Intercellular communication in bacteria

Deciphering the crosstalk between inflammation and biofilm in chronic wound healing: Phytocompounds loaded bionanomaterials as therapeutics

In terms of the economics and public health, chronic wounds exert a significant detrimental impact on the health care system. Bacterial infections, which cause the formation of highly resistant biofilms that elude standard antibiotics, are the main cause of chronic, non-healing wounds. Numerous studies have shown that phytochemicals are effective in treating a variety of diseases, and traditional medicinal plants often include important chemical groups such alkaloids, phenolics, tannins, terpenes, steroids, flavonoids, glycosides, and fatty acids. These substances are essential for scavenging free radicals which helps in reducing inflammation, fending off infections, and hastening the healing of wounds. Bacterial species can survive in chronic wound conditions because biofilms employ quorum sensing as a communication technique which regulates the expression of virulence components. Fortunately, several phytochemicals have anti-QS characteristics that efficiently block QS pathways, prevent drug-resistant strains, and reduce biofilm development in chronic wounds. This review emphasizes the potential of phytocompounds as crucial agents for alleviating bacterial infections and promoting wound healing by reducing the inflammation in chronic wounds, exhibiting potential avenues for future therapeutic approaches to mitigate the healthcare burden provided by these challenging conditions.

Engineering microbial consortia with rationally designed cellular interactions

Synthetic microbial consortia represent a frontier of synthetic biology that promises versatile engineering of cellular functions. They are primarily developed through the design and construction of cellular interactions that coordinate individual dynamics and generate collective behaviors. Here we review recent advances in the engineering of synthetic communities through cellular-interaction programming. We first examine fundamental building blocks for intercellular communication and unidirectional positive and negative interactions. We then recap the assembly of the building blocks for creating bidirectional interactions in two-species ecosystems, which is followed by the discussion of engineering toward complex communities with increasing species numbers, under spatial contexts, and via model-guided design. We conclude by summarizing major challenges and future opportunities of engineered microbial ecosystems.

N-acyl Homoserine Lactone Mediated Quorum Sensing Exhibiting Plant Growth-promoting and Abiotic Stress Tolerant Bacteria Demonstrates Drought Stress Amelioration

Multiple plant growth-promoting attributes with N-acyl homoserine lactone (AHL)-mediated quorum sensing exhibiting bacterial strains can help plants to withstand varying abiotic and biotic stress conditions for improving the plant health and productivity. In total, 306 bacterial isolates were isolated from diverse locations and sites. In our exploration, bacterial isolates were screened based on AHL production, plant growth-promoting attributes, abiotic stress tolerance, and antagonistic activity against phytopathogenic fungi. Among the screened 306 isolates, 4 (11VPKHP4, 7VP51.8, P51.10, NBRI N7) were selected based on their efficiency in AHL production, biofilm formation, enduring different abiotic stress conditions, exhibiting plant growth-promoting attributes, and antagonistic activity. Based on 16S rRNA gene sequencing analyses of the selected 4 isolates belong to Pseudomonas genera. Selected isolates 11VPKHP4, 7VP51.8, P51.10, and NBRI N7 were also proficient in biosurfactant production, emulsification, suggesting that all isolates fabricate emulsifiers. The plant growth promotion potential of selected 4 bacterial isolates showed significant growth enhancement in all the vegetative parameters of Zea mays under control as well as drought stress condition. Biochemical parameters and defense enzymes under drought stress conditions were also modulated in the PGPR treated plants as compared to their uninoculated respective controls. With quorum sensing, multiple PGPR attributes, stress tolerance, biofilm formation, and EPS production the selected isolates have the potential to facilitate enhanced plant growth, rhizosphere colonization, maintenance of soil moisture content under normal and diverse stresses.

Effect of Channel Radius on Fluorescent Nanoparticle Based Molecular Communication

The effect of the communication channel size on the transport and subsequent detection of chemical messengers is investigated on millimetric and micrometric channels. The transport of the information carriers, being characterized by an advective and a diffusive contribution, was simulated by varying the flow velocity and the diffusion coefficient. Then, to evaluate the information quality, the Intersymbol Interference (ISI) between two consecutive signals at a specific release delay was estimated. This allowed us to verify that operating under micrometric channel conditions has a larger flow velocity range to obtain completely separated successive signals and smaller release delays can be used between signals. The theoretical results were confirmed by developing a prototype molecular communication platform operating under microfluidic conditions, which enables communication through fluorescent nanoparticles, namely Carbon Quantum Dots (CQDs).

Bioactive small molecules produced by the human gut microbiome modulate Vibrio cholerae sessile and planktonic lifestyles

Humans live in symbiosis with a diverse community of microorganisms, which has evolved to carry out many specific tasks that benefit the host, including protection against invading pathogens. Within the chemical diversity of the gastrointestinal tract, small molecules likely constitute chemical cues for the communication between the microbiota and pathogens. Therefore, we sought to investigate if molecules produced by the human gut microbiota show biological activity against the human pathogen Vibrio cholerae. To probe the effects of the gut metabolome on V. cholerae, we investigated its response to small-molecule extracts from human feces, from a complex bacterial community cultivated in vitro, and from culture supernatants of Enterocloster citroniae, Bacteroides thetaiotaomicron, and Bacteroides vulgatus. Using RNA sequencing, we determined the impact of the human gut metabolome on V. cholerae global gene expression. Among the genes downregulated in the presence of the fecal extract, the most overrepresented functional category was cell motility, which accounted for 39% of repressed genes. Repression of V. cholerae motility by the fecal extract was confirmed phenotypically, and E. citroniae extracts reproduced this phenotype. A complex in vitro microbial community led to increased motility, as did extracts from B. vulgatus, a species present in this community. Accordingly, mucin penetration was also repressed by fecal and E. citroniae extracts, suggesting that the phenotypes observed may have implications for host colonization. Together with previous studies, this work shows that small molecules from the gut metabolome may have a widespread, significant impact on microbe-microbe interactions established in the gut environment.

Functional Integration and Individuality in Prokaryotic Collective Organisations

Both physiological and evolutionary criteria of biological individuality are underpinned by the idea that an individual is a functionally integrated whole. However, a precise account of functional integration has not been provided so far, and current notions are not developed in the details, especially in the case of composite systems. To address this issue, this paper focuses on the organisational dimension of two representative associations of prokaryotes: biofilms and the endosymbiosis between prokaryotes. Some critical voices have been raised against the thesis that biofilms are biological individuals. Nevertheless, it has not been investigated which structural and functional obstacles may prevent them from being fully integrated physiological or evolutionary units. By contrast, the endosymbiotic association of different species of prokaryotes has the potential for achieving a different type of physiological integration based on a common boundary and interlocked functions. This type of association had made it possible, under specific conditions, to evolve endosymbionts into fully integrated organelles. This paper therefore has three aims: first, to analyse the organisational conditions and the physiological mechanisms that enable integration in prokaryotic associations; second, to discuss the organisational differences between biofilms and prokaryotic endosymbiosis and the types of integration they achieve; finally, to provide a more precise account of functional integration based on these case studies.

The role of two-component regulatory systems in environmental sensing and virulence in Salmonella

Adaptation to environments with constant fluctuations imposes challenges that are only overcome with sophisticated strategies that allow bacteria to perceive environmental conditions and develop an appropriate response. The gastrointestinal environment is a complex ecosystem that is home to trillions of microorganisms. Termed microbiota, this microbial ensemble plays important roles in host health and provides colonization resistance against pathogens, although pathogens have evolved strategies to circumvent this barrier. Among the strategies used by bacteria to monitor their environment, one of the most important are the sensing and signalling machineries of two-component systems (TCSs), which play relevant roles in the behaviour of all bacteria. Salmonella enterica is no exception, and here we present our current understanding of how this important human pathogen uses TCSs as an integral part of its lifestyle. We describe important aspects of these systems, such as the stimuli and responses involved, the processes regulated, and their roles in virulence. We also dissect the genomic organization of histidine kinases and response regulators, as well as the input and output domains for each TCS. Lastly, we explore how these systems may be promising targets for the development of antivirulence therapeutics to combat antibiotic-resistant infections.

Plant-Based Phytochemicals as Possible Alternative to Antibiotics in Combating Bacterial Drug Resistance

The unprecedented use of antibiotics that led to development of resistance affect human health worldwide. Prescription of antibiotics imprudently and irrationally in different diseases progressed with the acquisition and as such development of antibiotic resistant microbes that led to the resurgence of pathogenic strains harboring enhanced armors against existing therapeutics. Compromised the treatment regime of a broad range of antibiotics, rise in resistance has threatened human health and increased the treatment cost of diseases. Diverse on metabolic, genetic and physiological fronts, rapid progression of resistant microbes and the lack of a strategic management plan have led researchers to consider plant-derived substances (PDS) as alternative or in complementing antibiotics against the diseases. Considering the quantitative characteristics of plant constituents that attribute health beneficial effects, analytical procedures for their isolation, characterization and phytochemical testing for elucidating ethnopharmacological effects has being worked out for employment in the treatment of different diseases. With an immense potential to combat bacterial infections, PDSs such as polyphenols, alkaloids and tannins, present a great potential for use, either as antimicrobials or as antibiotic resistance modifiers. The present study focuses on the mechanisms by which PDSs help overcome the surge in resistance, approaches for screening different phytochemicals, methods employed in the identification of bioactive components and their testing and strategies that could be adopted for counteracting the lethal consequences of multidrug resistance.

Not Just a Pathogen? Description of a Plant-Beneficial Pseudomonas syringae Strain

Plants develop in a microbe-rich environment and must interact with a plethora of microorganisms, both pathogenic and beneficial. Indeed, such is the case of Pseudomonas, and its model organisms P. fluorescens and P. syringae, a bacterial genus that has received particular attention because of its beneficial effect on plants and its pathogenic strains. The present study aims to compare plant-beneficial and pathogenic strains belonging to the P. syringae species to get new insights into the distinction between the two types of plant–microbe interactions. In assays carried out under greenhouse conditions, P. syringae pv. syringae strain 260-02 was shown to promote plant-growth and to exert biocontrol of P. syringae pv. tomato strain DC3000, against the Botrytis cinerea fungus and the Cymbidium Ringspot Virus. This P. syringae strain also had a distinct volatile emission profile, as well as a different plant-colonization pattern, visualized by confocal microscopy and gfp labeled strains, compared to strain DC3000. Despite the different behavior, the P. syringae strain 260-02 showed great similarity to pathogenic strains at a genomic level. However, genome analyses highlighted a few differences that form the basis for the following hypotheses regarding strain 260-02. P. syringae strain 260-02: (i) possesses non-functional virulence genes, like the mangotoxin-producing operon Mbo; (ii) has different regulation pathways, suggested by the difference in the autoinducer system and the lack of a virulence activator gene; (iii) has genes encoding DNA methylases different from those found in other P. syringae strains, suggested by the presence of horizontal-gene-transfer-obtained methylases that could affect gene expression.

Detection of Bacterial Quorum Sensing Molecules

Bacterial cells use the quorum sensing system to communicate with each other. The gram-negative species very often use N-acyl homoserine lactones for this purpose. One of the easiest ways to detect these molecules is the use of particular reporter strains, which possess different kinds of reporter genes under the control of AHL-responsive promoters. Here we present some of the possibilities available today, even for not specialized researchers.

Detection of bacterial contamination in food matrices by integration of quorum sensing in a paper-strip test

There are an estimated 48 million cases of foodborne illness in the United States every year. In general, these illnesses are the result of unintentional contamination and improper food handling. Because bacterial contamination plays a major role in food spoilage and, hence, in foodborne illnesses, it is important to design easy, portable methods to detect bacteria in food. Quorum sensing (QS) enables bacteria to communicate with one another and by doing so they can modulate their behavior in a cell-density dependent manner. In bacteria, quorum sensing molecules (QSMs) are known to control several factors such as virulence factor production, antibiotic production, biofilm formation, and gene regulation. Herein, we demonstrate the applicability of whole cell biosensing systems for the early identification of food contamination via detection of QSMs. Additionally, we have developed a portable system for detection of bacterial contamination using microdots of immobilized whole cell-based biosensors on paper that boast nanomolar level detection of QSMs in two different food matrices, namely beef and milk. Limits of detection ranged from 1 × 10-7 M to 1 × 10-9 M with relative standard deviations (RSDs) of 1-16%. This rapid, easy, and portable test could be a useful tool for use in the field and during all stages of food manipulation, i.e., from farms to distribution, storage, sales, and preparation prior to consumption, to ensure that food is free of bacterial contamination.

Extraction of Small Molecules from Fecal Samples and Testingof Their Activity on Microbial Physiology

The human body is colonized by vast communities of microbes, collectively known as microbiota, or microbiome. Although microbes colonize every surface of our bodies that is exposed to the external environment, the biggest collection of microbes colonizing humans and other mammals can be found in the gastrointestinal tract. Given the fact that the human gut is colonized by several hundred microbial species, our group hypothesized that the chemical diversity of this environment should be significant, and that many of the molecules present in that environment would have important signaling roles. Therefore, we devised a protocol to extract these molecules from human feces and test their signaling properties. Potentially bioactive extracts can be tested through addition to culture medium and analyses of bacterial growth and gene expression, among other properties. The protocol described herein provides an easy and rapid method for the extraction and testing of metabolites from fecal samples using Salmonella enterica as a model organism. This protocol can also be adapted to the extraction of small molecules from other matrices, such as cultured mammalian cells, tissues, body fluids, and axenic microbial cultures, and the resulting extracts can be tested against various microbial species.

Chitosan nanoencapsulation of flavonoids enhances their quorum sensing and biofilm formation inhibitory activities against an E.coli Top 10 biosensor

Phytochemicals have been found to be promising alternatives to conventional antibiotic therapies for the control of bacterial infections, as they may entail less selective pressure and hence reduce the development of resistance. This study involved examining the inhibition of biofilm formation and of quorum sensing (QS), and the cytotoxicity on mammalian cells of two flavonoids, quercetin and baicalein, in free form and associated into chitosan-based nanocapsules. This was done by use of a transformed E. coli Top 10 biosensor strain, while the cytotoxicity was evaluated on MDCK-C7 cells. In free form, application both flavonoids exhibited slight inhibitory activity on the QS response and biofilm formation, a scenario that was improved positively upon encapsulation with chitosan (Mw ∼115,000 g/mol and DA ∼42%). The association efficiency of 99% (quercetin) and 87% (baicalein) was determined, and each formulation had an average diameter of 190 ± 4 and 187 ± 2 nm, and zeta (ζ) potential of +48.1 ± 2.03 and +48.4 ± 3.46 mV, respectively. Both types of systems were stable against aggregation in M9 and MEM media. The in vitro release kinetics data of both flavonoids seemed to be similar with only ∼20% released over the first 5 h, or ∼10% over the first 4 h, respectively, with subsequent sudden release increase up to ∼40% in both cases. The free phytochemicals seemed to be cytotoxic to MDCK-C7 cells at higher doses, however, upon nanoencapsulation, a cytoprotective effect was evidenced. We have gained proof-of-principle of the advantages of encapsulation of two bioactive flavonoids.

Microbial stress mediated intercellular nanotubes in an anaerobic microbial consortium digesting cellulose

The anaerobic digestion process is a multi - step reaction dependent on concerted activities such as exchange of metabolites among physiologically different microbial communities. This study investigated the impact of iron oxide nanoparticles on the anaerobic sludge microbiota. It was shown there were three distinct microbial phases following addition of the nanoparticles: microbial stress and cell death of approximately one log order of magnitude, followed by microbial rewiring, and recovery. Furthermore, it was noted that cellular stress led to the establishment of intercellular nanotubes within the microbial biomass. Intercellular nanotube - mediated communication among genetically engineered microorganisms and ad hoc assembled co - cultures have been previously reported. This study presents evidence of intercellular nanotube formation within an environmental sample - i.e., anaerobic sludge microbiota subjected to stress. Our observations suggested a mode of microbial communication in the anaerobic digestion process not previously explored and which may have implications on bioreactor design and microbial functions.

Effect of organic acids on biofilm formation and quorum signaling of pathogens from fresh fruits and vegetables

Organic acids are known to be used as food preservatives due to their antimicrobial potential. This study evaluated the ability of three organic acids, namely, acetic acid, citric acid and lactic acid to manage E. coli and Salmonella sp. from fresh fruits and vegetables. Effect of these organic acids on biofilm forming ability and anti-quorum potential was also investigated. The effect of organic acids on inactivation of E. coli and Salmonella sp. on the surface of a selected vegetable (cucumber) was determined. The minimum inhibitory concentration of the organic acids were found to be 1.5, 2 and 0.2% in E. coli while it was observed to be 1, 1.5 and 1% in Salmonella sp. for acetic, citric and lactic acids respectively. Maximum inhibition of biofilm formation was recorded at 39.13% with lactic acid in E. coli and a minimum of 22.53% with citric acid in Salmonella sp. EPS production was affected in E. coli with lactic acid showing reduction by 13.42% while citric acid and acetic acid exhibited only 6.25% and 10.89% respectively. Swimming and swarming patterns in E. coli was notably affected by both acetic and lactic acids. Lactic and acetic acids showed higher anti-quorum sensing (QS) potential when compared to citric acid. 2% lactic acid showed a maximum inhibition of violacein production by 37.7%. Organic acids can therefore be used as potential quorum quenching agents in food industry. 2% lactic acid treatment on cucumber demonstrated that it was effective in inactivating E. coli and Salmonella sp. There was 1 log reduction in microbial count over a period of 6 days after the lactic acid treatment. Thus, organic acids can act as effective potential sanitizers in reducing the microbial load associated with fresh fruits and vegetables.

Microbial interactions during sugar cane must fermentation for bioethanol production: does quorum sensing play a role?

Microbial interactions represent important modulatory role in the dynamics of biological processes. During bioethanol production from sugar cane must, the presence of lactic acid bacteria (LAB) and wild yeasts is inevitable as they originate from the raw material and industrial environment. Increasing the concentration of ethanol, organic acids, and other extracellular metabolites in the fermentation must are revealed as wise strategies for survival by certain microorganisms. Despite this, the co-existence of LAB and yeasts in the fermentation vat and production of compounds such as organic acids and other extracellular metabolites result in reduction in the final yield of the bioethanol production process. In addition to the competition for nutrients, reduction of cellular viability of yeast strain responsible for fermentation, flocculation, biofilm formation, and changes in cell morphology are listed as important factors for reductions in productivity. Although these consequences are scientifically well established, there is still a gap about the physiological and molecular mechanisms governing these interactions. This review aims to discuss the potential occurrence of quorum sensing mechanisms between bacteria (mainly LAB) and yeasts and to highlight how the understanding of such mechanisms can result in very relevant and useful tools to benefit the biofuels industry and other sectors of biotechnology in which bacteria and yeast may co-exist in fermentation processes.

Microbial Biofilms and Chronic Wounds

Background is provided on biofilms, including their formation, tolerance mechanisms, structure, and morphology within the context of chronic wounds. The features of biofilms in chronic wounds are discussed in detail, as is the impact of biofilm on wound chronicity. Difficulties associated with the use of standard susceptibility tests (minimum inhibitory concentrations or MICs) to determine appropriate treatment regimens for, or develop new treatments for use in, chronic wounds are discussed, with alternate test methods specific to biofilms being recommended. Animal models appropriate for evaluating biofilm treatments are also described. Current and potential future therapies for treatment of biofilm-containing chronic wounds, including probiotic therapy, virulence attenuation, biofilm phenotype expression attenuation, immune response suppression, and aggressive debridement combined with antimicrobial dressings, are described.

The Shift of an Intestinal "Microbiome" to a "Pathobiome" Governs the Course and Outcome of Sepsis Following Surgical Injury

Sepsis following surgical injury remains a growing and worrisome problem following both emergent and elective surgery. Although early resuscitation efforts and prompt antibiotic therapy have improved outcomes in the first 24 to 48  h, late onset sepsis is now the most common cause of death in modern intensive care units. This time shift may be, in part, a result of prolonged exposure of the host to the stressors of critical illness which, over time, erode the health promoting intestinal microbiota and allow for virulent pathogens to predominate. Colonizing pathogens can then subvert the immune system and contribute to the deterioration of the host response. Here, we posit that novel approaches integrating the molecular, ecological, and evolutionary dynamics of the evolving gut microbiome/pathobiome during critical illness are needed to understand and prevent the late onset sepsis that develops following prolonged critical illness.

Microbial Communities Are Well Adapted to Disturbances in Energy Input

Although microbial systems are well suited for studying concepts in ecological theory, little is known about how microbial communities respond to long-term periodic perturbations beyond diel oscillations. Taking advantage of an ongoing microcosm experiment, we studied how methanotrophic microbial communities adapted to disturbances in energy input over a 20-day cycle period. Sequencing of bacterial 16S rRNA genes together with quantification of microbial abundance and ecosystem function were used to explore the long-term dynamics (510 days) of methanotrophic communities under continuous versus cyclic chemical energy supply. We observed that microbial communities appeared inherently well adapted to disturbances in energy input and that changes in community structure in both treatments were more dependent on internal dynamics than on external forcing. The results also showed that the rare biosphere was critical to seeding the internal community dynamics, perhaps due to cross-feeding or other strategies. We conclude that in our experimental system, internal feedbacks were more important than external drivers in shaping the community dynamics over time, suggesting that ecosystems can maintain their function despite inherently unstable community dynamics. IMPORTANCE Within the broader ecological context, biological communities are often viewed as stable and as only experiencing succession or replacement when subject to external perturbations, such as changes in food availability or the introduction of exotic species. Our findings indicate that microbial communities can exhibit strong internal dynamics that may be more important in shaping community succession than external drivers. Dynamic "unstable" communities may be important for ecosystem functional stability, with rare organisms playing an important role in community restructuring. Understanding the mechanisms responsible for internal community dynamics will certainly be required for understanding and manipulating microbiomes in both host-associated and natural ecosystems.

Bacterial Cell-Cell Communication in the Host via RRNPP Peptide-Binding Regulators

Human microbiomes are composed of complex and dense bacterial consortia. In these environments, bacteria are able to react quickly to change by coordinating their gene expression at the population level via small signaling molecules. In Gram-positive bacteria, cell–cell communication is mostly mediated by peptides that are released into the extracellular environment. Cell–cell communication based on these peptides is especially widespread in the group Firmicutes, in which they regulate a wide array of biological processes, including functions related to host–microbe interactions. Among the different agents of communication, the RRNPP family of cytoplasmic transcriptional regulators, together with their cognate re-internalized signaling peptides, represents a group of emerging importance. RRNPP members that have been studied so far are found mainly in species of bacilli, streptococci, and enterococci. These bacteria are characterized as both human commensal and pathogenic, and share different niches in the human body with other microorganisms. The goal of this mini-review is to present the current state of research on the biological relevance of RRNPP mechanisms in the context of the host, highlighting their specific roles in commensalism or virulence.

Potential Emergence of Multi-quorum Sensing Inhibitor Resistant (MQSIR) Bacteria

Expression of certain bacterial genes only at a high bacterial cell density is termed as quorum-sensing (QS). Here bacteria use signaling molecules to communicate among themselves. QS mediated genes are generally involved in the expression of phenotypes such as bioluminescence, biofilm formation, competence, nodulation, and virulence. QS systems (QSS) vary from a single in Vibrio spp. to multiple in Pseudomonas and Sinorhizobium species. The complexity of QSS is further enhanced by the multiplicity of signals: (1) peptides, (2) acyl-homoserine lactones, (3) diketopiperazines. To counteract this pathogenic behaviour, a wide range of bioactive molecules acting as QS inhibitors (QSIs) have been elucidated. Unlike antibiotics, QSIs don't kill bacteria and act at much lower concentration than those of antibiotics. Bacterial ability to evolve resistance against multiple drugs has cautioned researchers to develop QSIs which may not generate undue pressure on bacteria to develop resistance against them. In this paper, we have discussed the implications of the diversity and multiplicity of QSS, in acting as an arsenal to withstand attack from QSIs and may use these as reservoirs to develop multi-QSI resistance.

VarR controls colonization and virulence in the marine macroalgal pathogen Nautella italica R11

There is increasing evidence to suggest that macroalgae (seaweeds) are susceptible to infectious disease. However, to date, little is known about the mechanisms that facilitate the colonization and virulence of microbial seaweed pathogens. One well-described example of a seaweed disease is the bleaching of the red alga Delisea pulchra, which can be caused by the bacterium Nautella italica R11, a member of the Roseobacter clade. This pathogen contains a unique luxR-type gene, varR, which we hypothesize controls its colonization and virulence. We show here that a varR knock-out strain is deficient in its ability to cause disease in D. pulchra and is defective in biofilm formation and attachment to a common algal polysaccharide. Moreover complementation of the varR gene in trans can restore these functions to the wild type levels. Proteomic analysis of bacterial cells in planktonic and biofilm growth highlight the potential importance of nitrogen scavenging, mobilization of energy reserves, and stress resistance in the biofilm lifestyle of N. italica R11. Moreover, we show that VarR regulates the expression of a specific subset of biofilm-associated proteins. Taken together these data suggest that VarR controls colonization and persistence of N. italica R11 on the surface of a macroalgal host and that it is an important regulator of virulence.

Novel linear polymers able to inhibit bacterial quorum sensing

Bacterial phenotypes, such as biofilm formation, antibiotic resistance and virulence expression, are associated with quorum sensing. Quorum sensing is a density-dependent regulatory system of gene expression controlled by specific signal molecules, such as N-acyl homoserine lactones (AHLs), produced and released by bacteria. This study reports the development of linear polymers capable to attenuate quorum sensing by adsorption of AHLs. Linear polymers were synthesized using MMA as backbone monomer and methacrylic acid and itaconic acid as functional monomers. Two different quorum sensing-controlled phenotypes, Vibrio fischeri bioluminescence and Aeromonas hydrophila biofilm formation, were evaluated to test the polymers' efficiency. Results showed that both phenotypes were significantly affected by the polymers, with the itaconic acid-containing material being more effective than the methacrylic acid one. The polymer inhibitory effects were reverted by the addition of lactones, confirming attenuation of quorum sensing through sequestration of signal molecules. The polymers also showed no cytotoxicity when tested using a mammalian cell line.

RovS and its associated signaling peptide form a cell-to-cell communication system required for Streptococcus agalactiae pathogenesis

Bacteria can communicate with each other to coordinate their biological functions at the population level. In a previous study, we described a cell-to-cell communication system in streptococci that involves a transcriptional regulator belonging to the Rgg family and short hydrophobic peptides (SHPs) that act as signaling molecules. Streptococcus agalactiae, an opportunistic pathogenic bacterium responsible for fatal infections in neonates and immunocompromised adults, has one copy of the shp/rgg locus. The SHP-associated Rgg is called RovS in S. agalactiae. In this study, we found that the SHP/RovS cell-to-cell communication system is active in the strain NEM316 of S. agalactiae, and we identified different partners that are involved in this system, such as the Eep peptidase, the PptAB, and the OppA1-F oligopeptide transporters. We also identified a new target gene controlled by this system and reexamined the regulation of a previously proposed target gene, fbsA, in the context of the SHP-associated RovS system. Furthermore, our results are the first to indicate the SHP/RovS system specificity to host liver and spleen using a murine model, which demonstrates its implication in streptococci virulence. Finally, we observed that SHP/RovS regulation influences S. agalactiae's ability to adhere to and invade HepG2 hepatic cells. Hence, the SHP/RovS cell-to-cell communication system appears to be an essential mechanism that regulates pathogenicity in S. agalactiae and represents an attractive target for the development of new therapeutic strategies.

IMPORTANCE

Rgg regulators and their cognate pheromones, called small hydrophobic peptides (SHPs), are present in nearly all streptococcal species. The general pathways of the cell-to-cell communication system in which Rgg and SHP take part are well understood. However, many other players remain unidentified, and the direct targets of the system, as well as its link to virulence, remain unclear. Here, we identified the different players involved in the SHP/Rgg system in S. agalactiae, which is the leading agent of severe infections in human newborns. We have identified a direct target of the Rgg regulator in S. agalactiae (called RovS) and examined a previously proposed target, all in the context of associated SHP. For the first time, we have also demonstrated the implication of the SHP/RovS mechanism in virulence, as well as its host organ specificity. Thus, this cell-to-cell communication system may represent a future target for S. agalactiae disease treatment.

Combating pathogenic microorganisms using plant-derived antimicrobials: a minireview of the mechanistic basis

The emergence of antibiotic resistance in pathogenic bacteria has led to renewed interest in exploring the potential of plant-derived antimicrobials (PDAs) as an alternative therapeutic strategy to combat microbial infections. Historically, plant extracts have been used as a safe, effective, and natural remedy for ailments and diseases in traditional medicine. Extensive research in the last two decades has identified a plethora of PDAs with a wide spectrum of activity against a variety of fungal and bacterial pathogens causing infections in humans and animals. Active components of many plant extracts have been characterized and are commercially available; however, research delineating the mechanistic basis of their antimicrobial action is scanty. This review highlights the potential of various plant-derived compounds to control pathogenic bacteria, especially the diverse effects exerted by plant compounds on various virulence factors that are critical for pathogenicity inside the host. In addition, the potential effect of PDAs on gut microbiota is discussed.

Antivirulence activity of the human gut metabolome

The mammalian gut contains a complex assembly of commensal microbes termed microbiota. Although much has been learned about the role of these microbes in health, the mechanisms underlying these functions are ill defined. We have recently shown that the mammalian gut contains thousands of small molecules, most of which are currently unidentified. Therefore, we hypothesized that these molecules function as chemical cues used by hosts and microbes during their interactions in health and disease. Thus, a search was initiated to identify molecules produced by the microbiota that are sensed by pathogens. We found that a secreted molecule produced by clostridia acts as a strong repressor of Salmonella virulence, obliterating expression of the Salmonella pathogenicity island 1 as well as host cell invasion. It has been known for decades that the microbiota protects its hosts from invading pathogens, and these data suggest that chemical sensing may be involved in this phenomenon. Further investigations should reveal the exact biological role of this molecule as well as its therapeutic potential.

Microbes can communicate through the production and sensing of small molecules. Within the complex ecosystem formed by commensal microbes living in and on the human body, it is likely that these molecular messages are used extensively during the interactions between different microbial species as well as with host cells. Deciphering such a molecular dialect will be fundamental to our understanding of host-microbe interactions in health and disease and may prove useful for the design of new therapeutic strategies that target these mechanisms of communication.

AHL-priming functions via oxylipin and salicylic acid

Collaborative action between the host plant and associated bacteria is crucial for the establishment of an efficient interaction. In bacteria, the synchronized behavior of a population is often achieved by a density-dependent communication called quorum sensing. This behavior is based on signaling molecules, which influence bacterial gene expression. N-acyl homoserine lactones (AHLs) are such molecules in many Gram-negative bacteria. Moreover, some AHLs are responsible for the beneficial effect of bacteria on plants, for example the long chain N-3-oxo-tetradecanoyl-L-homoserine lactone (oxo-C14-HSL) can prime Arabidopsis and barley plants for an enhanced defense. This AHL-induced resistance phenomenon, named AHL-priming, was observed in several independent laboratories during the last two decades. Very recently, the mechanism of priming with oxo-C14-HSL was shown to depend on an oxylipin and salicylic acid (SA). SA is a key element in plant defense, it accumulates during different plant resistance responses and is the base of systemic acquired resistance. In addition, SA itself can prime plants for an enhanced resistance against pathogen attack. On the other side, oxylipins, including jasmonic acid (JA) and related metabolites, are lipid-derived signaling compounds. Especially the oxidized fatty acid derivative cis-OPDA, which is the precursor of JA, is a newly described player in plant defense. Unlike the antagonistic effect of SA and JA in plant-microbe interactions, the recently described pathway functions through a synergistic effect of oxylipins and SA, and is independent of the JA signaling cascade. Interestingly, the oxo-C14-HSL-induced oxylipin/SA signaling pathway induces stomata defense responses and cell wall strengthening thus prevents pathogen invasion. In this review, we summarize the findings on AHL-priming and the related signaling cascade. In addition, we discuss the potential of AHL-induced resistance in new strategies of plant protection.

Bacteria clustering by polymers induces the expression of quorum-sensing-controlled phenotypes

Bacteria deploy a range of chemistries to regulate their behaviour and respond to their environment. Quorum sensing is one method by which bacteria use chemical reactions to modulate pre-infection behaviour such as surface attachment. Polymers that can interfere with bacterial adhesion or the chemical reactions used for quorum sensing are therefore a potential means to control bacterial population responses. Here, we report how polymeric 'bacteria sequestrants', designed to bind to bacteria through electrostatic interactions and therefore inhibit bacterial adhesion to surfaces, induce the expression of quorum-sensing-controlled phenotypes as a consequence of cell clustering. A combination of polymer and analytical chemistry, biological assays and computational modelling has been used to characterize the feedback between bacteria clustering and quorum sensing signalling. We have also derived design principles and chemical strategies for controlling bacterial behaviour at the population level.

Maternal and environmental effects on symbiont-mediated antimicrobial defense

Bacteria produce a remarkable diversity of bioactive molecules with antimicrobial properties. Despite the importance of such compounds for human medicine, little is known about the factors influencing antibiotic production in natural environments. Recently, several insects have been found to benefit from symbiont-produced antimicrobial compounds for defense against pathogenic microbes. In the European beewolf, Philanthus triangulum (Hymenoptera, Crabronidae), bacteria of the genus Streptomyces provide protection against pathogens by producing antimicrobials on the larval cocoon during hibernation, thereby significantly enhancing the survival probability of the beewolf larva. To investigate the effects of abiotic and biotic factors on antibiotic production, we exposed beewolf cocoons to different environmental conditions and quantified the amount of Streptomyces-produced antibiotics by using gas chromatography/mass spectrometry (GC/MS). The results revealed no significant influence of temperature, humidity, or pathogen load on the antibiotic amount, indicating that antibiotic production is not affected by current environmental conditions but rather may be optimized to serve as a reliable long-term protection during the unpredictable phase of beewolf hibernation. However, the amount of antibiotics was positively correlated with the symbiont population size on the cocoon, which in turn is affected by the number of Streptomyces cells provided by the mother into the brood cell. Additionally, we found a positive correlation between the amount of hydrocarbons and the number and length of bacterial cells in the antennal gland secretion, suggesting that maternal investment affects symbiont growth and, thus, antibiotic production on the larval cocoon.

Structural basis for phosphorylated autoinducer-2 modulation of the oligomerization state of the global transcription regulator LsrR from Escherichia coli

Quorum-sensing systems are widely used by bacteria to control behavior in response to fluctuations in cell density. Several small diffusible molecules called autoinducers act as signaling molecules in quorum-sensing processes through interplay with sensors. Autoinducers modulate vital physiological functions such as nutrient acquisition, gene transcription, and virulence factor production. In Escherichia coli, LsrR serves as a global transcription regulator that responds to autoinducer-2 to regulate the expression of a variety of genes, including the lsr operon and the lsrR gene. Here, we report the crystal structure of full-length LsrR from E. coli, which has an N-terminal DNA-binding domain and a C-terminal ligand-binding domain connected by a β-strand. Although only two molecules are found in one asymmetric unit, two neighboring dimers pack to form a tetramer that is consistent with the oligomerization state of LsrR in solution. Mutagenesis experiments and gel shift assays indicated that Gln-33 and Tyr-26 might be involved in interactions between LsrR and DNA. The LsrR-binding site for phosphorylated autoinducer-2 was predicted by structural comparisons of LsrR with CggR and SorC. Cross-linking, size exclusion chromatography, and gel shift assays determined that phosphorylated autoinducer-2 triggered the disassembly of the LsrR tetramer into dimers and reduced the DNA binding ability of LsrR. Our findings reveal a mechanism for the change in the oligomerization state of LsrR in the presence of phosphorylated autoinducer-2. Based on these observations, we propose that phosphorylated autoinducer-2 triggers the disassembly of the LsrR tetramer to activate the transcription of its target genes.

Synthesis and biological evaluation of triazole-containing N-acyl homoserine lactones as quorum sensing modulators

Many bacterial species are capable of assessing their local population densities through a cell-cell signaling mechanism termed quorum sensing (QS). This intercellular communication process is mediated by small molecule or peptide ligands and their cognate protein receptors. Numerous pathogens use QS to initiate virulence once they achieve a threshold cell number on a host. Consequently, approaches to intercept QS have attracted considerable attention as potential anti-infective therapies. Our interest in the development of small molecule tools to modulate QS pathways motivated us to evaluate triazole-containing analogs of natural N-acyl L-homoserine lactone (AHL) signals as non-native QS agonists and antagonists in Gram-negative bacteria. We synthesized 72 triazole derivatives of five broad structure types in high yields and purities using efficient Cu(I)-catalyzed azide-alkyne couplings. These compounds were evaluated for their ability to activate or inhibit two QS receptors from two prevalent pathogens - LasR from Pseudomonas aeruginosa and AbaR from Acinetobacter baumannii- using bacterial reporter strains. Several triazole derivatives were identified that were capable of strongly modulating the activity of LasR and AbaR. These compounds represent a new and synthetically accessible class of AHL analogs, and could find utility as chemical tools to study QS and its role in bacterial virulence.

AidC, a novel N-acylhomoserine lactonase from the potato root-associated cytophaga-flavobacteria-bacteroides (CFB) group bacterium Chryseobacterium sp. strain StRB126

N-acylhomoserine lactones (AHLs) are used as quorum-sensing (QS) signal molecules by many gram-negative bacteria. We have reported that Chryseobacterium sp. strain StRB126, which was isolated from the root surface of potato, has AHL-degrading activity. In this study, we cloned and characterized the aidC gene from the genomic library of StRB126. AidC has AHL-degrading activity and shows homology to several metallo-β-lactamase proteins from Bacteroidetes, although not to any known AHL-degrading enzymes. Purified AidC, as a maltose-binding fusion protein, showed high degrading activity against all tested AHLs, whether short- or long-chain forms, with or without substitution at carbon 3. High-performance liquid chromatography (HPLC) analysis revealed that AidC functions as an AHL lactonase catalyzing AHL ring opening by hydrolyzing lactones. An assay to determine the effects of covalent and ionic bonding showed that Zn(2+) is important to AidC activity both in vitro and in vivo. In addition, the aidC gene could also be PCR amplified from several other Chryseobacterium strains. In conclusion, this study indicated that the aidC gene, encoding a novel AHL lactonase, may be widespread throughout the genus Chryseobacterium. Our results extend the diversity and known bacterial hosts of AHL-degrading enzymes.

Fatty acid-mediated signalling between two Pseudomonas species

We report the identification of fatty acids as mediators of intercellular signalling in Pseudomonas putida, and between Pseudomonas aeruginosa and P. putida. Tetradecanoic acid and fatty acids of similar chain length are present in supernatants of these strains and activate population density-dependent expression of ddcA, a gene involved in corn seed and root colonization by P. putida KT2440. Consistently, significant amounts of these compounds were also found in corn root exudates. The signalling pathway involves the two-component regulatory system formed by RoxS and RoxR, which had been previously shown to control expression of ddcA and of a set of genes related to the redox balance of P. putida cells. Production of the fatty acid signal in P. aeruginosa is under the control of the LasI/LasR and RhlI/RhlR quorum sensing systems. Our data indicate that in terms of cell-cell communication, P. putida KT2440 employs mechanisms closer to those of plant pathogens such as Xanthomonas spp. and fungi like Candida, which also rely on fatty acid derivatives.

Enhancing the utility of existing antibiotics by targeting bacterial behaviour?

The discovery of novel classes of antibiotics has slowed dramatically. This has occurred during a time when the appearance of resistant strains of bacteria has shown a substantial increase. Concern is therefore mounting over our ability to continue to treat infections in an effective manner using the antibiotics that are currently available. While ongoing efforts to discover new antibiotics are important, these must be coupled with strategies that aim to maintain as far as possible the spectrum of activity of existing antibiotics. In many instances, the resistance to antibiotics exhibited by bacteria in chronic infections is mediated not by direct resistance mechanisms, but by the adoption of modes of growth that confer reduced susceptibility. These include the formation of biofilms and the occurrence of subpopulations of 'persister' cells. As our understanding of these processes has increased, a number of new potential drug targets have been revealed. Here, advances in our ability to disrupt these systems that confer reduced susceptibility, and in turn increase the efficacy of antibiotic therapy, are discussed.

LitR of Vibrio salmonicida is a salinity-sensitive quorum-sensing regulator of phenotypes involved in host interactions and virulence

Vibrio (Aliivibrio) salmonicida is the causal agent of cold-water vibriosis, a fatal bacterial septicemia primarily of farmed salmonid fish. The molecular mechanisms of invasion, colonization, and growth of V. salmonicida in the host are still largely unknown, and few virulence factors have been identified. Quorum sensing (QS) is a cell-to-cell communication system known to regulate virulence and other activities in several bacterial species. The genome of V. salmonicida LFI1238 encodes products presumably involved in several QS systems. In this study, the gene encoding LitR, a homolog of the master regulator of QS in V. fischeri, was deleted. Compared to the parental strain, the litR mutant showed increased motility, adhesion, cell-to-cell aggregation, and biofilm formation. Furthermore, the litR mutant produced less cryptic bioluminescence, whereas production of acylhomoserine lactones was unaffected. Our results also indicate a salinity-sensitive regulation of LitR. Finally, reduced mortality was observed in Atlantic salmon infected with the litR mutant, implying that the fish were more susceptible to infection with the wild type than with the mutant strain. We hypothesize that LitR inhibits biofilm formation and favors planktonic growth, with the latter being more adapted for pathogenesis in the fish host.

Engineered cells as biosensing systems in biomedical analysis

Over the past two decades there have been great advances in biotechnology, including use of nucleic acids, proteins, and whole cells to develop a variety of molecular analytical tools for diagnostic, screening, and pharmaceutical applications. Through manipulation of bacterial plasmids and genomes, bacterial whole-cell sensing systems have been engineered that can serve as novel methods for analyte detection and characterization, and as more efficient and cost-effective alternatives to traditional analytical techniques. Bacterial cell-based sensing systems are typically sensitive, specific and selective, rapid, easy to use, low-cost, and amenable to multiplexing, high-throughput, and miniaturization for incorporation into portable devices. This critical review is intended to provide an overview of available bacterial whole-cell sensing systems for assessment of a variety of clinically relevant analytes. Specifically, we examine whole-cell sensing systems for detection of bacterial quorum sensing molecules, organic and inorganic toxic compounds, and drugs, and for screening of antibacterial compounds for identification of their mechanisms of action. Methods used in the design and development of whole-cell sensing systems are also reviewed.

Multi-species biofilms: living with friendly neighbors

Our knowledge regarding the nature and development of microbial biofilms has grown significantly since the first report of these communities by Antonie van Leeuwenhoek in the late 1600s. Nevertheless, most biofilm studies examine mono-species cultures, whereas nearly all biofilm communities in nature comprise a variety of microorganisms. The species that constitute a mixed biofilm and the interactions between these microorganisms critically influence the development and shape of the community. In this review, we focus on interactions occurring within a multi-species biofilm and their effects on the nature of the mixed community. In general, interspecies interactions involve communication, typically via quorum sensing, and metabolic cooperation or competition. Interactions among species within a biofilm can be antagonistic, such as competition over nutrients and growth inhibition, or synergistic. The latter can result in the development of several beneficial phenotypes. These include the promotion of biofilm formation by co-aggregation, metabolic cooperation where one species utilizes a metabolite produced by a neighboring species, and increased resistance to antibiotics or host immune responses compared to the mono-species biofilms. These beneficial interactions in mixed biofilms have important environmental, industrial, and clinical implications. The latter, for example, impacts the course and treatment of biofilm-related infections, such as those manifested in the lungs of cystic fibrosis patients.

Functional characterization of the quorum sensing regulator RsaL in the plant-beneficial strain Pseudomonas putida WCS358

In many bacteria, quorum sensing (QS) systems rely on a signal receptor and a synthase producing N-acyl-homoserine lactone(s) as the signal molecule(s). In some species, the rsaL gene, located between the signal receptor and synthase genes, encodes a repressor limiting signal synthase expression and hence signal molecule production. Here we investigate the molecular mechanism of action of the RsaL protein in the plant growth-promoting rhizobacterium Pseudomonas putida WCS358 (RsaL(WCS)). In P. putida WCS358, RsaL(WCS) displayed a strong repressive effect on the promoter of the QS signal synthase gene, ppuI, while it did not repress the same promoter in Pseudomonas aeruginosa. DNase I protection assays showed that purified RsaL(WCS) specifically binds to ppuI on a DNA region overlapping the predicted σ(70)-binding site, but such protection was observed only at high protein concentrations. Accordingly, electrophoretic mobility shift assays showed that the RsaL(WCS) protein was not able to form stable complexes efficiently with a probe encompassing the ppuI promoter, while it formed stable complexes with the promoter of lasI, the gene orthologous to ppuI in P. aeruginosa. This difference seems to be dictated by the lower dyad symmetry of the RsaL(WCS)-binding sequence on the ppuI promoter relative to that on the lasI promoter. Comparison of the results obtained in vivo and in vitro suggests that RsaL(WCS) needs a molecular interactor/cofactor specific for P. putida WCS358 to repress ppuI transcription. We also demonstrate that RsaL(WCS) regulates siderophore-mediated growth limitation of plant pathogens and biofilm formation, two processes relevant for plant growth-promoting activity.

XerR, a negative regulator of XccR in Xanthomonas campestris pv. campestris, relieves its repressor function in planta

We previously reported that XccR, a LuxR-type regulator of Xanthomonas campestris pv. campestris (Xcc), activates the downstream proline iminopeptidase virulence gene (pip) in response to certain host plant factor(s). In this report, we further show that the expression of the xccR gene was repressed in the culture medium by an NtrC-type response regulator, which we named XerR (XccR expression-related, repressor), and that this repression was relieved when the bacteria were grown in planta. Such a regulatory mechanism is reinforced by the observations that XerR directly bound to the xccR promoter in vitro, and that mutations at the phosphorylation-related residues of XerR resulted in the loss of its repressor function. Furthermore, the expression level of xccR increased even in XerR-overexpressing Xcc cells when they were vacuum infiltrated into cabbage plants. We also preliminarily characterized the host factor(s) involved in the above mentioned interactions between Xcc and the host plant, showing that a plant material(s) with molecular weight(s) less than 1 kDa abolished the binding of XerR to the xccR promoter, while the same material enhanced the binding of XccR to the luxXc box in the pip promoter. Taken together, our results implicate XerR in a new layer of the regulatory mechanism controlling the expression of the virulence-related xccR/pip locus and provide clues to the identification of plant signal molecules that interact with XerR and XccR to enhance the virulence of Xcc.

Malaria and trypanosome transmission: different parasites, same rules?

African trypanosomes produce different specialized stages for within-host replication and between-host transmission and therefore face a resource allocation trade-off between maintaining the current infection (survival) and investment into transmission (reproduction). Evolutionary theory predicts the resolution of this trade-off will significantly affect virulence and infectiousness. The application of life history theory to malaria parasites has provided novel insight into their strategies for survival and reproduction; how this framework can now be applied to trypanosomes is discussed. Specifically, predictions for how parasites trade-off investment in survival and transmission in response to variation in the within-host environment are outlined. An evolutionary approach has the power to explain why patterns of investment vary between strains and during infections, giving important insights into parasite biology.

Chemical signaling in the gastrointestinal tract

Chemical signaling via the production of small molecules such as hormones has been studied in detail in higher organisms. These molecules have important functions in maintaining physiological homeostasis as well as allowing organisms to respond to external insults. Virtually every living cell produces hormone-like diffusible small molecules that can be used to convey messages to neighboring cells-a vital step in adaptation, development, and survival within populations. Although most of our knowledge on cellular chemical communication comes from studies of multicellular eukaryotes, it is now understood that bacteria can also communicate using sophisticated signaling systems, in a way analogous to those used by higher organisms. Many of these microbes live in close association with higher eukaryotes, in mutualistic or commensal relationships. We suggest that there may be a wealth of unidentified bioactive small molecules in the human body, originating from both microbial and human cells and that have important biological functions. Because chemical signaling has important roles for the biology of both microbes and humans, detecting, identifying, and studying these chemical signals can further our understanding of the chemical interplay between microbiota and their hosts and provide us with an unexplored source of molecules that could be used for human benefit.

PpoR, an orphan LuxR-family protein of Pseudomonas putida KT2440, modulates competitive fitness and surface motility independently of N-acylhomoserine lactones

Pseudomonas putida KT2440 does not produce any of the common molecules involved in quorum sensing signalling described in other bacteria. However, as is the case in other microorganisms, the genome of this strain contains an open reading frame (PP_4647) coding for a transcriptional regulator belonging to the LuxR protein family. In this article, we present evidence indicating that this protein, named PpoR, modulates swarming motility in KT2440 and plays a role in the survival of this strain in the presence of potential competitors. These functions appear to be independent of known N-acylhomoserine lactones (AHLs), since we show that P. putida KT2440 does not produce significant quantities of these molecules under any condition tested and PpoR does not influence the expression of quorum sensing-dependent promoters even in the presence of exogenous AHLs. A ppoR mutant shows increased sensitivity to the iron chelator 2,2'-dipyridyl, while iron supplementation compensates the fitness loss of the mutant in competition with other Pseudomonas. All these data suggest that PpoR participates in both inter- and intraspecific processes relevant to the fitness of P. putida related to iron acquisition, and not necessarily mediated by canonical quorum sensing signal molecules.

Quenching the quorum sensing system: potential antibacterial drug targets

Emergence of antibiotic and multi-drug resistant pathogenic bacteria has created the need for new drugs and drug targets. During pathogenesis bacteria release signals which regulate virulence and pathogenicity related genes. Such bacteria co-ordinate their virulent behaviour in a cell density dependent phenomenon termed as quorum sensing (QS). In contrast, microbes interfere with QS system by quenching the signals, termed quorum quenching (QQ). As a consequence of disrupted QS, pathogens become susceptible to antibiotics and drugs. In this article, the biodiversity of organisms with potential to quench QS signals and the use of QQ molecules as antibacterial drugs have been reviewed.