Immune quorum sensing dictates IFN-I response dynamics in human plasmacytoid dendritic cells.

Type I interferons (IFN-Is) are key in fighting viral infections, but also serve major roles beyond antiviral immunity. Crucial is the tight regulation of IFN-I responses, while excessive levels are harmful to the cells. In essence, immune responses are generated by single cells making their own decisions, which are based on the signals they perceive. Additionally, immune cells must anticipate the future state of their environment, thereby weighing the costs and benefits of each possible outcome, in the presence of other potentially competitive decision makers (i.e., IFN-I producing cells). A rather new cellular communication mechanism called quorum sensing describes the effect of cell density on cellular secretory behaviors, which fits well with matching the right amount of IFN-Is produced to fight an infection. More competitive decision makers must contribute relatively less and vice versa. Intrigued by this concept, we assessed the effects of immune quorum sensing in pDCs, specialized immune cells known for their ability to mass produce IFN-Is. Using conventional microwell assays and droplet-based microfluidics assays, we were able the characterize the effect of quorum sensing in human primary immune cells in vitro. These insights open new avenues to manipulate IFN-I response dynamics in pathological conditions affected by aberrant IFN-I signaling.

A Streptococcus Quorum Sensing System Enables Suppression of Innate Immunity.

Some bacterial pathogens utilize cell-cell communication systems, such as quorum sensing (QS), to coordinate genetic programs during host colonization and infection. The human-restricted pathosymbiont Streptococcus pyogenes (group A streptococcus [GAS]) uses the Rgg2/Rgg3 QS system to modify the bacterial surface, enabling biofilm formation and lysozyme resistance. Here, we demonstrate that innate immune cell responses to GAS are substantially altered by the QS status of the bacteria. We found that macrophage activation, stimulated by multiple agonists and assessed by cytokine production and NF-κB activity, was substantially suppressed upon interaction with QS-active GAS but not QS-inactive bacteria. Neither macrophage viability nor bacterial adherence, internalization, or survival were altered by the QS activation status, yet tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6), and interferon beta (IFN-ß) levels and NF-κB reporter activity were drastically lower following infection with QS-active GAS. Suppression required contact between viable bacteria and macrophages. A QS-regulated biosynthetic gene cluster (BGC) in the GAS genome, encoding several putative enzymes, was also required for macrophage modulation. Our findings suggest a model wherein upon contact with macrophages, QS-active GAS produce a BGC-derived factor capable of suppressing inflammatory responses. The suppressive capability of QS-active GAS is abolished after treatment with a specific QS inhibitor. These observations suggest that interfering with the ability of bacteria to collaborate via QS can serve as a strategy to counteract microbial efforts to manipulate host defenses.IMPORTANCEStreptococcus pyogenes is restricted to human hosts and commonly causes superficial diseases such as pharyngitis; it can also cause severe and deadly manifestations including necrotizing skin disease or severe postinfectious sequelae like rheumatic heart disease. Understanding the complex mechanisms used by this pathogen to manipulate host defenses could aid in developing new therapeutics to treat infections. Here, we examine the impact of a bacterial cell-cell communication system, which is highly conserved across S. pyogenes, on host innate immune responses. We find that S. pyogenes uses this system to suppress macrophage proinflammatory cytokine responses in vitro Interference with this communication system could serve as a strategy to disarm bacteria and maintain an effective immune response.

The Role of luxS in Histophilus somni Virulence and Biofilm Formation.

S-Ribosylhomocysteinase (LuxS) is required for the synthesis of the autoinducer-2 (AI-2) quorum-sensing signaling molecule in many Gram-negative bacteria. The bovine (and ovine) opportunistic pathogen Histophilus somni contains luxS and forms a biofilm containing an exopolysaccharide (EPS) in the matrix. Since biofilm formation is regulated by quorum sensing in many bacteria, the roles of luxS in H. somni virulence and biofilm formation were investigated. Although culture supernatants from H. somni were ineffective at inducing bioluminescence in the Vibrio harveyi reporter strain BB170, H. somniluxS complemented the biosynthesis of AI-2 in the luxS-deficient Escherichia coli strain DH5α. H. somni strain 2336 luxS was inactivated by transposon mutagenesis. RNA expression profiles revealed that many genes were significantly differentially expressed in the luxS mutant compared to that in the wild-type, whether the bacteria were grown planktonically or in a biofilm. Furthermore, the luxS mutant had a truncated and asialylated lipooligosaccharide (LOS) and was substantially more serum sensitive than the wild-type. Not surprisingly, the luxS mutant was attenuated in a mouse model for H. somni virulence, and some of the altered phenotypes were partially restored after the mutation was complemented with a functional luxS However, no major differences were observed between the wild-type and the luxS mutant in regard to outer membrane protein profiles, biofilm formation, EPS production, or intracellular survival. These results indicate that luxS plays a role in H. somni virulence in the context of LOS biosynthesis but not biofilm formation or other phenotypic properties examined.

Micro-rheological properties of lung homogenates correlate with infection severity in a mouse model of Pseudomonas aeruginosa lung infection.

Lung infections caused by Pseudomonas aeruginosa pose a serious threat to patients suffering from, among others, cystic fibrosis, chronic obstructive pulmonary disease, or bronchiectasis, often leading to life-threatening complications. The establishment of a chronic infection is substantially related to communication between bacteria via quorum-sensing networks. In this study, we aimed to assess the role of quorum-sensing signaling molecules of the Pseudomonas quinolone signal (PQS) and to investigate the viscoelastic properties of lung tissue homogenates of PA-infected mice in a prolonged acute murine infection model. Therefore, a murine infection model was successfully established via intra-tracheal infection with alginate-supplemented Pseudomonas aeruginosa NH57388A. Rheological properties of lung homogenates were analyzed with multiple particle tracking (MPT) and quorum-sensing molecules were quantified with LC-MS/MS. Statistical analysis of bacterial load and quorum-sensing molecules showed a strong correlation between these biomarkers in infected lungs. This was accompanied by noticeable changes in the consistency of lung homogenates with increasing infection severity. Furthermore, viscoelastic properties of the lung homogenates strongly correlated with bacterial load and quorum sensing molecules. Considering the strong correlation between the viscoelasticity of lung homogenates and the aforementioned biomarkers, the viscoelastic properties of infected lungs might serve as reliable new biomarker for the evaluation of the severity of P. aeruginosa infections in murine models.

Intracellular survival and innate immune evasion of Burkholderia cepacia: Improved understanding of quorum sensing-controlled virulence factors, biofilm, and inhibitors.

Burkholderia cepacia complex (Bcc) are opportunistic pathogens implicated with nosocomial infections, and high rates of morbidity and mortality, especially in individuals with cystic fibrosis (CF). B. cepacia are naturally resistant to different classes of antibiotics, and can subvert the host innate immune responses by producing quorum sensing (QS) controlled virulence factors and biofilms. It still remains a conundrum as to how exactly the bacterium survives the intracellular environment within the host cells of CF patients and immunocompromised individuals although the bacterium can invade human lung epithelial cells, neutrophils, and murine macrophages. The mechanisms associated with intracellular survival in the airway epithelial cells and the role of QS and virulence factors in B. cepacia infections in cystic fibrosis remain largely unclear. The current review focuses on understanding the role of QS-controlled virulence factors and biofilms, and provides additional impetus to understanding the potentials of QS-inhibitory strategies against B. cepacia.

Modulation of Gut Microbiota through Dietary Phytochemicals as a Novel Anti-infective Strategy.

Quorum Sensing (QS) is a phenomenon in which bacterial cells communicate with each other with the help of several low molecular weight compounds. QS is largely dependent on population density, and it triggers when the concentration of quorum sensing molecules accumulate in the environment and crosses a particular threshold. Once a certain population density is achieved and the concentration of molecules crosses a threshold, the bacterial cells show a collective behavior in response to various chemical stimuli referred to as "auto-inducers". The QS signaling is crucial for several phenotypic characteristics responsible for bacterial survival such as motility, virulence, and biofilm formation. Biofilm formation is also responsible for making bacterial cells resistant to antibiotics. The human gut is home to trillions of bacterial cells collectively called "gut microbiota" or "gut microbes". Gut microbes are a consortium of more than 15,000 bacterial species and play a very crucial role in several body functions such as metabolism, development and maturation of the immune system, and the synthesis of several essential vitamins. Due to its critical role in shaping human survival and its modulating impact on body metabolisms, the gut microbial community has been referred to as "the forgotten organ" by O`Hara et al. (2006) [1]. Several studies have demonstrated that chemical interaction between the members of bacterial cells in the gut is responsible for shaping the overall microbial community. Recent advances in phytochemical research have generated a lot of interest in finding new, effective, and safer alternatives to modern chemical-based medicines. In the context of antimicrobial research various plant extracts have been identified with Quorum Sensing Inhibitory (QSI) activities among bacterial cells. This review focuses on the mechanism of quorum sensing and quorum sensing inhibitors isolated from natural sources.

Multiple Signaling Pathways Involved in Human Dendritic Cell Maturation Are Affected by the Fungal Quorum-Sensing Molecule Farnesol.

The quorum-sensing molecule farnesol is produced by the opportunistic human fungal pathogen Candida albicans Aside from its primary function of blocking the transition from yeast to hyphal morphotype, it has an immunomodulatory role on human dendritic cells (DC) through the alteration of surface markers, cytokine secretion, and their ability to activate T cells. Nonetheless, the molecular mechanisms by which farnesol modulates DC differentiation and maturation remained unknown. In this study, we demonstrate through transcriptional and functional assays that farnesol influences several signaling pathways during DC differentiation and in response to TLR agonists. In particular, farnesol increases the expression of the Ag-presenting glycoprotein CD1d through the nuclear receptors PPARγ and RARα, as well as p38 MAPK. However, the higher expression of CD1d did not confer these DC with an enhanced capacity to activate CD1d-restricted invariant NKT cells. In the presence of farnesol, there is reduced secretion of the Th1-inducing cytokine, IL-12, and increased release of proinflammatory cytokines, as well as the anti-inflammatory cytokine IL-10. These changes are partially independent of nuclear receptor activity but, in the case of TNF-α and IL-10, dependent on NF-κB and MAPK pathways. Interestingly, renewal of the IL-12/IL-10 milieu restores the ability of farnesol-differentiated DC to activate invariant NKT, Th1, and FOXP3+ regulatory T cells. Our results show that farnesol modulates nuclear receptors, NF-κB, and MAPK-signaling pathways, thereby impairing the capacity of DC to activate several T cells subsets and potentially conferring C. albicans, an advantage in overcoming DC-mediated immunity.

Small Is Mighty-Chemical Communication Systems in Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an opportunistic pathogen that causes a variety of acute and chronic infections. Usually a commensal on the host body, P. aeruginosa is capable of transforming into a virulent pathogen upon sensing favorable changes in the host immune system or stress cues. P. aeruginosa infections are hard to eradicate, because this pathogen has developed strong resistance to most conventional antibiotics; in addition, in chronic infections it commonly forms a biofilm matrix, which provides bacterial cells a protected environment to withstand various stresses including antibiotics. Given its importance as a human pathogen and its notorious antimicrobial tolerance, P. aeruginosa has been the subject of intensive investigations internationally. Research progress over the last two decades has unveiled a range of chemical communication systems in this pathogen. These diversified chemical communication systems endow P. aeruginosa a superb ability and remarkable flexibility to coordinate and modulate accordingly the transcriptional expression of various sets of genes associated with virulence and other physiologic activities in response to environmental changes. A fair understanding of the chemical signaling mechanisms with which P. aeruginosa governs virulence gene expression may hold the key to developing alternative therapeutic interventions that control and prevent bacterial infections.

Control of Staphylococcus aureus Quorum Sensing by a Membrane-Embedded Peptidase.

Gram-positive bacteria process and release small peptides, or pheromones, that act as signals for the induction of adaptive traits, including those involved in pathogenesis. One class of small signaling pheromones is the cyclic autoinducing peptides (AIPs), which regulate expression of genes that orchestrate virulence and persistence in a range of microbes, including staphylococci, listeriae, clostridia, and enterococci. In a genetic screen for Staphylococcus aureus secreted virulence factors, we identified an S. aureus mutant containing an insertion in the gene SAUSA300_1984 (mroQ), which encodes a putative membrane-embedded metalloprotease. A ΔmroQ mutant exhibited impaired induction of Toll-like receptor 2-dependent inflammatory responses from macrophages but elicited greater production of the inflammatory cytokine interleukin-1ß and was attenuated in a murine skin and soft tissue infection model. The ΔmroQ mutant phenocopies an S. aureus mutant containing a deletion of the accessory gene regulatory system (Agr), wherein both strains have significantly reduced production of secreted toxins and virulence factors but increased surface protein A abundance. The Agr system controls virulence factor gene expression in S. aureus by sensing the accumulation of AIP via the histidine kinase AgrC and the response regulator AgrA. We provide evidence to suggest that MroQ acts within the Agr pathway to facilitate the optimal processing or export of AIP for signal amplification through AgrC/A and induction of virulence factor gene expression. Mutation of MroQ active-site residues significantly reduces AIP signaling and attenuates virulence. Altogether, this work identifies a new component of the Agr quorum-sensing circuit that is critical for the production of S. aureus virulence factors.

Rethinking Communication in the Immune System: The Quorum Sensing Concept.

Quorum sensing was first described as the communication process bacteria employ to coordinate changes in gene expression and therefore, their collective behavior in response to population density. Emerging new evidence suggests that quorum sensing can also contribute to the regulation of immune cell responses. Quorum sensing might be achieved by the ability of immune cells to perceive the density of their own populations or those of other cells in their environment; responses to alterations in cell density might then be coordinated via changes in gene expression and protein signaling. Quorum sensing mechanisms can regulate T and B cell as well as macrophage function. We posit that perturbations in quorum sensing may undermine the balance between diverse immune cell populations and predispose the host to immune abnormalities.

Pseudomonas aeruginosa quorum-sensing metabolite induces host immune cell death through cell surface lipid domain dissolution.

Bacterial quorum-sensing autoinducers are small chemicals released to control microbial community behaviours. N-(3-oxo-dodecanoyl) homoserine lactone, the autoinducer of the Pseudomonas aeruginosa LasI-LasR circuitry, triggers significant cell death in lymphocytes. We found that this molecule is incorporated into the mammalian plasma membrane and induces dissolution of eukaryotic lipid domains. This event expels tumour necrosis factor receptor 1 into the disordered lipid phase for its spontaneous trimerization without its ligand and drives caspase 3-caspase 8-mediated apoptosis. In vivo, P. aeruginosa releases N-(3-oxo-dodecanoyl) homoserine lactone to suppress host immunity for its own better survival; conversely, blockage of caspases strongly reduces the severity of the infection. This work reveals an unknown communication method between microorganisms and the mammalian host and suggests interventions of bacterial infections by intercepting quorum-sensing signalling.

A Metabolism-Based Quorum Sensing Mechanism Contributes to Termination of Inflammatory Responses.

Recruitment of immune cells with antimicrobial activities is essential to fight local infections but has the potential to trigger immunopathology. Whether the immune system has the ability to sense inflammation intensity and self-adjust accordingly to limit tissue damage remains to be fully established. During local infection with an intracellular pathogen, we have shown that nitric oxide (NO) produced by recruited monocyte-derived cells was essential to limit inflammation and cell recruitment. Mechanistically, we have provided evidence that NO dampened monocyte-derived cell cytokine and chemokine production by inhibiting cellular respiration and reducing cellular ATP:ADP ratio. Such metabolic control operated at the tissue level but only when a sufficient number of NO-producing cells reached the site of infection. Thus, NO production and activity act as a quorum sensing mechanism to help terminate the inflammatory response.

How adaptive immunity constrains the composition and fate of large bacterial populations.

Features of the CRISPR-Cas system, in which bacteria integrate small segments of phage genome (spacers) into their DNA to neutralize future attacks, suggest that its effect is not limited to individual bacteria but may control the fate and structure of whole populations. Emphasizing the population-level impact of the CRISPR-Cas system, recent experiments show that some bacteria regulate CRISPR-associated genes via the quorum sensing (QS) pathway. Here we present a model that shows that from the highly stochastic dynamics of individual spacers under QS control emerges a rank-abundance distribution of spacers that is time invariant, a surprising prediction that we test with dynamic spacer-tracking data from literature. This distribution depends on the state of the competing phage-bacteria population, which due to QS-based regulation may coexist in multiple stable states that vary significantly in their phage-to-bacterium ratio, a widely used ecological measure to characterize microbial systems.

Induction of CD4 T cell memory by local cellular collectivity.

Cell differentiation is directed by signals driving progenitors into specialized cell types. This process can involve collective decision-making, when differentiating cells determine their lineage choice by interacting with each other. We used live-cell imaging in microwell arrays to study collective processes affecting differentiation of naïve CD4+ T cells into memory precursors. We found that differentiation of precursor memory T cells sharply increases above a threshold number of locally interacting cells. These homotypic interactions involve the cytokines interleukin-2 (IL-2) and IL-6, which affect memory differentiation orthogonal to their effect on proliferation and survival. Mathematical modeling suggests that the differentiation rate is continuously modulated by the instantaneous number of locally interacting cells. This cellular collectivity can prioritize allocation of immune memory to stronger responses.

Vibrio vulnificus quorum-sensing molecule cyclo(Phe-Pro) inhibits RIG-I-mediated antiviral innate immunity.

The recognition of pathogen-derived ligands by pattern recognition receptors activates the innate immune response, but the potential interaction of quorum-sensing (QS) signaling molecules with host anti-viral defenses remains largely unknown. Here we show that the Vibrio vulnificus QS molecule cyclo(Phe-Pro) (cFP) inhibits interferon (IFN)-ß production by interfering with retinoic-acid-inducible gene-I (RIG-I) activation. Binding of cFP to the RIG-I 2CARD domain induces a conformational change in RIG-I, preventing the TRIM25-mediated ubiquitination to abrogate IFN production. cFP enhances susceptibility to hepatitis C virus (HCV), as well as Sendai and influenza viruses, each known to be sensed by RIG-I but did not affect the melanoma-differentiation-associated gene 5 (MDA5)-recognition of norovirus. Our results reveal an inter-kingdom network between bacteria, viruses and host that dysregulates host innate responses via a microbial quorum-sensing molecule modulating the response to viral infection.

Signaling by a Conserved Quorum Sensing Pathway Contributes to Growth Ex Vivo and Oropharyngeal Colonization of Human Pathogen Group A Streptococcus.

Bacterial virulence factor production is a highly coordinated process. The temporal pattern of bacterial gene expression varies in different host anatomic sites to overcome niche-specific challenges. The human pathogen group A streptococcus (GAS) produces a potent secreted protease, SpeB, that is crucial for pathogenesis. Recently, we discovered that a quorum sensing pathway comprised of a leaderless short peptide, SpeB-inducing peptide (SIP), and a cytosolic global regulator, RopB, controls speB expression in concert with bacterial population density. The SIP signaling pathway is active in vivo and contributes significantly to GAS invasive infections. In the current study, we investigated the role of the SIP signaling pathway in GAS-host interactions during oropharyngeal colonization. The SIP signaling pathway is functional during growth ex vivo in human saliva. SIP-mediated speB expression plays a crucial role in GAS colonization of the mouse oropharynx. GAS employs a distinct pattern of SpeB production during growth ex vivo in saliva that includes a transient burst of speB expression during early stages of growth coupled with sustained levels of secreted SpeB protein. SpeB production aids GAS survival by degrading LL37, an abundant human antimicrobial peptide. We found that SIP signaling occurs during growth in human blood ex vivo. Moreover, the SIP signaling pathway is critical for GAS survival in blood. SIP-dependent speB regulation is functional in strains of diverse emm types, indicating that SIP signaling is a conserved virulence regulatory mechanism. Our discoveries have implications for future translational studies.

Quorum-sensing molecule dihydroxy-2,3-pentanedione and its analogs as regulators of epithelial integrity.

BACKGROUND AND OBJECTIVE: Quorum-sensing molecules regulate the behavior of bacteria within biofilms and at the same time elicit an immune response in host tissues. Our aim was to investigate the regulatory role of dihydroxy-2,3-pentanedione (DPD), the precursor of universal autoinducer-2 (AI-2), and its analogs (ethyl-DPD, butyl-DPD and isobutyl-DPD) in the integrity of gingival epithelial cells. MATERIAL AND METHODS: Human gingival keratinocytes were incubated with four concentrations (10 µmol L-1 , 1 µmol L-1 , 100 nmol L-1 and 10 nmol L-1 ) of DPD and its analogs for 24 hours. The numbers of viable cells were determined using a proliferation kit, matrix metalloproteinase (MMP)-2 and -9 activities were determined by gelatin zymography, and expression of occludin protein and occludin mRNA were determined by western blotting and RT-qPCR, respectively. RESULTS: Increased cell proliferation was observed in gingival keratinocytes incubated with 100 nmol L-1 of butyl-DPD. MMP-9 activity was elevated in cells incubated with 10 µmol L-1 of ethyl-DPD. On the other hand, MMP-2 activity did not show any significant change when gingival keratinocytes were incubated with or without DPD or analogs. Western blot analyses demonstrated five forms (105, 61, 52.2, 44 and 37 kDa) of occludin. Incubation with 1 µmol L-1 and 100 nmol L-1 of DPD and with 10 nmol L-1 of ethyl-DPD increased dimeric (105 kDa) forms of occludin, while incubation with 100 nmol L-1 of isobutyl-DPD increased monomeric (61 kDa) forms. DPD and ethyl-DPD decreased, and 100 nmol L-1 of isobutyl-DPD and 10 nmol L-1 of butyl-DPD increased, the monomeric (52.2 kDa and 44 kDa) forms of occludin, whereas ethyl-DPD decreased and isobutyl-DPD increased, the low-molecular-weight (37 kDa) forms. According to RT-qPCR analysis, the exposure of gingival keratinocytes to 10 µmol L-1 of isobutyl-DPD up-regulated expression of occludin. CONCLUSION: The results indicate that isobutyl-DPD has the potential to enhance the integrity of the epithelium by stimulating the formation of occluding, without affecting the proliferation or gelatinolytic enzyme activities of the exposed cells. The modulatory effect of an AI-2 analog on the epithelial cell response is shown for the first time.

Efeito de extratos orgânicos de variedades de cebola sobre o quorum sensing bacteriano / Effect of organic extracts of onion varieties on bacterial quorum sensing

Muitos genes bacterianos são regulados pelo mecanismo de comunicação denominado quorum sensing (QS). Neste sistema, moléculas sinalizadoras ativam um comportamento de grupo, conforme a densidade celular, permitindo o controle da expressão gênica. Estudos sugerem o potencial de compostos extraídos de plantas sobre o QS, a exemplo da quercetina, um flavonol presente em concentrações elevadas em algumas frutas e hortaliças. Este composto é o flavonoide majoritário presente em cebola (Allium cepa), mas não existem estudos que mostrem a atividade anti-QS de extratos orgânicos deste vegetal. Este trabalho avaliou o potencial antimicrobiano e anti-QS de extratos orgânicos de cebola branca e cebola roxa, assim como de alguns de seus componentes majoritários identificados, em fenótipos regulados pelo QS como a produção de violaceína em Chrormobacterium violaceum ATCC 12472, a motilidade tipo swarming e a formação de biofilmes em Pseudomonas aeruginosa PAO1 e Serratia marcescens MG1. Extratos de cebola branca e roxa foram obtidos por extração em fase sólida utilizando coluna de poliamida e seus compostos identificados e quantificados pelas técnicas de Cromatografia líquida- ionização por elétron spray-espectrometria de massas e cromatografia líquida de alta eficiência acoplada a detector de arranjo de diodo. A atividade antimicrobiana foi avaliada pelas curvas de multiplicação de cada micro-organismo. O efeito dos compostos quercetina aglicona (inibidor do QS já relatado na literatura e encontrado no extrato de cebola roxa) e quercetina-3-ß-D-glicosideo (um dos compostos majoritários encontrados em ambos extratos) sobre os micro-organismos utilizados neste estudo foi também avaliado. Foram obtidos três extratos: cebola branca em metanol (CB-MeOH), cebola branca em metanol amônia (CBMeOH/ NH4) e cebola roxa em metanol (CR-MeOH). Os compostos quercetina 3,4'- diglicosídeio, quercetina-4-glicosídeo, quercetina-3-ß-D-glicosideo e quercetina aglicona foram os predominantes nos extratos das duas variedades de cebola. Cianidina-3-O-glicosideo também foi identificada no extrato de cebola roxa. A concentração inibitória mínima (MIC) dos extratos foi igual ou superior a 125 µg/ml (p/v) de extrato seco. Não foi observada inibição significativa da produção de violaceína em C. violaceum pelos extratos orgânicos de cebola e nem pela quercetina-3-ß-D-glicosideo, nas concentrações sub-inibitórias avaliadas. No entanto, a quercetina aglicona inibiu significativamente a produção de violaceína em todas as concentrações. A glicosilação da quercetina pode ter afetado sua atividade sobre a inibição da produção de violaceina, já que estudos mostram menor atividade biológica deste composto quando glicosilado. Para a motilidade tipo swarming em P. aeruginosa PAO1 houve inibição significativa pelo extrato de cebola roxa, em todas as concentrações estudadas. Os demais extratos não apresentaram inibição contra este micro-organismo. Para S. marcescens MG1, foi observada inibição da motilidade swarming somente na concentração de 125 µg/ml de CBMeOH/ NH4. As análises de comparação entre os dois tipos de quercetina revelaram que, embora para as duas bactérias testadas os dois compostos apresentaram atividade inibitória sobre a motilidade tipo swarming, a quercetina-3-ß-D-glicosideo foi menos eficiente que a quercetina aglicona na concentração de 125 µg/ml. A formação de biofilmes não foi influenciada pelos extratos e, inesperadamente, não se detectou inibição da formação de biofilmes por ambos tipos de quercetina avaliados. De forma geral, os extratos orgânicos de cebola mostraram pouco efeito sobre os fenótipos controlados pelo quorum sensing e a glicosilação da quercetina provavelmente explica a baixa atividade antimicrobiana e anti-QS dos extratos

Many bacterial genes are regulated by a communication mechanism called quorum sensing (QS). In this system, signaling molecules activate a group behavior according to cell density, allowing the control of gene expression. Studies suggest the inhibitory potential of compounds extracted from plants on the QS system, like quercetin, a flavonol present in high concentrations in some fruits and vegetables. This compound is the main flavonoid found in onion (Allium cepa); however, there are no studies showing the anti-QS activity of organic extracts of this plant. The objective of this work was to evaluate the antimicrobial and anti-QS potential of organic extracts of white and red onions, and their major components studied in QS-regulated phenotypes such as violacein production in Chromobacterium violaceum, swarming motility and biofilm formation in Pseudomonas aeruginosa PAO1 and Serratia marcescens MG1.White and red onion extracts were obtained by solid phase extraction using a polyamide column and its compounds were identified and quantified by Liquid Chromatography - Electron Spray-Mass Spectrometry and high performance liquid chromatography coupled to diode array detector. O The antimicrobial activity was evaluated by growth curves of each microorganism. The effect of non-glycosylated quercetin (a QS inhibitor already reported in the literature and found in red onion extract) and quercetin-3-ß-D-glycoside (one of the major compounds found in both extracts) on the microorganisms used in this study was also evaluated. Three extracts were obtained: white onion in methanol (CB-MeOH), white onion in methanol ammonia (CB-MeOH / NH4) and red onion in methanol (CR-MeOH). Our results showed that quercetin 3,4'- diglycoside, quercetin-4-glycoside, quercetin-3-ß-D-glycoside and non-glycosylated quercetin were predominant in the extracts of the two onion varieties. Cyanidin-3-O-glycoside has also been identified in the purple onion extract. The minimum inhibitory concentration (MIC) of extracts was equal or greater than 125 µg / ml (w / v) of dry extract. There was no significant inhibition of violacein production in C. violaceum by organic onion extracts or by quercetin-3-ß- D-glycoside at the sub-inhibitory concentrations evaluated. However, non-glycosylated quercetin showed a significant inhibition of violacein production in all tested concentrations. The glycosylation of Quercetin could have altered its inhibition activity towards violacein production, and in fact, some studies have shown less biological activity of some phenolic compounds when they have been glycosylated. For swarming motility in P. aeruginosa PAO1 there was significant inhibition by red onion extract, in all studied concentrations. The other extracts did not present inhibition against this microorganism. For S. marcescens MG1, inhibition of swarming motility was observed only at the concentration of 125 µg / ml of CB-MeOH / NH4. Comparative analyses between the two types of quercetin showed that, although for the two bacteria tested the two compounds showed inhibitory activity on swarming motility, quercetin-3-ß-D-glycoside was less efficient than non-glycosylated quercetin in the concentration of 125 µg / ml. Biofilm formation was not influenced by the extracts and unexpectedly, both types of quercetin evaluated did not show inhibition towards biofilm formation. In general, organic onion extracts showed little effect on quorum sensing controlled phenotypes and glycosylation of quercetin probably explains the low antimicrobial and anti-QS activity of the extracts

A quorum sensing-independent path to stumpy development in Trypanosoma brucei.

For persistent infections of the mammalian host, African trypanosomes limit their population size by quorum sensing of the parasite-excreted stumpy induction factor (SIF), which induces development to the tsetse-infective stumpy stage. We found that besides this cell density-dependent mechanism, there exists a second path to the stumpy stage that is linked to antigenic variation, the main instrument of parasite virulence. The expression of a second variant surface glycoprotein (VSG) leads to transcriptional attenuation of the VSG expression site (ES) and immediate development to tsetse fly infective stumpy parasites. This path is independent of SIF and solely controlled by the transcriptional status of the ES. In pleomorphic trypanosomes varying degrees of ES-attenuation result in phenotypic plasticity. While full ES-attenuation causes irreversible stumpy development, milder attenuation may open a time window for rescuing an unsuccessful antigenic switch, a scenario that so far has not been considered as important for parasite survival.