Quorum sensing (QS) is a type of cell-to-cell communication that allows the bacteria to communicate via small molecules to coordinate activities such as growth, biofilm formation, virulence, and stress response as a population. QS depends on the accumulation of signal molecules as the bacterial population increases. After a critical threshold of the signal molecules are reached, the bacteria induce a cellular response allowing the bacteria to coordinate their activities as a population. In Vibrio anguillarum, three parallel quorum-sensing phosphorelay systems channels information via three hybrid sensor kinases VanN, VanQ, and CqsS that function as receptors for signal molecules produced by the synthases VanM, VanS, and CqsA, respectively. The phosphorelay systems converge onto a single regulatory pathway via the phosphotransferase VanU, which phosphorylates the response regulator VanO. Together with the alternative sigma factor RpoN, VanO activates the expression of a small RNA, Qrr1 (Quorum regulatory RNA), which in conjunction with the small RNA chaperone Hfq, destabilizes vanT mRNA, which encode the major quorum-sensing regulator in V. anguillarum. This thesis furthers the knowledge on the quorum-sensing phosphorelay systems in V. anguillarum. In this study, three additional qrr genes were identified, which were expressed during late logarithmic growth phase. The signal synthase VanM activated the expression of the Qrr1-4, which stands in contrast to Qrr regulation in other vibrios. Moreover, in addition to VanO, we predict the presence of a second response regulator which can be phosphorylated by VanU and repress Qrr1-4 expression. Thus, VanU functions as a branch point that can regulate the quorum-sensing regulon by activating or repressing VanT expression. Furthermore, VanT was shown to directly activate VanM expression and thus forming a negative regulatory loop, in which VanM represses VanT expression indirectly via Qrr1-4. In addition, VanM expression was negatively regulated post-transcriptionally by Hfq. Furthermore, a universal stress protein UspA repressed VanM expression via the repression of VanT expression. We showed that UspA binds Hfq, thus we suggest that UspA plays a role in sequestering Hfq and indirectly affect gene expression. This thesis also investigated the mechanism by which V. anguillarum can attach to and colonize fish skin tissue. We show unequivocally that fish skin epithelial cells can internalize bacteria, thus keeping the skin clear from pathogens. In turn, V. anguillarum utilized the lipopolysaccharide O-antigen to evade internalization by the fish skin epithelial cells. This study provides new insights into the molecular mechanism by which pathogen interacts with marine animals to cause disease.

Autoinducer 2 (AI-2) molecules produced by bacterial cells are thought to be involved in controlling a variety of bacterial cellular processes by coordinated gene and protein expression. Previous work in our laboratory has shown that ground beef contains compounds that can interfere with AI-2-mediated bioluminescence expression in Vibrio. harveyi. The underlying hypothesis of this work was that AI-2 molecules affect the protein expression in Escherichia coli O157:H7 and AI-2 inhibitory molecules negate the influence of AI-2 molecules. The main objectives of this study were to identify, characterize, and isolate the factors responsible for inhibition of AI-2 molecules from ground beef extracts, elucidate the role of LuxS/AI-2 cell signaling system in E. coli O157:H7 protein expression, and determine if inhibitory factors present in ground beef extract can negate the influence of AI-2 molecules on the protein expression. Using a solvent extraction procedure and gas chromatography analysis, AI-2 inhibitory factors present in ground beef extracts were identified as both medium and long chain fatty acids. When identified fatty acids were tested at different concentrations for AI-2 inhibition, AI-2 inhibition ranging from 25% to 90% was observed. Both ground beef extracts and mixture of selected fatty acids also resulted in 2- to 4-fold reduced AI-2 influenced biofilm formation by E. coli K12 cells. Identification of LuxS/AI-2-mediated protein expression in E. coli O157:H7 was conducted using two dimensional gel electrophoresis. Protein expression analysis showed that the LuxS/AI-2 system modulates the expression of proteins involved in different cellular processes such as carbohydrate and amino acid metabolism, stress response, and formation of flagella and motility. When AI-2 inhibitory factors were added along with AI-2 molecules, the expression patterns of three AI-2-influenced proteins (GlmS, SpeE, and NikA) were changed suggesting that AI-2 inhibitors can negate the influence of AI-2 molecules on protein expression of selected proteins. Collectively, these results highlight that proteins associated with different cellular processes in E. coli O157:H7 can be modulated depending on whether cells are in contact with AI-2 molecules in the presence or absence of AI-2 inhibitory factors.

Pseudomonas chlororaphis (aureofaciens) 30-84 is a biocontrol bacterium effective against take-all disease of wheat. Phenazine (PZ) production by strain 30-84 is the primary mechanism responsible for pathogen inhibition and the rhizosphere persistence of 30-84. The PhzR/PhzI system of strain 30-84 directly regulates PZ production and mutations in this QS system are defective in biofilm formation. Genetic complementation or direct addition of AHL signal restored biofilm formation to a phzI mutant. Mutations in PZ biosynthesis were equally defective in biofilm formation. Addition of PZ or genetic complementation of the PZ biosynthetic mutation restored biofilm formation. QS and PZ production also were involved in the establishment of populations on wheat seeds and plant roots. Presence of 10% wild type strain 30-84 in mixtures with QS or PZ mutants restored root colonization. These data demonstrate that QS and specifically PZ production are essential for biofilm formation by strain 30-84. This is a new role for PZs in the rhizosphere community.Strain 30-84 produces primarily phenazine-1-carboxylic acid (PCA) and 2-hydroxy-PCA (2-OH-PCA). We generated derivatives of strain 30-84 that produced the same total amount of PZs as the wild type but produced only PCA, or more efficiently converted PCA to 2-OH-PCA. These derivatives with altered PZ ratios differed from the wild type in initial attachment, biofilm architecture, and dispersal. Increased 2-OH-PCA production increased initial attachment, although both alterations resulted in thicker biofilms and reduced dispersal rates. Loss of 2-OH-PCA production resulted in a significant reduction in pathogen inhibition. My findings indicate that alterations in the endogenous ratios of PZs have wide-ranging effects on the biology of strain 30-84. I initiated studies to understand the mechanisms by which PZs affect surface attachment and biofilm development. Addition of PZs to metabolically inactivated cells improved adhesion compared to the inactive cells alone, suggesting that PZs may improve initial binding to surfaces. Results from whole genome transcription profiles of wild type strain 30-84 to a PZ mutant indicate that genes potentially involved in biofilm formation were up-regulated in the presence of PZs. These results provide initial evidence that PZs may modulate cell adhesion and biofilm formation via multiple mechanisms.

Quorum sensing is a cell communication mechanism used to coordinate group behaviour based on population density. A mathematical model of quorum sensing in bacterial biofilms is developed, consisting of a nonlinear diffusion reaction system describing the effects of a growing biofilm on bacterial quorum sensing behaviour. In numerical experiments, the influence of the hydrodynamic environment and nutrient conditions on biofilm growth and quorum sensing behaviour are studied, and flow-facilitated inter-colony communication and spatiotemporal quorum sensing induction patterns are observed. The model is extended to include an impact of quorum sensing on biofilm growth, through the explicit description of EPS, the protective biomass layer surrounding bacterial biofilm cells. The circumstances under which quorum sensing-regulated EPS production is a beneficial strategy for cells are identified. Biofilm colonies that use this strategy have lower cell populations than non-quorum sensing colonies, but may secure nutrients in a space-limited environment and outcompete neighbouring colonies.

Biofilms are complex and highly adapted communities of microorganisms found attached to surfaces. Among the best characterized infectious multi-cellular biofilms is the oral community known as dental plaque. Streptococcus mutans resides in the oral biofilm, and is one of the main causative agents of dental caries. Streptococci are known to monitor their population density using a peptide pheromone (CSP)/two component signalling system (ComDE) in a process classically known as quorum sensing (QS). Previous work in S. mutans has implicated the QS system in genetic competence, the stress response, bacteriocin production and biofilm formation. Our objective in this work was to thoroughly characterize the transcriptional and phenotypic response to CSP in S. mutans, and determine its role in biofilm formation. We have shown that the CSP pheromone is more than simply a QS signal, and is also an inducible ‘alarmone’ capable of communicating stress in the population. We have demonstrated that elevated concentrations of CSP such as those that occur during stress trigger autolysis in a fraction of the population. Importantly, we have shown that autolysis in S. mutans occurs via a novel mechanism of action: intracellular accumulation of a self-acting bacteriocin. We have also identified and characterized the autolysis immunity protein, which is differentially regulated from the bacteriocin to allow survival at low cell density. A second regulatory system was shown to govern expression of autolysis immunity in the absence of CSP signaling, and also contribute to the oxidative stress response in the biofilm. Finally, we present evidence that autolysis is involved in the release of DNA in the biofilm, which contributes to the architecture of the extracellular matrix and may provide a mechanism for the dissemination of fitness-enhancing genes under stress. Together, our data provides a mechanistic link between phenotypes previously ascribed to the CSP pheromone in S. mutans.

Biofilms are complex and highly adapted communities of microorganisms found attached to surfaces. Among the best characterized infectious multi-cellular biofilms is the oral community known as dental plaque. Streptococcus mutans resides in the oral biofilm, and is one of the main causative agents of dental caries. Streptococci are known to monitor their population density using a peptide pheromone (CSP)/two component signalling system (ComDE) in a process classically known as quorum sensing (QS). Previous work in S. mutans has implicated the QS system in genetic competence, the stress response, bacteriocin production and biofilm formation. Our objective in this work was to thoroughly characterize the transcriptional and phenotypic response to CSP in S. mutans, and determine its role in biofilm formation. We have shown that the CSP pheromone is more than simply a QS signal, and is also an inducible ‘alarmone’ capable of communicating stress in the population. We have demonstrated that elevated concentrations of CSP such as those that occur during stress trigger autolysis in a fraction of the population. Importantly, we have shown that autolysis in S. mutans occurs via a novel mechanism of action: intracellular accumulation of a self-acting bacteriocin. We have also identified and characterized the autolysis immunity protein, which is differentially regulated from the bacteriocin to allow survival at low cell density. A second regulatory system was shown to govern expression of autolysis immunity in the absence of CSP signaling, and also contribute to the oxidative stress response in the biofilm. Finally, we present evidence that autolysis is involved in the release of DNA in the biofilm, which contributes to the architecture of the extracellular matrix and may provide a mechanism for the dissemination of fitness-enhancing genes under stress. Together, our data provides a mechanistic link between phenotypes previously ascribed to the CSP pheromone in S. mutans.

Honors Project--Smith College, Northampton, Mass., 2010. / Includes bibliographical references.

Made available in DSpace on 2015-05-14T12:56:01Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 944601 bytes, checksum: c211a613a7ca088bee34c901c685111c (MD5) Previous issue date: 2011-12-16 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The objective of this research was to investigate the existence of antimicrobial plant compounds and anti-quorum sensing in oral biofilms. The plants used in this study were: Schinus terebinthifolius Raddi (aroeira da praia); Lippia sidoides Scham (alecrim pimenta); Abarema cochiliacarpos (Gomes) B. & J.W. Grimes (barbatimão); Anacardium occidentale Linn (cajueiro); Psidium guajava Linn (goiabeira); Tabebuia avellanedae Lorentz ex Griseb (ipê-roxo); Hymenaea courbaril L. (jatobá) e a Bowdichia virgiloides Kunth (sucupira), Anadenanthera macrocarpa Benth (angico), Abarema cochiliacarpos (Gomes) B. & J.W. Grimes (barbatimão), Ziziphus joazeiro Mart (joazeiro), Sida cordifolia L. (malva branca), Chenopodium ambrosioides L. (mastruz), Punica granatum L. (romã) and Zingiber officinale (gengibre). The extracts were submitted in good yield to the partition with solvents in order of increasing polarity, hexane, dichloromethane and ethyl acetate. The antimicrobial activity was determined by the techniques of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) and use the model QSIs (Quorum Sensing Inhibitor System) proposed by Rasmussen and colleagues (2005). The results proved promising for the development of phytomedicines. The anti-quorun sensing was significant for Abarema cochiliacarpos (Gomes) B. & JW Grimes (barbatimão), Anacardium occidentale Linn (cajueiro), Psidium guajava Linn (goiabeira) and the fruit of Hymenaea courbaril L. (Jatobá). The results support the hypothesis that extracts and partitions of some medicinal plants have anti-quorum sensing activity as a mechanism of antimicrobial activity. / O objetivo desta pesquisa foi investigar a existência de compostos vegetais que apresentem atividade antimicrobiana e anti-quorum sensing em micro-organismos formadores de biofilmes bucais. As plantas utilizadas nesta pesquisa foram: Schinus terebinthifolius Raddi (aroeira da praia); Lippia sidoides Scham (alecrim pimenta); Abarema cochiliacarpos (Gomes) B. & J.W. Grimes (barbatimão); Anacardium occidentale Linn (cajueiro); Psidium guajava Linn (goiabeira); Tabebuia avellanedae Lorentz ex Griseb (ipê-roxo); Hymenaea courbaril L. (jatobá) e a Bowdichia virgiloides Kunth (sucupira), Anadenanthera macrocarpa Benth (angico), Abarema cochiliacarpos (Gomes) B. & J.W. Grimes (barbatimão), Ziziphus joazeiro Mart (joazeiro), Sida cordifolia L. (malva branca), Chenopodium ambrosioides L. (mastruz), Punica granatum L. (romã) e Zingiber officinale (gengibre). Os extratos de Schinus terebinthifolius Raddi (aroeira da praia); Lippia sidoides Scham (alecrim pimenta); Abarema cochiliacarpos (Gomes) B. & J.W. Grimes (barbatimão); Anacardium occidentale Linn (cajueiro); Psidium guajava Linn (goiabeira); Tabebuia avellanedae Lorentz ex Griseb (ipê-roxo); Hymenaea courbaril L. (jatobá) e a Bowdichia virgiloides Kunth (sucupira), Abarema cochiliacarpos (Gomes) B. & J.W. Grimes (barbatimão), Ziziphus joazeiro Mart (joazeiro) foram submetidos à partição com solventes em ordem crescente de polaridade, hexano, diclorometano e acetato de etila. A atividade antimicrobiana foi determinada pela técnica da microdiluição em caldo e utilizou o modelo de QSIs (sistema Inibidor de Quorum Sensing) proposto por Rasmussen e colaboradores (2005). Os resultados obtidos revelaram-se promissores para o desenvolvimento de fitomedicamentos. A atividade anti-quorun sensing foi significante para Abarema cochiliacarpos (Gomes) B. & J.W. Grimes (barbatimão); Anacardium occidentale Linn (cajueiro); Psidium guajava Linn (goiabeira); e o fruto do Hymenaea courbaril L. (jatobá). Os resultados suportam a hipótese que extratos e partições de algumas plantas medicinais possuem atividade anti-quorum sensing como mecanismo de atividade antimicrobiana.

Submitted by Reginaldo Soares de Freitas (reginaldo.freitas@ufv.br) on 2017-04-12T12:06:06Z No. of bitstreams: 1 texto completo.pdf: 2872306 bytes, checksum: 4524f2b2277b755bcb654e67c07705cb (MD5) / Made available in DSpace on 2017-04-12T12:06:06Z (GMT). No. of bitstreams: 1 texto completo.pdf: 2872306 bytes, checksum: 4524f2b2277b755bcb654e67c07705cb (MD5) Previous issue date: 2014-02-27 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Quorum sensing (QS) é um mecanismo de comunicação entre células microbianas mediado por moléculas sinalizadoras, denominadas autoindutores (AIs). Embora não produza o AI-1, que são moléculas de acil homoserina lactonas (AHLs), Salmonella responde às AHLs produzidas por outros micro-organismos por sintetizar a proteína de resposta ao AI-1, denominada de SdiA. Contudo, o gene sdiA ainda não tem sua função estabelecida em Salmonella. Neste trabalho foi avaliada a influência de n-dodecanoil homoserina lactona (C12-AHL) e de furanonas, análogas de AHLs, sobre o crescimento, motilidade, adesão, formação de biofilme em poliestireno e síntese de proteínas por Salmonella Enteritidis PT4 em anaerobiose. Na presença ou na ausência de 50 nM de C12-AHL e, ou de furanonas, o crescimento, a motilidade em massa ou por espalhamento (swarming) e a motilidade por contração ou por espasmos (twitching) deste patógeno não foram alterados. Conforme verificado pela determinação do potencial de adesão e formação de biofilme em microplacas, enumeração de células aderidas e microscopia confocal de epifluorescência, foi constatada que a presença de C12-AHL induziu a formação de biofilme em superfície de poliestireno após 36 h de cultivo. Além disso, constatou-se o efeito antagonista das furanonas sobre a formação de biofilme nas mesmas condições. As proteínas sintetizadas por Salmonella Enteritidis PT4 na presença e na ausência de 50 nM de C12-AHL após 7 h de cultivo em anaerobiose foram extraídas, separadas por eletroforese bidimensional e preditas por comparação do ponto isoelétrico e massa molecular disponíveis em bancos de dados. Foi verificado que, na presença de C12-AHL houve síntese de maior número de proteínas, sendo que muitas delas estão relacionadas com o sistema QS e com a formação de biofilme. Os resultados obtidos evidenciam que a presença de C12-AHL exógena influencia o metabolismo de Salmonella Enteritidis PT4, cultivada em condições de anaerobiose e indicam uma nova linha de investigação que pode contribuir para o esclarecimento dos mecanismos relacionados com a patogenicidade de Salmonella. / Quorum sensing (QS) is a communication mechanism employed by microbial cells by means of signaling molecules, called autoinducers (AIs). Even though Salmonella does not produce AI-1, also known as acyl homoserine lactones (AHLs), it responds to AHLs produced by other microorganisms through an AI-1 response protein called SdiA. However, the sdiA gene has not established function in Salmonella. In this work the influence of n-dodecanoyl homoserine lactone (C12-AHL) and furanones, AHLs analogs, was assessed on the growth, motility, adhesion, biofilm formation on polystyrene and protein synthesis by Salmonella Enteritidis PT4 under anaerobic environment. In the presence or absence of 50 nM C12-AHL and/or furanones, growth, swarming and twitching motilities of this pathogen were not altered. We found that the presence of C12-AHL induced biofilm formation after 36 h of cultivation as determined by the adhesion potential and biofilm formation on microplates, the enumeration of adhered cells and by confocal epifluorescence microscopy. The results were evaluated by determining the adhesion potential and biofilm formation on microplates, the enumeration of adhered cells and by confocal epifluorescence microscopy. The antagonistic effect of furanones was also determined by using the same conditions. The proteins synthesized by Salmonella Enteritidis PT4 in the presence or absence of 50 nM C12-AHL, after 7 h of incubation in anaerobic conditions, were extracted, separated by bidimensional electrophoresis and predicted on the basis of their isoelectric point and molecular masses available on databases. In the presence of C12-AHL, there was an increased synthesis of proteins, with the majority related to the QS system and biofilm formation. The results demonstrate that the presence of exogenous C12-AHL affects the metabolism of Salmonella Enteritidis PT4, cultivated under anaerobic conditions and point towards a new line of investigation which could contribute to the understanding of the pathogenicity mechanisms in Salmonella.

Submitted by Nathália Faria da Silva (nathaliafsilva.ufv@gmail.com) on 2017-06-19T12:02:55Z No. of bitstreams: 1 texto completo.pdf: 1276596 bytes, checksum: e320e36eac8ecfc4d887e0c9f618329e (MD5) / Made available in DSpace on 2017-06-19T12:02:55Z (GMT). No. of bitstreams: 1 texto completo.pdf: 1276596 bytes, checksum: e320e36eac8ecfc4d887e0c9f618329e (MD5) Previous issue date: 2005-04-29 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Por meio de um mecanismo denominado quorum sensing, diversas bactérias podem comunicar-se umas com as outras e coordenar a expressão gênica de acordo com a densidade populacional de forma semelhante aos organismos multicelulares. Este trabalho teve como objetivo investigar a produção de moléculas sinalizadoras de quorum, conhecidas como homoserinas lactonas aciladas (AHLs), por bactérias psicrotróficas proteolíticas isoladas de leite cru refrigerado. Verificou-se também se a atividade proteolítica de Pseudomonas fluorescens 07A é dependente da densidade populacional. Foram avaliados 53 isolados psicrotróficos proteolíticos e sete estirpes ATCC quanto à produção de AHL em meio sólido utilizando as bactérias Agrobacterium tumefaciens A136, Chromobacterium violaceum CV026 e Escherichia coli pSB403 como sistemas monitores ou sensores da presença de AHL. A produção de moléculas sinalizadoras foi freqüente entre os isolados psicrotróficos proteolíticos, constatando-se que, aproximadamente, 89 % das estirpes avaliadas foram produtoras de AHL. A extração de moléculas autoindutoras foi realizada em dois isolados, Serratia liquefaciens 016 e P. fluorescens 07A e, a presença de AHL nos extratos foi confirmada pelas estirpes monitoras C. violaceum CV026 e A. tumefaciens KYC55, respectivamente. A produção de AHL pelo isolado P. fluorescens 07A foi determinada em meio de cultivo à base de triptona suplementado com 0,25 % de cálcio (TYEP + CaCl 2 0,25 %) e em meio de constituição mínima (MMS), utilizando a estirpe monitora A. tumefaciens KYC55. Em meio TYEP + CaCl 2 0,25 % a produção de AHL(s) alcançou o máximo ao final da fase logarítmica, quando a população de P. fluorescens 07A foi da ordem de 10 9 UFC mL -1 . Esta mesma população resultou em uma concentração de AHL(s) cerca de três vezes menor no MMS. Os resultados indicaram que a produção de AHL pode estar sob controle de autoindução. O crescimento e a atividade proteolítica do isolado 07A foram avaliados em seis meios de cultivo. Não foi observada diferença no crescimento do isolado nos diferentes meios, exceto no MMS, onde a estirpe cresceu de forma mais lenta. Entretanto, a produção de proteases foi maior em meio TYEP + CaCl 2 0,25 % e em leite desnatado reconstituído (LDR 12 %). A detecção da atividade proteolítica só foi possível quando a cultura atingiu concentração populacional elevada em todos os meios avaliados, excetuando-se no meio MMS, quando a atividade proteolítica foi detectada em uma densidade populacional 10 vezes menor. Nos meios a base de triptona o pH aumentou para valores próximos a 8,5. Meios contendo íons Ca +2 apresentaram grande fluorescência ao final do período de incubação, indicando maior produção do sideróforo pioverdina. A adição de duas AHLs sintéticas não afetou o crescimento e a atividade proteolítica da estirpe P. fluorescens 07A em meio TYEP + CaCl 2 0,25 %. Quando comparada ao controle, a atividade proteolítica específica de P. fluorescens 07A não foi afetada significativamente, ao nível de 5% de probabilidade, pela suplementação do meio TYEP + CaCl 2 0,25 % com extrato de AHL obtido da própria estirpe. O quorum sensing pode não estar regulando a atividade proteolítica em P. fluorescens 07A. / In a process called quorum sensing, a range of bacterial cells communicate and coordinate the expression of multiple phenotypes according to cell density. This work aimed to evaluate the production of signaling molecules known as N-acyl homoserine lactone (AHL) by psychrotrophic proteolytic bacteria isolated from raw milk as well as to verify if the proteolytic activity of Pseudomonas fluorescens 07A is under the control of quorum sensing. Fifty three psychrotrophic isolates and seven ATCC cultures were screened for the production of AHL in agar systems using the monitor strains Agrobacterium tumefaciens A136, Chromobacterium violaceum CV026 and Escherichia coli pSB403. It was verified that almost 89 % of the psychrotrophic isolates tested elicited a positive response in at least one of the monitor strains of AHL. Signaling molecules were extracted from Serratia liquefaciens 016 and P. fluorescens 07A and the presence of AHL was detected with the monitor strains C. violaceum CV026 and A. tumefaciens KYC55, respectively. The production of AHL by the strain 07A was evaluated in a complex medium (TYEP + CaCl 2 0,25 %) and a defined medium (MMS). A. tumefaciens KYC55 was used to quantify AHL in this assay. In TYEP + CaCl 2 0,25 % the AHL concentration reached a maximum value at the end of xvthe stationary phase, when the population of P. fluorescens 07A was 10 9 CFU mL -1 . When MMS medium was used, a similar cell population was achieved, but the AHL concentration was three fold lower. The results indicated that AHL production by P. fluorescens 07A could be regulated by autoinduction. The growth and proteolytic activity of the P. fluorescens 07A were monitored in six different media. It was not observed difference in the specific growth rate of the strain in the media tested, except in MMS, where it grew slower. On the other hand, the protease activity was higher in the complex medium (TYEP + CaCl 2 0,25 %) and reconstituted skimmed milk (LDR 12 %). Proteolytic activity was only detected when the culture reached high population densities (10 8 UFC mL -1 ) in all media evaluated. In MMS, proteolytic activity was detected at lower population densities (10 7 UFC ml -1 ). An increase in pH values was observed during cultivation in media containing tryptone. The emission of fluorescence was intensified in media containing calcium. The addition of two synthetic AHLs did not affect the growth rates and the proteolytic activity of the strain 07A in TYEP + CaCl 2 0,25 %. Furthermore, the addition of AHL extracted from the culture did not increase the specific proteolytic activity in TYEP + CaCl 2 0,25 %. These results indicate that quorum sensing may not have a direct role in the regulation of protease production in P. fluorescens 07A.