Quorum-sensing (QS) regulates the production of key virulence factors in Pseudomonas aeruginosa and other important pathogenic bacteria. In this report, extracts of leaves and bark of Combretum albiflorum (Tul.) Jongkind (Combretaceae) were found to quench the production of QS-dependent factors in P. aeruginosa PAO1. Chromatographic fractionation of the crude active extract generated several active fractions containing flavonoids, as shown by their typical spectral features. Purification and structural characterization of one of the active compounds led to the identification of the flavan-3-ol catechin [(2R,3S)-2-(3,4-dihydroxyphenyl)-3,4-dihydro-1(2H)-benzopyran-3,5,7-triol]. The identity of catechin as one of the active molecules was confirmed by comparing the high-pressure liquid chromatography profiles and the mass spectrometry spectra obtained for a catechin standard and for the active C. albiflorum fraction. Moreover, standard catechin had a significant negative effect on pyocyanin and elastase productions and biofilm formation, as well as on the expression of the QS-regulated genes lasB and rhlA and of the key QS regulatory genes lasI, lasR, rhlI, and rhlR. The use of RhlRand LasR-based biosensors indicated that catechin might interfere with the perception of the QS signal N-butanoyl-L-homoserine lactone by RhlR, thereby leading to a reduction of the production of QS factors. Hence, catechin, along with other flavonoids produced by higher plants, might constitute a first line of defense against pathogenic attacks by affecting QS mechanisms and thereby virulence factor production.Pseudomonas aeruginosa is a gram-negative bacterium infecting insects, plants, animals, and humans (65). As an opportunistic pathogen, P. aeruginosa is a major cause of nosocomial diseases and mortality in immunocompromised patients and particularly in patients with cystic fibrosis, diffused panbronchitis, and pulmonary deficiencies (21, 54). Successful infection of diverse hosts is due to the profusion and diversity of virulence factors secreted by P. aeruginosa such as proteases, exopolysaccharides and redox-active compounds, as well as to its capacity to form biofilms (9, 60, 62).Many pathogenic bacteria trigger the production of their virulence factors in a population density-dependent manner, a cell-to-cell communication mechanism known as quorum sensing (QS) (24). This mechanism enables bacteria to detect their population density through the production, release, and perception of small diffusible molecules called autoinducers and to coordinate gene expression accordingly (7,9,13,24,84). In P. aeruginosa, two QS systems (las and rhl) drive the production (by the synthetases LasI and RhlI) and the perception (by the transcription factors LasR and RhlR) of the acyl-homoserine lactones (AHL) N-(3-oxododecanoyl)-L-homoserine lactone (3-oxo-C12-HSL) and N-butanoyl-L-homoserine lactone (C4-HSL), respectively (9, 62). Once LasR interacts with 3-oxo-C12-HSL, it induces the las system (by increasing lasI expression) and triggers the p...

A total of fourteen (14) species of wild edible fruits from Burkina Faso were analyzed for their phenolic and flavonoid contents, and their antioxidant activities using the DPPH, FRAP and ABTS methods. The data obtained show that the total phenolic and total flavonoid levels were significantly higher in the acetone than in the methanol extracts. Detarium microcarpum fruit had the highest phenolic and the highest flavonoid content, followed by that of Adansonia digitata, Ziziphus mauritiana, Ximenia americana and Lannea microcarpa. Significant amounts of total phenolics were also detected in the other fruit species in the following order of decreasing levels: Tamarindus indica > Sclerocarya birrea > Dialium guineense > Gardenia erubescens > Diospyros mespiliformis > Parkia biglobosa > Ficus sycomorus > Vitellaria paradoxa. Detarium microcarpum fruit also showed the highest antioxidant activity using the three antioxidant assays. Fruits with high antioxidant activities were also found to possess high phenolic and flavonoid contents. There was a strong correlation between total phenolic and flavonoid levels and antioxidant activities.

Preliminary screening of the Malagasy plant Combretum albiflorum for compounds attenuating the production of quorum sensing (QS)-controlled virulence factors in bacteria led to the identification of active fractions containing flavonoids. In the present study, several flavonoids belonging to the flavone, flavanone, flavonol and chalcone structural groups were screened for their capacity to reduce the production of QS-controlled factors in the opportunistic pathogen Pseudomonas aeruginosa (strain PAO1). Flavanones (i.e. naringenin, eriodictyol and taxifolin) significantly reduced the production of pyocyanin and elastase in P. aeruginosa without affecting bacterial growth. Consistently, naringenin and taxifolin reduced the expression of several QS-controlled genes (i.e. lasI, lasR, rhlI, rhlR, lasA, lasB, phzA1 and rhlA) in P. aeruginosa PAO1. Naringenin also dramatically reduced the production of the acylhomoserine lactones N-(3-oxododecanoyl)-Lhomoserine lactone (3-oxo-C12-HSL) and N-butanoyl-L-homoserine lactone (C4-HSL), which is driven by the lasI and rhlI gene products, respectively. In addition, using mutant strains deficient for autoinduction (DlasI and DrhlI) and LasR-and RhlR-based biosensors, it was shown that QS inhibition by naringenin not only is the consequence of a reduced production of autoinduction compounds but also results from a defect in the proper functioning of the RlhR-C4-HSL complex. Widely distributed in the plant kingdom, flavonoids are known for their numerous and determinant roles in plant physiology, plant development and in the success of plant-rhizobia interactions, but, as shown here, some of them also have a role as inhibitors of the virulence of pathogenic bacteria by interfering with QS mechanisms. INTRODUCTIONIn many pathogenic bacteria the production of virulence factors is triggered in a population density-dependent manner through quorum sensing (QS), a cell-to-cell communication mechanism that enables bacteria to coordinate virulence factor production by means of the synthesis, release and perception of small diffusible molecules called autoinducers (Antunes et al., 2010;Bjarnsholt et al., 2010;Case et al., 2008;Ng & Bassler, 2009). For instance, in the plant and mammal opportunistic pathogen Pseudomonas aeruginosa, two main QS systems (lasI/R and rhlI/R), responsible for the synthesis and perception of the acylhomoserine lactones (AHLs) N-(3-oxododecanoyl)-L-homoserine lactone (3-oxo-C12-HSL) and N-butanoyl-L-homoserine lactone (C4-HSL), respectively , control the expression Abbreviations: AHL, acylhomoserine lactone; C4-HSL, N-butanoyl-Lhomoserine lactone; ESI-MS, electrospray ionization-MS; HHL, Nhexanoyl-L-homoserine lactone; 3-oxo-C12-HSL, N-(3-oxododecanoyl)-L-homoserine lactone; PI, propidium iodide; QS, quorum sensing.3These authors contributed equally to this work. of an arsenal of virulence factors. The transcription factors LasR and RhlR interact with and are activated by 3-oxo-C12-HSL and C4-HSL, respectively, triggering the production of biofilms, LasB elastase...

International audienceThe leaves of three Mentha species harvested in Algeria, Mentha spicata L. (MS), Mentha pulegium L. (MP) and Mentha rotundifolia (L.) Huds (MR) were examined for their content in polyphenols and for some activities-linked biological properties these could impart. The contents in total phenolics (TPC) and flavonoids (TFC) were evaluated by the Folin–Ciocalteu and the aluminum chloride methods, respectively. Whereas MS showed the highest TPC (12.0 ± 0.3 mg gallic acid equivalents/g of dry weight), MR had the highest content in TFC (3.3 ± 0.1 mg quercetin equivalents of dry weight). The pharmacological properties of these extracts were evaluated by assessing in vitro their antioxidant and antityrosinase activities. The modulation of mushroom tyrosinase activity was measured by colorimetry of the melanins formed in the presence of tyrosine. MS exhibited the strongest radical scavenging activity (RSA) in all assays: (i) the IC50s values to neutralize the 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) cation radicals (ABTS+) and the 2,2′-diphenyl-1-picrylhydrazyl radicals (DPPH) were 10.3 ± 0.9 and 16.2 ± 0.2 μg/mL, respectively; and (ii) its original electrochemically measured superoxide quenching index value is 188 ± 37 μg/mL (AI50). MR however showed the highest tyrosinase inhibitory activity (IC50 = 108 ± 20 μg/mL). A silica gel thin-layer chromatography (TLC) technique revealed the presence of caffeic and rosmarinic acids and diosmin in all extracts. These results were confirmed by high performance liquid chromatography with diode array detection (HPLC/DAD