acinetobacter baumannii is a Gram-negative opportunistic nosocomial pathogen, which can cause ventilator-related and blood infection in critically ill patients. The resistance of A. baumannii clinical isolates to common antimicrobials and their tolerance to desiccation have emerged as a serious problem to public health. In the process of pathogenesis, bacteria release signals, which regulate virulence and pathogenicity-related genes. Such bacteria coordinate their virulent behavior in a cell density-dependent phenomenon called quorum sensing (QS). In contrast, the two main approaches of QS interference, quorum sensing inhibitors (QSIs) and quorum quenching (QQ) enzymes, have been developed to reduce the virulence of bacteria, thus reducing the pressure to produce bacterial drug resistance. Therefore, QSIs or QQ enzymes, which interfere with these processes, might potentially inhibit bacterial QS and ultimately biofilm formation. In this review, we aim to describe the state-of-art in the QS process in A. baumannii and elaborate on the use of QSIs or QQ enzymes as antimicrobial drugs in various potential sites of the QS pathway.

Identification of FDA-approved antivirulence drugs targeting the Pseudomonas aeruginosa quorum sensing effector protein PqsE

The ability of the bacterial pathogen Pseudomonas aeruginosa to cause both chronic and acute infections mainly relies on its capacity to finely modulate the expression of virulence factors through a complex network of regulatory circuits, including the pqs quorum sensing (QS) system. While in most QS systems the signal molecule/receptor complexes act as global regulators that modulate the expression of QS-controlled genes, the main effector protein of the pqs system is PqsE. This protein is involved in the synthesis of the QS signal molecules 2-alkyl-4(1H)-quinolones (AQs), but it also modulates the expression of genes involved in virulence factors production and biofilm formation via AQ-independent pathway(s). P. aeruginosa pqsE mutants disclose attenuated virulence in plant and animal infection models, hence PqsE is considered a good target for the development of antivirulence drugs against P. aeruginosa.

In this study, the negative regulation exerted by PqsE on its own transcription has been exploited to develop a screening system for the identification of PqsE inhibitors in a library of FDA-approved drugs. This led to the identification of nitrofurazone and erythromycin estolate, two antibiotic compounds that reduce the expression of PqsE-dependent virulence traits and biofilm formation in the model strain P. aeruginosa PAO1 at concentrations far below those affecting the bacterial growth rate. Notably, both drugs reduce the production of the PqsE-controlled virulence factor pyocyanin also in P. aeruginosa strains isolated from cystic fibrosis patients, and do not antagonize the activity of antibiotics commonly used to treat P. aeruginosa infection.

Screening of Bacterial Quorum Sensing Inhibitors in a Vibrio fischeri LuxR-Based Synthetic Fluorescent E. coli Biosensor

A library of 23 pure compounds of varying structural and chemical characteristics was screened for their quorum sensing (QS) inhibition activity using a synthetic fluorescent Escherichia coli biosensor that incorporates a modified version of lux regulon of Vibrio fischeri. Four such compounds exhibited QS inhibition activity without compromising bacterial growth, namely, phenazine carboxylic acid (PCA), 2-heptyl-3-hydroxy-4-quinolone (PQS), 1H-2-methyl-4-quinolone (MOQ) and genipin. When applied at 50 µM, these compounds reduced the QS response of the biosensor to 33.7% ± 2.6%, 43.1% ± 2.7%, 62.2% ± 6.3% and 43.3% ± 1.2%, respectively. A series of compounds only showed activity when tested at higher concentrations. This was the case of caffeine, which, when applied at 1 mM, reduced the QS to 47% ± 4.2%. In turn, capsaicin, caffeic acid phenethyl ester (CAPE), furanone and polygodial exhibited antibacterial activity when applied at 1mM, and reduced the bacterial growth by 12.8% ± 10.1%, 24.4% ± 7.0%, 91.4% ± 7.4% and 97.5% ± 3.8%, respectively. Similarly, we confirmed that trans-cinnamaldehyde and vanillin, when tested at 1 mM, reduced the QS response to 68.3% ± 4.9% and 27.1% ± 7.4%, respectively, though at the expense of concomitantly reducing cell growth by 18.6% ± 2.5% and 16% ± 2.2%, respectively. Two QS natural compounds of Pseudomonas aeruginosa, namely PQS and PCA, and the related, synthetic compounds MOQ, 1H-3-hydroxyl-4-quinolone (HOQ) and 1H-2-methyl-3-hydroxyl-4-quinolone (MHOQ) were used in molecular docking studies with the binding domain of the QS receptor TraR as a target. We offer here a general interpretation of structure-function relationships in this class of compounds that underpins their potential application as alternatives to antibiotics in controlling bacterial virulence.

Antioxidant, anti-quorum sensing, biofilm inhibitory activities and chemical composition of Patchouli essential oil: in vitro and in silico approach

The interest in naturally occurring essential oils from medicinal plants has increased extremely over the last decade markedly because they possess antimicrobial and antioxidant protective properties against different chronic diseases. Extensive survival of drug-resistant infectious bacteria depends on quorum sensing (QS) signaling network which raises the need for alternative antibacterial compounds. The aim of this study was to examine the phytochemical compounds of patchouli essential oil (PEO) and to assess its antioxidant activity. Antioxidant studies estimated by 2, 2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging method showed that the PEO has effective antioxidant activity (IC₅₀ 19.53 µg/mL). QS inhibitory activity of PEO was examined by employing the biosensor strain, Chromobacterium violaceum CV12472. At sub-lethal concentrations, PEO potentially reduced the QS regulated violacein synthesis in CV12472 without inhibiting its cell proliferation. Moreover, it also effectively reduced the production of some QS regulated virulence factors and biofilm development in P. aeruginosa PAO1 without hindering its growth. Phytochemical analysis of PEO was done by GC/MS technique. Molecular docking of PEO major compounds with QS (LasR and FabI) and biofilm regulator proteins (MvfR and Sialidase) of PAO1 was evaluated. These phytocompounds showed potential hydrogen binding interactions with these proteins. The overall results, in vitro and in silico, suggest that PEO could be applied as biocontrol agent against antibiotic resistance pathogens. Communicated by Ramaswamy H. Sarma.

Hit Identification of New Potent PqsR Antagonists as Inhibitors of Quorum Sensing in Planktonic and Biofilm Grown Pseudomonas aeruginosa

Current treatments for Pseudomonas aeruginosa infections are becoming less effective because of the increasing rates of multi-antibiotic resistance. Pharmacological targeting of virulence through inhibition of quorum sensing (QS) dependent virulence gene regulation has considerable therapeutic potential. In P. aeruginosa, the pqs QS system regulates the production of multiple virulence factors as well as biofilm maturation and is a promising approach for developing antimicrobial adjuvants for combatting drug resistance. In this work, we report the hit optimisation for a series of potent novel inhibitors of PqsR, a key regulator of the pqs system, bearing a 2-((5-methyl-5H-[1,2,4]triazino[5,6-b]indol-3-yl)thio) acetamide scaffold. The initial hit compound 7 (PAO1-L IC₅₀ 0.98 ± 0.02 μM, PA14 inactive at 10 μM) was obtained through a virtual screening campaign performed on the PqsR ligand binding domain using the University of Nottingham Managed Chemical Compound Collection. Hit optimisation gave compounds with enhanced potency against strains PAO1-L and PA14, evaluated using P. aeruginosa pqs-based QS bioreporter assays. Compound 40 (PAO1-L IC₅₀ 0.25 ± 0.12 μM, PA14 IC₅₀ 0.34 ± 0.03 μM) is one of the most potent PqsR antagonists reported showing significant inhibition of P. aeruginosa pyocyanin production and pqs system signaling in both planktonic cultures and biofilms. The co-crystal structure of 40 with the PqsR ligand binding domain revealed the specific binding interactions occurring between inhibitor and this key regulatory protein.

Rosmarinus officinalis L. (Rosemary) Extracts Containing Carnosic Acid and Carnosol are Potent Quorum Sensing Inhibitors of Staphylococcus aureus Virulence

Staphylococcus aureus is an opportunistic pathogen and a common cause of skin infection. S. aureus also plays a role in the pathogenesis of the chronic inflammatory skin disease, atopic dermatitis. S. aureus virulence involves activation of the quorum sensing agr operon. In this paper, we show that the diterpene carnosic acid, present in R. officinalis L. (rosemary) leaves, is a specific inhibitor of S. aureus agr expression as low as 5 μM. Carnosol and rosmarinic acid are two other phytochemicals present in rosemary leaves. Carnosol, but not rosmarinic acid, is also a potent agr expression inhibitor. Natural rosemary extracts containing carnosic acid and carnosol inhibit S. aureus agr expression, both in luciferase reporter strains and in wild type strains isolated from patients with atopic dermatitis. Specific inhibition of S. aureus virulence using topical formulations of rosemary extract may offer a practical approach to preventing and treating flares of atopic dermatitis.

Sesquiterpene lactones from Polydora serratuloides and their quorum sensing inhibitory activity

The leaves of Polydora serratuloides, with the synonym Vernonia perrottetii are widely used as purgative agents for gastrointestinal problems, and other members of Vernonieae have been used in African traditional medicine for decades. A new sesquiterpene lactone of the keto-hirsutinolide type, 13-acetoxy-1(4β),5(6)β-diepoxy-8α-(senecioyloxy)-3-oxo-1,7(11)-germacradiene-12,6-olide 1, was isolated from the hexane extract of its leaves, in addition to the known 13-acetoxy-1,4β-epoxy-8α-(senecioyloxy)-3-oxo-1,5,7(11)-germacratriene-12,6-olide 2. Three common flavonoids (apigenin 3, luteolin 4 and velutin 5) were also isolated. The antibacterial and quorum sensing inhibitory activities of compounds 1 and 2 and crudes extracts showed limited activity on Bacillus subtilis and Staphylococcus aureus, with no activity on Gram negative bacteria. However, quorum sensing (QSI) experiments indicated that 1 and 2, and the four crude extracts had interesting inhibitory activity on the biosensor organism, Chromobacterium violaceum ATCC 12472 in the range of 0.33-5.25 mg mL^-1, with compound 1 being the most effective at 0.33 mg mL^-1.

Identification of Quorum Sensing Activators and Inhibitors in The Marine Sponge Sarcotragus spinosulus

Marine sponges, a well-documented prolific source of natural products, harbor highly diverse microbial communities. Their extracts were previously shown to contain quorum sensing (QS) signal molecules of the N-acyl homoserine lactone (AHL) type, known to orchestrate bacterial gene regulation. Some bacteria and eukaryotic organisms are known to produce molecules that can interfere with QS signaling, thus affecting microbial genetic regulation and function. In the present study, we established the production of both QS signal molecules as well as QS inhibitory (QSI) molecules in the sponge species Sarcotragus spinosulus. A total of eighteen saturated acyl chain AHLs were identified along with six unsaturated acyl chain AHLs. Bioassay-guided purification led to the isolation of two brominated metabolites with QSI activity. The structures of these compounds were elucidated by comparative spectral analysis of ¹HNMR and HR-MS data and were identified as 3-bromo-4-methoxyphenethylamine (1) and 5,6-dibromo-N,N-dimethyltryptamine (2). The QSI activity of compounds 1 and 2 was evaluated using reporter gene assays for long- and short-chain AHL signals (Escherichia coli pSB1075 and E. coli pSB401, respectively). QSI activity was further confirmed by measuring dose-dependent inhibition of proteolytic activity and pyocyanin production in Pseudomonas aeruginosa PAO1. The obtained results show the coexistence of QS and QSI in S. spinosulus, a complex signal network that may mediate the orchestrated function of the microbiome within the sponge holobiont.

In silico Selection and Experimental Validation of FDA-Approved Drugs as Anti-quorum Sensing Agents

The emergence of antibiotic resistant bacterial pathogens is increasing at an unprecedented pace, calling for the development of new therapeutic options. Small molecules interfering with virulence processes rather than growth hold promise as an alternative to conventional antibiotics. Anti-virulence agents are expected to decrease bacterial virulence and to pose reduced selective pressure for the emergence of resistance. In the opportunistic pathogen Pseudomonas aeruginosa the expression of key virulence traits is controlled by quorum sensing (QS), an intercellular communication process that coordinates gene expression at the population level. Hence, QS inhibitors represent promising anti-virulence agents against P. aeruginosa. Virtual screenings allow fast and cost-effective selection of target ligands among vast libraries of molecules, thus accelerating the time and limiting the cost of conventional drug-discovery processes, while the drug-repurposing approach is based on the identification of off-target activity of FDA-approved drugs, likely endowed with low cytotoxicity and favorable pharmacological properties. This study aims at combining the advantages of virtual screening and drug-repurposing approaches to identify new QS inhibitors targeting the pqs QS system of P. aeruginosa. An in silico library of 1,467 FDA-approved drugs has been screened by molecular docking, and 5 hits showing the highest predicted binding affinity for the pqs QS receptor PqsR (also known as MvfR) have been selected. In vitro experiments have been performed by engineering ad hoc biosensor strains, which were used to verify the ability of hit compounds to decrease PqsR activity in P. aeruginosa. Phenotypic analyses confirmed the impact of the most promising hit, the antipsychotic drug pimozide, on the expression of P. aeruginosa PqsR-controlled virulence traits. Overall, this study highlights the potential of virtual screening campaigns of FDA-approved drugs to rapidly select new inhibitors of important bacterial functions.

Virtual Screening and Biomolecular Interactions of CviR-Based Quorum Sensing Inhibitors Against Chromobacterium violaceum

The rise of bacterial multi drug resistance becomes a global threat to the mankind. Therefore it is essential to find out alternate strategies to fight against these "super bugs." Quorum sensing (QS) is a cell-to-cell communication mechanism by which many bacteria regulate their biofilm and virulence factors expression to execute their pathogenesis. Hence, interfering the quorum sensing is an effective alternate strategy against various pathogens. In this study, we aimed to find out potential CviR-mediated quorum sensing inhibitors (QSIs) against Chromobacterium violaceum. Virtual screening from a natural products database, in vitro biofilm and violacein inhibition assays have been performed. Biofilm formation was investigated using confocal microscopy and gene expression studies were carried out using qRT-PCR. Further, to study the biomolecular interaction of QSIs with purified CviR Protein (a LuxR homologue), microscale thermophoresis (MST) analysis was performed. Results suggested that phytochemicals SPL, BN1, BN2, and C7X have potential GScore when compared to cognate ligand and reduced the biofilm formation and violacein production significantly. Especially, 100 μM of BN1 drastically reduced the biofilm formation about 82.61%. qRT-PCR studies revealed that cviI, cviR, vioB, vioC, vioD genes were significantly down regulated by QSIs. MST analysis confirmed the molecular interactions between QSIs and purified CviR protein which cohere with the docking results. Interestingly, we found that BN2 has better interaction with CviR (Kd = 45.07 ±1.90 nm). Overall results suggested that QSIs can potentially interact with CviR and inhibit the QS in a dose dependent manner. Since, LuxR homologs present in more than 100 bacterial species, these QSIs may be developed as broad spectrum anti-infective drugs in future.

Assessment of the Quorum Sensing Inhibition Activity of a Non-Toxic Chitosan in an N-Acyl Homoserine Lactone (AHL)-Based Escherichia coli Biosensor

New approaches to deal with drug-resistant pathogenic bacteria are urgent. We studied the antibacterial effect of chitosans against an Escherichia coli quorum sensing biosensor reporter strain and selected a non-toxic chitosan to evaluate its quorum sensing (QS) inhibition activity and its effect on bacterial aggregation. To this end, chitosans of varying degree of acetylation (DA) (12 to 69%) and molecular weight (Mw) (29 to 288 kDa) were studied. Only chitosans of low DA (~12%) inhibited bacterial growth, regardless of their Mw. A chitosan with medium degree of polymerization (named MDP) DA30, with experimental DA 42% and Mw 115 kDa was selected for further QS inhibition and scanning electron microscopy (SEM) imaging studies. MDP DA30 chitosan exhibited QS inhibition activity in an inverse dose-dependent manner (≤12.5 µg/mL). SEM images revealed that this chitosan, when added at low concentration (≤30.6 µg/mL), induced substantial bacterial aggregation, whereas at high concentration (234.3 µg/mL), it did not. Aggregation explains the QS inhibition activity as the consequence of retardation of the diffusion of N-acylated homoserine lactones (AHLs).

Cobalt Complex with Thiazole-Based Ligand as New Pseudomonas aeruginosa Quorum Quencher, Biofilm Inhibitor and Virulence Attenuator

Pseudomonas aeruginosa is one of the most dreaded human pathogens, because of its intrinsic resistance to a number of commonly used antibiotics and ability to form sessile communities (biofilms). Innovative treatment strategies are required and that can rely on the attenuation of the pathogenicity and virulence traits. The interruption of the mechanisms of intercellular communication in bacteria (quorum sensing) is one of such promising strategies. A cobalt coordination compound (Co(HL)₂) synthesized from (E)-2-(2-(pyridin-2-ylmethylene)hydrazinyl)-4-(p-tolyl)thiazole (HL) is reported herein for the first time to inhibit P. aeruginosa 3-oxo-C12-HSL-dependent QS system (LasI/LasR system) and underling phenotypes (biofilm formation and virulence factors). Its interactions with a possible target, the transcriptional activator protein complex LasR-3-oxo-C12-HSL, was studied by molecular modeling with the coordination compound ligand having stronger predicted interactions than those of co-crystallized ligand 3-oxo-C12-HSL, as well as known-binder furvina. Transition metal group 9 coordination compounds may be explored in antipathogenic/antibacterial drug design.

Biofilms in the Food Industry: Health Aspects and Control Methods

Diverse microorganisms are able to grow on food matrixes and along food industry infrastructures. This growth may give rise to biofilms. This review summarizes, on the one hand, the current knowledge regarding the main bacterial species responsible for initial colonization, maturation and dispersal of food industry biofilms, as well as their associated health issues in dairy products, ready-to-eat foods and other food matrixes. These human pathogens include Bacillus cereus (which secretes toxins that can cause diarrhea and vomiting symptoms), Escherichia coli (which may include enterotoxigenic and even enterohemorrhagic strains), Listeria monocytogenes (a ubiquitous species in soil and water that can lead to abortion in pregnant women and other serious complications in children and the elderly), Salmonella enterica (which, when contaminating a food pipeline biofilm, may induce massive outbreaks and even death in children and elderly), and Staphylococcus aureus (known for its numerous enteric toxins). On the other hand, this review describes the currently available biofilm prevention and disruption methods in food factories, including steel surface modifications (such as nanoparticles with different metal oxides, nanocomposites, antimicrobial polymers, hydrogels or liposomes), cell-signaling inhibition strategies (such as lactic and citric acids), chemical treatments (such as ozone, quaternary ammonium compounds, NaOCl and other sanitizers), enzymatic disruption strategies (such as cellulases, proteases, glycosidases and DNAses), non-thermal plasma treatments, the use of bacteriophages (such as P100), bacteriocins (such us nisin), biosurfactants (such as lichenysin or surfactin) and plant essential oils (such as citral- or carvacrol-containing oils).

Corrigendum: Cross-Talk between Staphylococcus aureus and Other Staphylococcal Species via the agr Quorum Sensing System

[This corrects the article on p. 1733 in vol. 7, PMID: 27877157.].

Strain identification and quorum sensing inhibition characterization of marine-derived Rhizobium sp. NAO1

A novel strategy for combating pathogens is through the ongoing development and use of anti-quorum sensing (QS) treatments such as therapeutic bacteria or their anti-QS substances. Relatively little is known about the bacteria that inhabit the open ocean and of their potential anti-pathogenic attributes; thus, in an initiative to identify these types of therapeutic bacteria, planktonic microbes from the North Atlantic Ocean were collected, isolated, cultured and screened for anti-QS activity. Screening analysis identified one such strain, Rhizobium sp. NAO1. Extracts of Rhizobium sp. NAO1 were identified via ultra-performance liquid chromatography (UPLC) analysis. They were shown to contain N-acyl homoserine lactone (AHL)-based QS analogues (in particular, the N-butyryl homoserine lactone (C4-AHL) analogue) and could disrupt biofilm formation by Pseudomonas aeruginosa PAO1. QS inhibition was confirmed using confocal scanning laser microscopy and growth curves, and it was shown to occur in a dose-dependent manner without affecting bacterial growth. Secondary metabolites of Rhizobium sp. NAO1 inhibited PAO1 pathogenicity by downregulating AHL-mediated virulence factors such as elastase activity and siderophore production. Furthermore, as a result of biofilm structure damage, the secondary metabolite products of Rhizobium sp. NAO1 significantly increased the sensitivity of PAO1 to aminoglycoside antibiotics. Our results demonstrated that Rhizobium sp. strain NAO1 has the ability to disrupt P. aeruginosa PAO1 biofilm architecture, in addition to attenuating P. aeruginosa PAO1 virulence factor production and pathogenicity. Therefore, the newly identified ocean-derived Rhizobium sp. NAO1 has the potential to serve as a QS inhibitor and may be a new microbial resource for drug development.

Furvina inhibits the 3-oxo-C12-HSL-based quorum sensing system of Pseudomonas aeruginosa and QS-dependent phenotypes

Disruption of cell-cell communication or quorum sensing (QS) is considered a stimulating approach for reducing bacterial pathogenicity and resistance. Although several QS inhibitors (QSIs) have been discovered so far their clinical use remains distant. This problem can be circumvented by searching for QSI among drugs already approved for the treatment of different diseases. In this context, antibiotics have earned special attention. Whereas at high concentrations antibiotics exert a killing effect, at lower concentrations they may act as signaling molecules and as such can modulate gene expression. In this study, the antibiotic furvina was shown to be able to cause inhibition of the 3-oxo-C12-HSL-dependent QS system of Pseudomonas aeruginosa. Furvina interacts with the LasI/LasR system. The data were validated by modeling studies. Furvina can also reduce biofilm formation and decrease the production of QS-controlled virulence factors.

Cross-Talk between Staphylococcus aureus and Other Staphylococcal Species via the agr Quorum Sensing System

Staphylococci are associated with both humans and animals. While most are non-pathogenic colonizers, Staphylococcus aureus is an opportunistic pathogen capable of causing severe infections. S. aureus virulence is controlled by the agr quorum sensing system responding to secreted auto-inducing peptides (AIPs) sensed by AgrC, a two component histidine kinase. agr loci are found also in other staphylococcal species and for Staphylococcus epidermidis, the encoded AIP represses expression of agr regulated virulence genes in S. aureus. In this study we aimed to better understand the interaction between staphylococci and S. aureus, and show that this interaction may eventually lead to the identification of new anti-virulence candidates to target S. aureus infections. Here we show that culture supernatants of 37 out of 52 staphylococcal isolates representing 17 different species inhibit S. aureus agr. The dog pathogen, Staphylococcus schleiferi, expressed the most potent inhibitory activity and was active against all four agr classes found in S. aureus. By employing a S. aureus strain encoding a constitutively active AIP receptor we show that the activity is mediated via agr. Subsequent cloning and heterologous expression of the S. schleiferi AIP in S. aureus demonstrated that this molecule was likely responsible for the inhibitory activity, and further proof was provided when pure synthetic S. schleiferi AIP was able to completely abolish agr induction of an S. aureus reporter strain. To assess impact on S. aureus virulence, we co-inoculated S. aureus and S. schleiferi in vivo in the Galleria mellonella wax moth larva, and found that expression of key S. aureus virulence factors was abrogated. Our data show that the S. aureus agr locus is highly responsive to other staphylococcal species suggesting that agr is an inter-species communication system. Based on these results we speculate that interactions between S. aureus and other colonizing staphylococci will significantly influence the ability of S. aureus to cause infection, and we propose that other staphylococci are potential sources of compounds that can be applied as anti-virulence therapy for combating S. aureus infections.

Flavones as Quorum Sensing Inhibitors Identified by a Newly Optimized Screening Platform Using Chromobacterium violaceum as Reporter Bacteria

Quorum sensing (QS) is the process by which bacteria produce and detect signal molecules to coordinate their collective behavior. This intercellular communication is a relevant target for anti-biofilm therapies. Here we have optimized a screening-applicable assay to search for new quorum sensing inhibitors from natural compound libraries. In this system, QS is correlated with the production of violacein, which is directly controlled by the LuxI/LuxR system in Chromobacterium violaceum ATCC 31532. The parallel use of C. violaceum Tn5-mutant CV026, which depends on auto-inducer addition, allows simultaneous discrimination of compounds that act as quenchers of the AHL signal (quorum quenchers). The incorporation of a redox stain into the platform allowed further distinction between QS inhibitors, quorum quenchers and antibacterial compounds. A pilot screening was performed with 465 natural and synthetic flavonoids. All the most active compounds were flavones and they displayed potencies (IC₅₀) in the range of 3.69 to 23.35 μM. These leads were particularly promising as they inhibited the transition from microcolonies into mature biofilms from Escherichia coli and Pseudomonas aeruginosa strains. This approach can be very effective in identifying new antimicrobials posing lesser risks of resistance.

Quorum Sensing Inhibition and Structure-Activity Relationships of β-Keto Esters

Traditional therapeutics to treat bacterial infections have given rise to multi-drug resistant pathogens, which pose a major threat to human and animal health. In several pathogens, quorum sensing (QS)-a cell-cell communication system in bacteria-controls the expression of genes responsible for pathogenesis, thus representing a novel target in the fight against bacterial infections. Based on the structure of the autoinducers responsible for QS activity and other QS inhibitors, we hypothesize that β-keto esters with aryl functionality could possess anti-QS activity. A panel of nineteen β-keto ester analogs was tested for the inhibition of bioluminescence (a QS-controlled phenotype) in the marine pathogen Vibrio harveyi. Initial screening demonstrated the need of a phenyl ring at the C-3 position for antagonistic activity. Further additions to the phenyl ring with 4-substituted halo groups or a 3- or 4-substituted methoxy group resulted in the most active compounds with IC50 values ranging from 23 µM to 53 µM. The compounds additionally inhibit green fluorescent protein production by E. coli JB525. Evidence is presented that aryl β-keto esters may act as antagonists of bacterial quorum sensing by competing with N-acyl homoserine lactones for receptor binding. Expansion of the β-keto ester panel will enable us to obtain more insight into the structure-activity relationships needed to allow for the development of novel anti-virulence agents.

New Perspectives on the Use of Phytochemicals as an Emergent Strategy to Control Bacterial Infections Including Biofilms

The majority of current infectious diseases are almost untreatable by conventional antibiotic therapy given the advent of multidrug-resistant bacteria. The degree of severity and the persistence of infections are worsened when microorganisms form biofilms. Therefore, efforts are being applied to develop new drugs not as vulnerable as the current ones to bacterial resistance mechanisms, and also able to target bacteria in biofilms. Natural products, especially those obtained from plants, have proven to be outstanding compounds with unique properties, making them perfect candidates for these much-needed therapeutics. This review presents the current knowledge on the potentialities of plant products as antibiotic adjuvants to restore the therapeutic activity of drugs. Further, the difficulties associated with the use of the existing antibiotics in the treatment of biofilm-related infections are described. To counteract the biofilm resistance problems, innovative strategies are suggested based on literature data. Among the proposed strategies, the use of phytochemicals to inhibit or eradicate biofilms is highlighted. An overview on the use of phytochemicals to interfere with bacterial quorum sensing (QS) signaling pathways and underlying phenotypes is provided. The use of phytochemicals as chelating agents and efflux pump inhibitors is also reviewed.

Attenuation of quorum sensing-mediated virulence of Acinetobacter baumannii by Glycyrrhiza glabra flavonoids

AIM

To develop an alternative quorum quenching therapy against multidrug-resistant Acinetobacter baumannii.

METHODS & RESULTS

Activity-guided partially purified fraction (F1) from Glycyrrhiza glabra significantly (p < 0.05) reduced quorum sensing regulated virulence factors of A. baumannii viz. motility, biofilm formation and production of antioxidant enzymes. Mechanistically, F1 downregulated the expression of autoinducer synthase gene, abaI, and consequently reduced (92%) the production of 3-OH-C12-HSL as determined by ESI-MS. Q-TOF and Q-TRAP analyses suggested the presence of flavonoids viz. licoricone, glycyrin and glyzarin as the active ingredients.

CONCLUSION

This is the first report on quorum quenching activity of G. glabra linked to its flavonoids that downregulated the expression of abaI and attenuated quorum sensing regulated virulence of A. baumannii.

Quorum sensing inhibition, relevance to periodontics

Quorum sensing helps bacteria to communicate with each other and in coordinating their behavior. Many diseases of human beings, plants, and animals are mediated by quorum sensing. Various approaches are being tried to inhibit this communication to control the diseases caused by bacteria. Periodontal pathogens also communicate through quorum sensing and new approaches to treat periodontal disease using quorum sensing inhibition need to explored.

2-Furaldehyde diethyl acetal from tender coconut water (Cocos nucifera) attenuates biofilm formation and quorum sensing-mediated virulence of Chromobacterium violaceum and Pseudomonas aeruginosa

The aim of this study was to evaluate the anti-biofilm and quorum sensing inhibitory (QSI) potential of tender coconut water (TCW) against Chromobacterium violaceum and Pseudomonas aeruginosa. TCW significantly inhibited the QS regulated violacein, virulence factors and biofilm production without affecting their growth. qRT-PCR analysis revealed the down-regulation of autoinducer synthase, transcriptional regulator and virulence genes. Mass-spectrometric analysis of a petroleum ether extract of the TCW hydrolyte revealed that 2-furaldehyde diethyl acetal (2FDA) and palmitic acid (PA) are the major compounds. In vitro bioassays confirmed the ability of 2FDA to inhibit the biofilm formation and virulence factors. In addition, the combination of PA with 2FDA resulted in potent inhibition of biofilm formation and virulence factors. The results obtained strongly suggest that TCW can be exploited as a base for designing a novel antipathogenic drug formulation to treat biofilm mediated infections caused by P. aeruginosa.

Applications of thin-layer chromatography in extraction and characterisation of ajoene from garlic bulbs

Novel and inexpensive methods of thin-layer chromatography (TLC) were employed for the extraction, characterisation and mechanism of quorum sensing inhibition by ajoene, a component from toluene garlic bulb (Allium sativum L.) extract (TGE). TLC profiling of TGE was carried out using ethyl acetate as solvent. Out of total spots extracted from TLC, four spots exhibited quorum sensing inhibitory (QSI) potential. Among those, spot 5 was identified as Z-ajoene by TLC and confirmed by NMR and MS. HPLC analysis indicated 97.7% purity for purified ajoene. TLC densitometric analysis quantified 221.08 μmol/g of ajoene in TGE and indicated that ajoene is stable at 4°C and at acidic pH. HPTLC profiling showed that ajoene exhibits QSI effect by inhibiting the production of both long-chain acyl homoserine lactones and Pseudomonas quinolone signal (PQS) by P. aeruginosa and also by inactivating PQS molecules.

Phenol, 2,4-bis(1,1-dimethylethyl) of marine bacterial origin inhibits quorum sensing mediated biofilm formation in the uropathogen Serratia marcescens

Intercellular communication in bacteria (quorum sensing, QS) is an important phenomenon in disease dissemination and pathogenesis, which controls biofilm formation also. This study reports the anti-QS and anti-biofilm efficacy of seaweed Gracilaria gracilis associated Vibrio alginolyticus G16 against Serratia marcescens. Purification and mass spectrometric analysis revealed the active principle as phenol, 2,4-bis(1,1-dimethylethyl) [PD]. PD affected the QS regulated virulence factor production in S. marcescens and resulted in a significant (p < 0.05) reduction in biofilm (85%), protease (41.9%), haemolysin (69.9%), lipase (84.3%), prodigiosin (84.5%) and extracellular polysaccharide (84.62%) secretion without hampering growth, as evidenced by XTT [2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide] assay. qPCR analysis confirmed the down-regulation of the fimA, fimC, flhD and bsmA genes involved in biofilm formation. Apart from biofilm inhibition and disruption, PD increased the susceptibility of S. marcescens to gentamicin when administered synergistically, which opens another avenue for combinatorial therapy where PD can be used to enhance the efficacy of conventional antibiotics.

Quorum quenching mediated approaches for control of membrane biofouling

Membrane biofouling is widely acknowledged as the most frequent adverse event in wastewater treatment systems resulting in significant loss of treatment efficiency and economy. Different strategies including physical cleaning and use of antimicrobial chemicals or antibiotics have been tried for reducing membrane biofouling. Such traditional practices are aimed to eradicate biofilms or kill the bacteria involved, but the greater efficacy in membrane performance would be achieved by inhibiting biofouling without interfering with bacterial growth. As a result, the search for environmental friendly non-antibiotic antifouling strategies has received much greater attention among scientific community. The use of quorum quenching natural compounds and enzymes will be a potential approach for control of membrane biofouling. This approach has previously proven useful in diseases and membrane biofouling control by triggering the expression of desired phenotypes. In view of this, the present review is provided to give the updated information on quorum quenching compounds and elucidate the significance of quorum sensing inhibition in control of membrane biofouling.

Inhibition of virulence gene expression in Staphylococcus aureus by novel depsipeptides from a marine photobacterium

During a global research expedition, more than five hundred marine bacterial strains capable of inhibiting the growth of pathogenic bacteria were collected. The purpose of the present study was to determine if these marine bacteria are also a source of compounds that interfere with the agr quorum sensing system that controls virulence gene expression in Staphylococcus aureus. Using a gene reporter fusion bioassay, we recorded agr interference as enhanced expression of spa, encoding Protein A, concomitantly with reduced expression of hla, encoding α-hemolysin, and rnaIII encoding RNAIII, the effector molecule of agr. A marine Photobacterium produced compounds interfering with agr in S. aureus strain 8325-4, and bioassay-guided fractionation of crude extracts led to the isolation of two novel cyclodepsipeptides, designated solonamide A and B. Northern blot analysis confirmed the agr interfering activity of pure solonamides in both S. aureus strain 8325-4 and the highly virulent, community-acquired strain USA300 (CA-MRSA). To our knowledge, this is the first report of inhibitors of the agr system by a marine bacterium.