Citrus limonoids interfere with Vibrio harveyi cell-cell signalling and biofilm formation by modulating the response regulator LuxO

Essential oils of Citrus limon, Cymbopogon citratus, and Lavandula officinalis disrupt E. coli biofilms by inducing cellular damage

Biofilms by E. coli is not only the primary cause of recurrent urinary tract infections, but also lead to medical device-associated infections. Ever increasing antibiotic resistance due to biofilms has sparked a search for plant-based replacements. This study examines anti-biofilm potential of essential oils from lemon (Citrus limon), lemongrass (Cymbopogon citratus), and lavender (Lavandula officinalis) against 26 clinical isolates of E. coli. Essential oils (EOs) were extracted by using hydro-distillation and characterized by GC-MS and FT-IR. Biofilms were quantitatively and qualitatively evaluated. The extracted essential oils had yields ranging from 1.37 to 1.45 (w/v). The main constituents were limonene (94%) in lemon, citral (42%) and ketones (9%) in lemongrass, and linalool (35%) and linalyl anthranilate (20%) in lavender EO. MICs and MBCs of EOs were in the range of 3.13-50 µg/ml. The oils under investigation exhibited notable biofilm inhibition and eradication properties. The percent inhibition and eradication varied between 6-89% and 3-82.25% (p < 0.0001), respectively. These findings suggest that essential oils from lemon, lemongrass, and lavender may offer a promising natural approach to combat E. coli biofilms and associated infections. Further research is warranted to elucidate the underlying mechanisms of action and to explore their clinical applications.

Advancing Antibiotic-Resistant Microbe Combat: Nanocarrier-Based Systems in Combination Therapy Targeting Quorum Sensing

The increase in antibiotic-resistant bacteria presents a significant risk to worldwide public health, emphasizing the necessity of novel approaches to address infections. Quorum sensing, an essential method of communication among bacteria, controls activities like the formation of biofilms, the production of virulence factors, and the synthesis of secondary metabolites according to the number of individuals in the population. Quorum quenching, which interferes with these processes, emerges as a vital approach to diminish bacterial virulence and prevent biofilm formation. Nanocarriers, characterized by their small size, high surface-area-to-volume ratio, and modifiable surface chemistry, offer a versatile platform for the disruption of bacterial communication by targeting various stages within the quorum sensing pathway. These features allow nanocarriers to infiltrate biofilms, disrupt cell membranes, and inhibit bacterial proliferation, presenting a promising alternative to traditional antibiotics. Integrating nanocarrier-based systems into combination therapies provides a multi-pronged approach to infection control, enhancing both the efficacy and specificity of treatment regimens. Nonetheless, challenges related to the stability, safety, and clinical effectiveness of nanomaterial-based antimicrobial treatments remain. Continued research and development are essential to overcoming these obstacles and fully harnessing the potential of nano-antimicrobial therapies. This review emphasizes the importance of quorum sensing in bacterial behavior and highlights the transformative potential of nanotechnology in advancing antimicrobial treatments, offering innovative solutions to combat antibiotic-resistant pathogens.

Quorum Quenching Approaches against Bacterial-Biofilm-Induced Antibiotic Resistance

With the widespread phenomenon of antibiotic resistance and the diffusion of multiple drug-resistant bacterial strains, enormous efforts are being conducted to identify suitable alternative agents against pathogenic microorganisms. Since an association between biofilm formation and antibiotic resistance phenotype has been observed, a promising strategy pursued in recent years focuses on controlling and preventing this formation by targeting and inhibiting the Quorum Sensing (QS) system, whose central role in biofilm has been extensively demonstrated. Therefore, the research and development of Quorum Quenching (QQ) compounds, which inhibit QS, has gradually attracted the attention of researchers and has become a new strategy for controlling harmful microorganisms. Among these, a number of both natural and synthetic compounds have been progressively identified as able to interrupt the intercellular communication within a microbial community and the adhesion to a surface, thus disintegrating mature/preformed biofilms. This review describes the role played by QS in the formation of bacterial biofilms and then focuses on the mechanisms of different natural and synthetic QS inhibitors (QSIs) exhibiting promising antibiofilm ability against Gram-positive and Gram-negative bacterial pathogens and on their applications as biocontrol strategies in various fields.

Biofouling in Membrane Bioreactors-Mitigation and Current Status: a Review

Biological fouling as termed biofouling is caused by varied living organisms and is difficult to eliminate from the environment thus becoming a major issue during membrane bioreactors. Biofouling in membrane bioreactors (MBRs) is a crucial problem in increasing liquid pressure due to reduced pore diameter, clogging of the membrane pores, and alteration of the chemical composition of the water which greatly limits the growth of MBRs. Thus, membrane biofouling and/or microbial biofilms is a hot research topic to improve the market competitiveness of the MBR technology. Though several antibiofouling strategies (addition of bioflocculant or sponge into MBRs) came to light, biological approaches are sustainable and more practicable. Among the biological approaches, quorum sensing-based biofouling control (so-called quorum quenching) is an interesting and promising tool in combating biofouling issues in the MBRs. Several review articles have been published in the area of membrane biofouling and mitigation approaches. However, there is no single source of information about biofouling and/or biofilm formation in different environmental settings and respective problems, antibiofilm strategies and current status, quorum quenching, and its futurity. Thus, the objectives of the present review were to provide latest insights on mechanism of membrane biofouling, quorum sensing molecules, biofilm-associated problems in different environmental setting and antibiofilm strategies, special emphasis on quorum quenching, and its futurity in the biofilm/biofouling control. We believe that these insights greatly help in the better understanding of biofouling and aid in the development of sustainable antibiofouling strategies.

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

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

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

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

Novel approaches for the treatment of infections due to multidrug-resistant bacterial pathogens

Antimicrobial resistance (AMR), which is a major challenge for global healthcare, emerging because of several reasons including overpopulation, increased global migration and selection pressure due to enhanced use of antibiotics. Antibiotics are the widely used therapeutic options to combat infectious diseases; however, unfortunately, inadequate and irregular antibiotic courses are also major contributing factors in the emergence of AMR. Additionally, persistent failure to develop and commercialize new antibiotics has created the scarcity of effective anti-infective drugs. Thus, there is an urgent need for a new class of antimicrobials and other novel approaches to curb the menace of AMR. Besides the conventional approaches, some novel approaches such as the use of antimicrobial peptides, bacteriophages, immunomodulation, host-directed therapy and antibodies have shown really promising potentials.

Recent Strategies to Combat Biofilms Using Antimicrobial Agents and Therapeutic Approaches

Biofilms are intricate bacterial assemblages that attach to diverse surfaces using an extracellular polymeric substance that protects them from the host immune system and conventional antibiotics. Biofilms cause chronic infections that result in millions of deaths around the world every year. Since the antibiotic tolerance mechanism in biofilm is different than that of the planktonic cells due to its multicellular structure, the currently available antibiotics are inadequate to treat biofilm-associated infections which have led to an immense need to find newer treatment options. Over the years, various novel antibiofilm compounds able to fight biofilms have been discovered. In this review, we have focused on the recent and intensively researched therapeutic techniques and antibiofilm agents used for biofilm treatment and grouped them according to their type and mode of action. We also discuss some therapeutic approaches that have the potential for future advancement.

Inhibition of polymicrobial biofilm formation by saw palmetto oil, lauric acid and myristic acid

Biofilms are communities of bacteria, fungi or yeasts that form on diverse biotic or abiotic surfaces, and play important roles in pathogenesis and drug resistance. A generic saw palmetto oil inhibited biofilm formation by Staphylococcus aureus, Escherichia coli O157:H7 and fungal Candida albicans without affecting their planktonic cell growth. Two main components of the oil, lauric acid and myristic acid, are responsible for this antibiofilm activity. Their antibiofilm activities were observed in dual-species biofilms as well as three-species biofilms of S. aureus, E. coli O157:H7 and C. albicans. Transcriptomic analysis showed that lauric acid and myristic acid repressed the expressions of haemolysin genes (hla and hld) in S. aureus, several biofilm-related genes (csgAB, fimH and flhD) in E. coli and hypha cell wall gene HWP1 in C. albicans, which supported biofilm inhibition. Also, saw palmetto oil, lauric acid and myristic acid reduced virulence of three microbes in a nematode infection model and exhibited minimal cytotoxicity. Furthermore, combinatorial treatment of fatty acids and antibiotics showed synergistic antibacterial efficacy against S. aureus and E. coli O157:H7. These results demonstrate that saw palmetto oil and its main fatty acids might be useful for controlling bacterial infections as well as multispecies biofilms.

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.

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.

The Chemistry of Antibiofilm Phytocompounds

Phytocompounds are long known for their therapeutic uses due to their competence as antimicrobial agents. The antimicrobial activity of these bioactive compounds manifests their ability as an antibiofilm agent and is thereby proved to be competent to treat the widespread biofilm-associated chronic infections. The rapid development of antibiotic resistance in bacteria has made the treatment of these infections almost impossible by conventional antibiotic therapy, which forced a switch-over to the use of phytocompounds. The present overview deals with the classification of a huge array of phytocompounds according to their chemical nature, detection of their target pathogen, and elucidation of their mode of action.

Exploring the Antivirulence Activity of Pulverulentone A, a Phloroglucinol-Derivative from Callistemon citrinus Leaf Extract, against Multi-Drug Resistant Pseudomonas aeruginosa

(1) Background: Bacterial resistance to antibiotics is a global life-threatening issue. Antivirulence therapy is a promising approach to combat bacterial infections as it disarms the bacteria from their virulence factors with reduced selective pressure and a lower chance of resistance. (2) Methods: Callistemon citrinus leaf extract and its major constituent, Pulverulentone A, were tested for their ability to inhibit biofilm, exopolysaccharides, pyocyanin and proteases produced by MDR P. aeruginosa. In addition, a Galleria mellonella larvae model was employed to evaluate the in vivo cytotoxicity of Pulverulentone A and its ability to combat Pseudomonas infection. Docking study was further performed to investigate Pulverulentone A druggability against main quorum sensing (QS) targets expressed by P. aeruginosa; (3) Results: Both C. citrinus extract and the isolated compound could inhibit biofilm formation, extracellular polymeric substances (EPS) and pigment production by the tested isolates. Unexpectedly, no significant inhibition was observed on proteases production. The in silico docking analysis revealed good interactions of Pulverulentone A with all QS targets examined (LasR, MyfR/PqsR, QscR). Pulverulentone A was safe up to 400 µg·mL⁻¹ in Galleria caterpillars. Moreover, pre-treatment of P. aeruginosa with Pulverulentone A slightly enhanced the survival of the infected larvae. (4) Conclusions: The present study proves Pulverulentone A safety with significant in vitro and in silico antivirulence potential against P. aeruginosa.

Natural Anti-biofilm Agents: Strategies to Control Biofilm-Forming Pathogens

Pathogenic microorganisms and their chronic pathogenicity are significant concerns in biomedical research. Biofilm-linked persistent infections are not easy to treat due to resident multidrug-resistant microbes. Low efficiency of various treatments and in vivo toxicity of available antibiotics drive the researchers toward the discovery of many effective natural anti-biofilm agents. Natural extracts and natural product-based anti-biofilm agents are more efficient than the chemically synthesized counterparts with lesser side effects. The present review primarily focuses on various natural anti-biofilm agents, i.e., phytochemicals, biosurfactants, antimicrobial peptides, and microbial enzymes along with their sources, mechanism of action via interfering in the quorum-sensing pathways, disruption of extracellular polymeric substance, adhesion mechanism, and their inhibitory concentrations existing in literature so far. This study provides a better understanding that a particular natural anti-biofilm molecule exhibits a different mode of actions and biofilm inhibitory activity against more than one pathogenic species. This information can be exploited further to improve the therapeutic strategy by a combination of more than one natural anti-biofilm compounds from diverse sources.

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.

Current and future perspectives for controlling Vibrio biofilms in the seafood industry: a comprehensive review

The contamination of seafood with Vibrio species can have severe repercussions in the seafood industry. Vibrio species can form mature biofilms and persist on the surface of several seafoods such as crabs, oysters, mussels, and shrimp, for extended duration. Several conventional approaches have been employed to inhibit the growth of planktonic cells and prevent the formation of Vibrio biofilms. Since Vibrio biofilms are mostly resistant to these control measures, novel alternative methods need to be urgently developed. In this review, we propose environmentally friendly approaches to suppress Vibrio biofilm formation using a hypothesized mechanism of action.

In-vivo biotransformation of citrus functional components and their effects on health

Citrus, one of the most popular fruits worldwide, contains various functional components, including flavonoids, dietary fibers (DFs), essential oils (EOs), synephrines, limonoids, and carotenoids. The functional components of citrus attract special attention due to their health-promoting effects. Food components undergo complex biotransformation by host itself and the gut microbiota after oral intake, which alters their bioaccessibility, bioavailability, and bioactivity in the host body. To better understand the health effects of citrus fruits, it is important to understand the in-vivo biotransformation of citrus functional components. We reviewed the biotransformation of citrus functional components (flavonoids, DFs, EOs, synephrines, limonoids, and carotenoids) in the body from their intake to excretion. In addition, we described the importance of biotransformation in terms of health effects. This review would facilitate mechanistic understanding of the health-promoting effect of citrus and its functional components, and also provide guidance for the development of health-promoting foods based on citrus and its functional components.

Purification and Evaluation of N-benzyl Cinnamamide from Red Seaweed Gracilaria fisheri as an Inhibitor of Vibrio harveyi AI-2 Quorum Sensing

Previously, we reported that the ethanol extract from red seaweed Gracilaria fisheri effectively decreased biofilm formation of Vibrio harveyi. In this study, the anti-biofilm active compounds in the ethanol extract were isolated and their structures identified. The anti-biofilm fractionation assay for minimum inhibitory concentration (MIC) produced two fractions which possessed maximal inhibitory activities toward the biofilm formation of V. harveyi strains 1114 and BAA 1116. Following chromatographic separation of the bioactive fractions, two pure compounds were isolated, and their structures were elucidated using FTIR, NMR, and HR-TOF-MS. The compounds were N-benzyl cinnamamide and α-resorcylic acid. The in vitro activity assay demonstrated that both compounds inhibited the biofilm formation of V. harveyi and possessed the anti-quorum sensing activity by interfering with the bioluminescence of the bacteria. However, the N-benzyl cinnamamide was more potent than α-resorcylic acid with a 10-fold lesser MIC. The present study reveals the beneficial property of the N-benzyl cinnamamide from the ethanol extract as a lead anti-microbial drug against V. harveyi.

Developing natural products as potential anti-biofilm agents

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

Quorum sensing inhibitors as antipathogens: biotechnological applications

The mechanisms through which microbes communicate using signal molecules has inspired a great deal of research. Microbes use this exchange of information, known as quorum sensing (QS), to initiate and perpetuate infectious diseases in eukaryotic organisms, evading the eukaryotic defense system by multiplying and expressing their pathogenicity through QS regulation. The major issue to arise from such networks is increased bacterial resistance to antibiotics, resulting from QS-dependent mediation of the formation of biofilm, the induction of efflux pumps, and the production of antibiotics. QS inhibitors (QSIs) of diverse origins have been shown to act as potential antipathogens. In this review, we focus on the use of QSIs to counter diseases in humans as well as plants and animals of economic importance. We also discuss the challenges encountered in the potential applications of QSIs.

Oenothera rosea L´Hér. ex Ait attenuates acute colonic inflammation in TNBS-induced colitis model in rats: in vivo and in silico myeloperoxidase role

Oenothera rosea L´Hér. ex Ait is a species traditionally used in the treatment of inflammation, headache, stomach pain, infections, among others. The aim of this study was evaluating the acute anti-inflammatory activity of the aqueous extract of O. rosea by 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis. Rats were randomized into six groups: (I) Sham; (II) EtOH; (III) TNBS; and (IV-VI) 250, 500 and 750 mg/Kg, respectively. The colonic injury was induced (groups III-VI) by intrarectal instillation of 0.25 mL of TNBS (10 mg) in 50% ethanol. Groups I and II received an enema (0.25 mL) of physiological saline solution or 50% ethanol, respectively. Treatments were administered by oral gavage 48, 24 and 1 h prior, and 24 h after the induction. The inflammatory response was assessed considering the macroscopic and microscopic damage, the serum nitric oxide (NO), the colonic IL-1β levels, and the myeloperoxidase (MPO) activity. Moreover, we performed an LC-MS-based metabolite profiling, and a docking on the MPO. Doses of 500 and 750 mg/Kg showed a protective effect in the TNBS-induced colonic damage. This activity was related to the downregulation of evaluated parameters. Also, considering previous reports, 29 metabolites of 91 detected were selected for the docking, of which Isolimonic acid (29) and Kaempferol 3-(2'',4''-diacetylrhamnoside) (10) showed the highest affinity to MPO. The aqueous extract of O. rosea protected the TNBS-induced colonic damage in rats, an effect that could be associated with the presence of polyphenolic compounds, alkaloids, and terpenes; as well as their ability to down-regulate MPO activity.

Biological Approaches for Disease Control in Aquaculture: Advantages, Limitations and Challenges

Although aquaculture activity has experienced a great development over the past three decades, infectious diseases have become a limiting factor for further intensification. Because the use of antibiotics has led to the widespread emergence of antibiotic resistance, the search for alternative environmentally friendly approaches is urgently needed. This Opinion paper offers an update on the successes and challenges of biological approaches for bacterial disease prevention and control in aquaculture. Although most of these approaches are still in research and development stages, some of them have shown promising results in field trials. Therefore, a better understanding of the mechanisms of action of these approaches will help to maximise their beneficial properties.

Anti-Quorum Sensing Natural Compounds

Increasing extent of pathogenic resistance to drugs has encouraged the seeking for new anti-virulence drugs. Many pharmacological and pharmacognostical researches are performed to identify new drugs or discover new structures for the development of novel therapeutic agents in the antibiotic treatments. Although many phytochemicals show prominent antimicrobial activity, their power lies in their anti-virulence properties. Quorum sensing (QS) is a bacterial intercellular communication mechanism, which depends on bacterial cell population density and controls the pathogenesis of many organisms by regulating gene expression, including virulence determinants. QS has become an attractive target for the development of novel anti-infective agents that do not rely on the use of antibiotics. Anti-QS compounds are known to have the ability to prohibit bacterial pathogenicity. Medicinal plants offer an attractive repertoire of phytochemicals with novel microbial disease-controlling potential, due to the spectrum of secondary metabolites present in extracts, which include phenolics, quinones, flavonoids, alkaloids, terpenoids, and polyacetylenes. They have recently received considerable attention as a new source of safe and effective QS inhibitory substances. The objective of this review is to give a brief account of the research reports on the plants and natural compounds with anti-QS potential.

Nature to the natural rescue: Silencing microbial chats

Communication is the sole means by which effective networking and co-existence is accomplished amongst living beings. Microbes have their own chit-chats. Science has overheard these microbial gossips and have concluded that these aren't just informal communications, but carefully coordinated signals that plan their effective strategies. Tracking one such signal molecule, N-acyl homoserine lactone (AHL), led to a fundamental understanding to microbial quorum sensing (QS). Furtherance of research sought for ways to cut off communication between these virulent forms, so as to hinder their combinatorial attacks through quorum sensing inhibitors (QSIs). A clear understanding of the inhibitors of these microbial communication systems is vital to destroy their networking and co-working. The current review, consolidates the solutions for QSIs offered from natural sources against these micro components, that are capable of slaughtering even nature's most fit entity-man. The applications of effective out sourcing of this QSI technologies and the need for development are discussed. The importance of silencing this microbial chatter to various aspects of human life and their implications are discussed and elaborated.

Chitosan/Cyclodextrin/TPP Nanoparticles Loaded with Quercetin as Novel Bacterial Quorum Sensing Inhibitors

The widespread emergence of antibiotic-resistant bacteria has highlighted the urgent need of alternative therapeutic approaches for human and animal health. Targeting virulence factors that are controlled by bacterial quorum sensing (QS), seems a promising approach. The aims of this study were to generate novel nanoparticles (NPs) composed of chitosan (CS), sulfo-butyl-ether-β-cyclodextrin (Captisol®) and/or pentasodium tripolyphosphate using ionotropic gelation technique, and to evaluate their potential capacity to arrest QS in bacteria. The resulting NPs were in the size range of 250-400 nm with CS70/5 and 330-600 nm with CS70/20, had low polydispersity index (<0.25) and highly positive zeta potential ranging from ζ ~+31 to +40 mV. Quercetin, a hydrophobic model flavonoid, could be incorporated proportionally with increasing amounts of Captisol® in the NPs formualtion, without altering significantly its physicochemical properties. Elemental analysis and FTIR studies revealed that Captisol® and quercetin were effectively integrated into the NPs. These NPs were stable in M9 bacterial medium for 7 h at 37 °C. Further, NPs containing Captisol® seem to prolong the release of associated drug. Bioassays against an E. coli Top 10 QS biosensor revealed that CS70/5 NPs could inhibit QS up to 61.12%, while CS70/20 NPs exhibited high antibacterial effects up to 88.32%. These results suggested that the interaction between NPs and the bacterial membrane could enhance either anti-QS or anti-bacterial activities.

Quorum-Sensing Systems as Targets for Antivirulence Therapy

The development of novel therapies to control diseases caused by antibiotic-resistant pathogens is one of the major challenges we are currently facing. Many important plant, animal, and human pathogens regulate virulence by quorum sensing, bacterial cell-to-cell communication with small signal molecules. Consequently, a significant research effort is being undertaken to identify and use quorum-sensing-interfering agents in order to control diseases caused by these pathogens. In this review, an overview of our current knowledge of quorum-sensing systems of Gram-negative model pathogens is presented as well as the link with virulence of these pathogens, and recent advances and challenges in the development of quorum-sensing-interfering therapies are discussed.

Beyond the bulk: disclosing the life of single microbial cells

Microbial single cell analysis has led to discoveries that are beyond what can be resolved with population-based studies. It provides a pristine view of the mechanisms that organize cellular physiology, unbiased by population heterogeneity or uncontrollable environmental impacts. A holistic description of cellular functions at the single cell level requires analytical concepts beyond the miniaturization of existing technologies, defined but uncontrolled by the biological system itself. This review provides an overview of the latest advances in single cell technologies and demonstrates their potential. Opportunities and limitations of single cell microbiology are discussed using selected application-related examples.

In vitro and in vivo exploration of palmitic acid from Synechococcus elongatus as an antibiofilm agent on the survival of Artemia franciscana against virulent vibrios

Biofilm formation of Vibrio spp. has been demonstrated as a potentially important mechanism contributing antibiotic treatment failure in aquaculture. In the present study, the effect of palmitic acid (PA) identified from Synechococcus elongatus was assessed for the inhibition of quorum sensing (QS) regulated biofilm formation in aquatic bacterial pathogens. The biofilm inhibitory concentration (BIC) of PA against Vibrio spp. was found to be 100µgml^-1. In this concentration, PA exhibited a significant inhibition in biofilm biomass of Vibrio harveyi MTCC 3438, V. parahaemolyticus ATCC 17802, V. vulnificus MTCC 1145 and V. alginolyticus ATCC 17749 without hindering their planktonic growth. Also, PA displayed gradual decrease in bioluminescence production of V. harveyi. The results of extracellular polymeric substances quantification, microbial adhesion to hydrocarbons and Fourier transform infrared spectroscopic (FT-IR) analyses suggested that PA positively interferes with the initial adhesion stages of biofilm formation. In addition, confocal and scanning electron microscopic analysis substantiates the antibiofilm efficacy of the PA. The transcriptomic analysis revealed the down-regulation of QS mediated response regulator genes expression in V. harveyi. Concomitantly, PA reduced the intestinal colonization of vibrios in brine shrimp larvae and thereby attenuates the biofilm assemblage and its associated virulence. In vivo studies using brine shrimp larvae manifested the reduction in adherence and virulence, which prompts further investigation about the potential of PA for the treatment of vibriosis.

In vitro antibiofilm efficacy of Piper betle against quorum sensing mediated biofilm formation of luminescent Vibrio harveyi

Vibrio harveyi is a potent biofilm former, which confers resistance to multiple antimicrobials, disinfectants, chemicals and biocides. The prevalence of biofilm mediated antibiotic resistance among aquatic bacterial pathogens stresses the search for novel alternative approach to treat vibriosis in aquaculture. Exploring suitable therapeutics from natural resources could be a novel area of research. Therefore, this work was executed to evaluate the inhibitory effect of Piper betle ethyl acetate extract (PBE) on bioluminescence production and biofilm formation of V. harveyi. Minimal inhibitory concentration (MIC) of PBE against planktonic V. harveyi was found to be 1600 μg ml^-1; furthermore, PBE inhibited the quorum sensing (QS) mediated bioluminescence production and biofilm formation in V. harveyi upto 98 and 74% respectively, at its sub-MIC concentration of 400 μg ml^-1 without affecting their cell viability. Similar results were obtained for exopolysaccharides production and swimming motility related to biofilm formation of V. harveyi, where PBE reduced EPS production upto 64%. Light and confocal laser scanning microscopic analyses further confirmed that the PBE effectively prevented the initial attachment as well as microcolonies formation of V. harveyi biofilm, when compared to their untreated controls. This study demonstrates the promising antibiofilm activity of PBE and confirms the ethnopharmacological potential of this plant against V. harveyi infections.

Er:YAG Laser Irradiation Induces Behavioral Changes in V. harveyi

OBJECTIVE

Laser technologies have many different applications in medical, agricultural, and industrial fields. Studies have shown several effects of laser energy on different bacterial species, in a wide variety of settings. Recent reports have found that one of the unique features of bacteria is their ability to communicate among themselves (quorum sensing). We sought to investigate whether low-energy laser irradiation affects bacterial behavior, which is regulated by quorum sensing.

METHODS

Laser irradiations were performed using Er:YAG laser (2940 nm wavelength) at output powers of 0.5, 1.5, 2.5, and 4 W on wild-type Vibrio harveyi. Bioluminescence, motility, and biofilm forming capability were assessed on the bacteria after irradiation.

RESULTS

After irradiation of bacteria, positive dose/output power dependencies were found in the bioluminescence omitted from tested experimental groups. Motility of colonies on semi-solid media was inhibited as irradiation output power was increased. However, after irradiation, biomass analysis of biofilm samples showed negligible differences between the irradiated samples and controls.

CONCLUSIONS

Results indicate the impact of low-energy laser irradiation on bacterial behavior such as quorum sensing and motility, without affecting bacterial growth patterns.

The Chemistry and Pharmacology of Citrus Limonoids

Citrus limonoids (CLs) are a group of highly oxygenated terpenoid secondary metabolites found mostly in the seeds, fruits and peel tissues of citrus fruits such as lemons, limes, oranges, pumellos, grapefruits, bergamots, and mandarins. Represented by limonin, the aglycones and glycosides of CLs have shown to display numerous pharmacological activities including anticancer, antimicrobial, antioxidant, antidiabetic and insecticidal among others. In this review, the chemistry and pharmacology of CLs are systematically scrutinised through the use of medicinal chemistry tools and structure-activity relationship approach. Synthetic derivatives and other structurally-related limonoids from other sources are include in the analysis. With the focus on literature in the past decade, the chemical classification of CLs, their physico-chemical properties as drugs, their biosynthesis and enzymatic modifications, possible ways of enhancing their biological activities through structural modifications, their ligand efficiency metrics and systematic graphical radar plot analysis to assess their developability as drugs are among those discussed in detail.

Highly Effective Inhibition of Biofilm Formation by the First Metagenome-Derived AI-2 Quenching Enzyme

Bacterial cell-cell communication (quorum sensing, QS) represents a fundamental process crucial for biofilm formation, pathogenicity, and virulence allowing coordinated, concerted actions of bacteria depending on their cell density. With the widespread appearance of antibiotic-resistance of biofilms, there is an increasing need for novel strategies to control harmful biofilms. One attractive and most likely effective approach is to target bacterial communication systems for novel drug design in biotechnological and medical applications. In this study, metagenomic large-insert libraries were constructed and screened for QS interfering activities (quorum quenching, QQ) using recently established reporter strains. Overall, 142 out of 46,400 metagenomic clones were identified to interfere with acyl-homoserine lactones (AHLs), 13 with autoinducer-2 (AI-2). Five cosmid clones with highest simultaneous interfering activities were further analyzed and the respective open reading frames conferring QQ activities identified. Those showed homologies to bacterial oxidoreductases, proteases, amidases and aminotransferases. Evaluating the ability of the respective purified QQ-proteins to prevent biofilm formation of several model systems demonstrated highest inhibitory effects of QQ-2 using the crystal violet biofilm assay. This was confirmed by heterologous expression of the respective QQ proteins in Klebsiella oxytoca M5a1 and monitoring biofilm formation in a continuous flow cell system. Moreover, QQ-2 chemically immobilized to the glass surface of the flow cell effectively inhibited biofilm formation of K. oxytoca as well as clinical K. pneumoniae isolates derived from patients with urinary tract infections. Indications were obtained by molecular and biochemical characterizations that QQ-2 represents an oxidoreductase most likely reducing the signaling molecules AHL and AI-2 to QS-inactive hydroxy-derivatives. Overall, we propose that the identified novel QQ-2 protein efficiently inhibits AI-2 modulated biofilm formation by modifying the signal molecule; and thus appears particularly attractive for medical and biotechnological applications.

The talking language in some major Gram-negative bacteria

Cell-cell interaction or quorum sensing (QS) is a vital biochemical/physiological process in bacteria that is required for various physiological functions, including nutrient uptake, competence development, biofilm formation, sporulation, as well as for toxin secretion. In natural environment, bacteria live in close association with other bacteria and interaction among them is crucial for survival. The QS-regulated gene expression in bacteria is a cell density-dependent process and the initiation process depends on the threshold level of the signaling molecule, N-acyl-homoserine lactone (AHL). The present review summarizes the QS signal and its respective circuit in Gram-negative bacteria. Most of the human pathogens belong to Gram-negative group, and only a few of them cause disease through QS system. Thus, inhibition of pathogenic bacteria is important. Use of antibiotics creates a selective pressure (antibiotics act as natural selection factor to promote one group of bacteria over another group) for emerging multidrug-resistant bacteria and will not be suitable for long-term use. The alternative process of inhibition of QS in bacteria using different natural and synthetic molecules is called quorum quenching. However, in the long run, QS inhibitors or blockers may also develop resistance, but obviously it will solve some sort of problems. In this review, we also have stated the mode of action of quorum-quenching molecule. The understanding of QS network in pathogenic Gram-negative bacteria will help us to solve many health-related problems in future.

Agents that inhibit bacterial biofilm formation

In the biofilm form, bacteria are more resistant to various antimicrobial treatments. Bacteria in a biofilm can also survive harsh conditions and withstand the host's immune system. Therefore, there is a need for new treatment options to treat biofilm-associated infections. Currently, research is focused on the development of antibiofilm agents that are nontoxic, as it is believed that such molecules will not lead to future drug resistance. In this review, we discuss recent discoveries of antibiofilm agents and different approaches to inhibit/disperse biofilms. These new antibiofilm agents, which contain moieties such as imidazole, phenols, indole, triazole, sulfide, furanone, bromopyrrole, peptides, etc. have the potential to disperse bacterial biofilms in vivo and could positively impact human medicine in the future.

Mini Review of Phytochemicals and Plant Taxa with Activity as Microbial Biofilm and Quorum Sensing Inhibitors

Microbial biofilms readily form on many surfaces in nature including plant surfaces. In order to coordinate the formation of these biofilms, microorganisms use a cell-to-cell communication system called quorum sensing (QS). As formation of biofilms on vascular plants may not be advantageous to the hosts, plants have developed inhibitors to interfere with these processes. In this mini review, research papers published on plant-derived molecules that have microbial biofilm or quorum sensing inhibition are reviewed with the objectives of determining the biosynthetic classes of active compounds, their biological activity in assays, and their families of occurrence and range. The main findings are the identification of plant phenolics, including benzoates, phenyl propanoids, stilbenes, flavonoids, gallotannins, proanthocyanidins and coumarins as important inhibitors with both activities. Some terpenes including monoterpenes, sesquiterpenes, diterpenes and triterpenes also have anti-QS and anti-biofilm activities. Relatively few alkaloids were reported. Quinones and organosulfur compounds, especially from garlic, were also active. A common feature is the polar nature of these compounds. Phytochemicals with these activities are widespread in Angiosperms in temperate and tropical regions, but gymnosperms, bryophytes and pteridophytes were not represented.

Mini Review of Phytochemicals and Plant Taxa with Activity as Microbial Biofilm and Quorum Sensing Inhibitors

Microbial biofilms readily form on many surfaces in nature including plant surfaces. In order to coordinate the formation of these biofilms, microorganisms use a cell-to-cell communication system called quorum sensing (QS). As formation of biofilms on vascular plants may not be advantageous to the hosts, plants have developed inhibitors to interfere with these processes. In this mini review, research papers published on plant-derived molecules that have microbial biofilm or quorum sensing inhibition are reviewed with the objectives of determining the biosynthetic classes of active compounds, their biological activity in assays, and their families of occurrence and range. The main findings are the identification of plant phenolics, including benzoates, phenyl propanoids, stilbenes, flavonoids, gallotannins, proanthocyanidins and coumarins as important inhibitors with both activities. Some terpenes including monoterpenes, sesquiterpenes, diterpenes and triterpenes also have anti-QS and anti-biofilm activities. Relatively few alkaloids were reported. Quinones and organosulfur compounds, especially from garlic, were also active. A common feature is the polar nature of these compounds. Phytochemicals with these activities are widespread in Angiosperms in temperate and tropical regions, but gymnosperms, bryophytes and pteridophytes were not represented.

Inhibition of quorum sensing, biofilm, and spoilage potential in Shewanella baltica by green tea polyphenols

We investigated the quorum sensing (QS) system of Shewanella baltica and the anti-QS related activities of green tea polyphenols (TP) against spoilage bacteria in refrigerated large yellow croaker. Autoinducer-2 (AI-2) and the diketopiperazines (DKPs) cyclo-(L-Pro-L-Leu) and cyclo-(L-Pro-L-Phe) were detected in the culture extract of S. baltica XH2, however, no N-acylhomoserine lactones (AHLs) activity was observed. Green TP at sub-inhibitory concentrations interfered with AI-2 and DKPs activities of S. baltica without inhibiting cell growth and promoted degradation of AI-2. The green TP treatment inhibited biofilm development, exopolysaccharide production and swimming motility of S. baltica in a concentration- dependent manner. In addition, green TP decreased extracellular protease activities and trimethylamine production in S. baltica. A transcriptional analysis showed that green TP repressed the luxS and torA genes in S. baltica, which agreed with the observed reductions in QS activity and the spoilage phenotype. Epigallocatechin gallate (EGCG)-enriched in green TP significantly inhibited AI-2 activity of S. baltica. These findings strongly suggest that green TP could be developed as a new QS inhibitor for seafood preservation to enhance shelf life.

Polymethoxyflavones Isolated from the Peel of Miaray Mandarin (Citrus miaray) Have Biofilm Inhibitory Activity in Vibrio harveyi

Citrus fruits are a good source of bioactive compounds with numerous beneficial biological activities. In the present study, fruits of the unexplored Miaray mandarin were used for the isolation of 10 bioactive compounds. Dried peels were sequentially extracted with hexane and chloroform in a Soxhlet-type apparatus for 8 h. The extracts were concentrated under vacuum and separated by flash chromatography to obtain nine polymethoxyflavones and a limonoid. The purity of each compound was analyzed by high-performance liquid chromatography (HPLC), and the compounds were identified by spectral analysis using MALDI-TOF-MS and NMR. The isolated compounds were identified as 5-hydroxy-3,7,3',4'-tetramethoxyflavone, 5,6,7,8,4'-pentamethoxyflavone (tangeretin), 3,5,6,7,8,3',4'-heptamethoxyflavone, 5,6,7,8,3',4'-hexamethoxyflavone (nobiletin), 3,5,7,8,3',4'-hexamethoxyflavone, 3,5,7,3',4'-pentamethoxyflavone (pentamethylquercetin), 5,7,4'-trimethoxyflavone, 5,7,8,4'-tetramethoxyflavone, 5,7,8,3',4'-pentamethoxyflavone, and limonin. These compounds were further tested for their ability to inhibit cell-cell signaling and biofilm formation in Vibrio harveyi. Among the evaluated polymethoxyflavones, 3,5,6,7,8,3',4'-heptamethoxyflavone and 3,5,7,8,3',4'-hexamethoxyflavone inhibited autoinducer-mediated cell-cell signaling and biofilm formation. These results suggest that Miaray mandarin fruits are a good source of polymethoxyflavones. This is the first report on the isolation of bioactive compounds from Miaray mandarin and evaluation of their biofilm inhibitory activity as well as isolation of pentamethylquercetin from the Citrus genus.

Attenuation of quorum sensing-mediated virulence in Gram-negative pathogenic bacteria: implications for the post-antibiotic era

Quorum quenching compounds blocked quorum sensing system of bacteria by several mechanisms (a, b, c and d).

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.