Quorum sensing (QS) regulates bacterial gene expression and studies suggest quercetin, a flavonol found in onion, as a QS inhibitor. There are no studies showing the anti-QS activity of plants containing quercetin in its native glycosylated forms. This study aimed to evaluate the antimicrobial and anti-QS potential of organic extracts of onion varieties and its representative phenolic compounds quercetin aglycone and quercetin 3-β-D-glucoside in the QS model bacteria Chromobacterium violaceum ATCC 12472, Pseudomonas aeruginosa PAO1, and Serratia marcescens MG1. Three phenolic extracts were obtained: red onion extract in methanol acidified with 2.5% acetic acid (RO-1), white onion extract in methanol (WO-1) and white onion extract in methanol ammonium (WO-2). Quercetin 4-O-glucoside and quercetin 3,4-O-diglucoside were identified as the predominant compounds in both onion varieties using HPLC-DAD and LC-ESI-MS/MS. However, quercetin aglycone, cyanidin 3-O-glucoside and quercetin glycoside were identified only in RO-1. The three extracts showed minimum inhibitory concentration (MIC) values equal to or above 125 µg/ml of dried extract. Violacein production was significantly reduced by RO-1 and quercetin aglycone, but not by quercetin 3-β-D-glucoside. Motility in P. aeruginosa PAO1 was inhibited by RO-1, while WO-2 inhibited S. marcescens MG1 motility only in high concentration. Quercetin aglycone and quercetin 3-β-D-glucoside were effective at inhibiting motility in P. aeruginosa PAO1 and S. marcescens MG1. Surprisingly, biofilm formation was not affected by any extracts or the quercetins tested at sub-MIC concentrations. In silico studies suggested a better interaction and placement of quercetin aglycone in the structures of the CviR protein of C. violaceum ATCC 12472 than the glycosylated compound which corroborates the better inhibitory effect of the former over violacein production. On the other hand, the two quercetins were well placed in the AHLs binding pockets of the LasR protein of P. aeruginosa PAO1. Overall onion extracts and quercetin presented antimicrobial activity, and interference on QS regulated production of violacein and swarming motility.

Capsicum peppers have not been investigated as sources of quorum sensing (QS) inhibitors. This study aimed to identify compounds in pimenta‐malagueta (Capsicum frutescens) and red pepper (Capsicum annuum) extracts and to evaluate their effect on violacein production in Chromobacterium violaceum ATCC 12472 and C. violaceum CV026, as well as biofilm formation (BF) in Pseudomonas aeruginosa PAO1 and Serratia marcescens MG1. Among the extracts, pimenta‐malagueta methanolic extract (PMME) was chosen because it contained capsaicin, dihydrocapsaicin, and luteolin in greater amount than the other extracts. In general, PMME partially inhibited bacterial growth at 2.5 and 5.0 mg/mL, as well as capsaicin at 100 µg/mL and luteolin at 62.5, 125, and 250 µg/mL. At lower concentrations, PMME and luteolin reduced violacein production in C. violaceum ATCC 12472 without affecting growth, a result that was not observed with capsaicin. We show that violacein inhibition by PMME is likely due to luteolin. In silico docking evaluation showed that luteolin binds to the CviR QS regulator. Crystal violet staining and confocal microscopy revealed that BF was increased by PMME and capsaicin, being remarkably superior for P. aeruginosa PAO1 at 30 °C. Capsaicin is not an effective QS inhibitor, while luteolin should be further investigated for its potential effects in QS regulated phenotypes. Practical Application Quorum sensing (QS) is a form of bacterial communication targeted for studies aiming to inhibit bacterial virulence. QS regulates phenotypes that influence microbial activities across many areas, including Food Science. Capsicum frutescens is a type of chili pepper consumed in Brazil, rich in bioactive compounds such as capsaicin (which gives its pungency) and luteolin (a phenolic compound). We show that C. frutescens extract and luteolin inhibit QS in a model bacterium, along with the possible molecular mechanism of inhibition. Capsaicin did not inhibit QS neither biofilm formation. Luteolin should be further investigated for its QS inhibition properties and biotechnological applications.

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Ao prof. Uelinton Pinto por ter me aceitado como aluna. Por me dar a confiança de aprender sobre um tema totalmente novo para mim. Por cada sugestão e crítica durante este processo. Ao Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), pelo apoio financiero durante o mestrado. À profa. Neuza Aymoto Hassimotto, pelo apoio e dedicação no preparo e avaliação dos extratos das pimentas. Ao prof. Eduardo Purgatto, pela disponibilização do padrão capsaicina. Aos técnicos, Elias da Silva Araujo e Aline de Oliveira Santos, pelo auxílio na liofilização das pimentas. Ao prof. Felipe Rebello Lourenço, pelo apoio e auxílio na análise estatística dos dados e por todas as sugestões. Aos profesores Marcelo Palma Sircili, Mariza Landgraf e Maria Cristina Dantas Vanetti, pelas oportunas sugestões e críticas deste trabalho no Exame de Qualificação. À profa. Vanessa Bueris, pelo auxílio, motivação e dedicação na realização do ensaio de Microscopia Confocal. Ao técnico Alexsander Seixas de Souza do Laboratório de Biologia Celular do Instituto Butantan, pelo auxílio no uso do microscopio confocal. À Katia e Lúcia, pelo treinamento e auxílio constantes na manipulação dos equipamentos, materiais e ambientes do Laboratório. A cada um dos meus colegas que trabalharam ou trabalham no Laboratório, por terem resolvido dúvidas em momentos difíceis, pela disponibilidade e por terem apontado novas ideias ao meu trabalho. Ao Jhosep, por ser o amigo e companheiro de horas incertas. Por ter discutido com entusiasmo várias ideias do meu trabalho. Palavras-chaves: Quorum sensing, extrato metanólico, pimenta-malagueta, capsaicina, produção de violaceína, formação de biofilme.