| Quorum sensing (QS) is a phenomenon present in many gram-negative bacteria that allows bacterial communication and controlled expression of a large suite of genes through quorum sensing signals-N-acyl homoserin lactones (AHLs). Generally, QS can occur between diverse species as well as within a single bacterial species, and can control a host of different processes, essentially serving as a simple communication network. A variety of different molecules can be used as signals. Common classes of signaling molecules are oligopeptides in gram-positive, AHL in gram-negative bacteria and a family of autoinducers known as autoinducers-2 in both gram-negative and gram-positive bacteria.Bacteria that use quorum sensing constantly produce and secrete certain signaling molecules-autoinducers or pheromones. These bacteria also have a receptor that can specifically detect the signaling molecule (called inducer). When the inducer binds the receptor, it activates transcription of certain genes, including those for inducer synthesis. There is a low likelihood of a bacterium detecting its own secreted inducer. Thus, in order for gene transcription to be activated, the cell must encounter signaling molecules secreted by other cells in its environment. When only a few other bacteria of the same kind are in the vicinity, diffusion reduces the concentration of the inducer in the surrounding medium to almost zero, so the bacteria produce little inducer. However, as the population grows the concentration of the inducer passes a threshold, causing more inducer to be synthesized. This forms a positive feedback loop, and the receptor becomes fully activated. Activation of the receptor induces the up regulation of other specific genes, causing all of the cells to begin transcription at approximately the same time. This coordinated behavior of bacterial cells can be useful in a variety of situations.In this study, in order to investigate quorum sensing in Pseudomonas aeruginosa and Serratia marcescens, two reporters E.coli (pHL01) and E.coli strain (pHL02), were genetically engineered. These utilize a simple, yet effective and inexpensive detection method based on lasR/lasI and rhlR/rhll quorum sensing systems. For E.coli (pHL01), the biosensor strain responded to the presence of long chain signals at detection limit 5×10-8M. The result of detecting AHLs from extractings of Pseudomonas aeruginosa ATCC9027 showed that this reporter strain was sensitive with AHLs from extractions. For the second reporter strain E.coli (pHL02), this reporter strain responded to the presence of short-chain signals at a detection limit of 5×10-8M. This was found to be comparable to AHLs extraction from Serratia marcescens H30. We examined this ability of this reporter strain for detection of AHLs from extractions of Pseudomonas aeruginosa ATCC9027, and the results demonstrated that the reporter strain could be applicable to a wide range of gram-negative bacteria producing short-chain AHLs.The studies about influencing of some factors on the B-galactosidase activity assay of reporter strain showed that the suitable conditions to apply the reporter strains were at pH 7.2 of LB cultures, culture's incubating temperature 37℃, and Culture's incubating time 18h.The application of E.coli (pHL02) for studying the effect of some factors on AHL production from P. aeruginosa and S. marcescens showed that AHL production was effected not only by strains, but also by surrounding condition such as medium, temperature, pH, carbon sources. The suitable conditions for producing AHL production from P. aeruginosa and S. marcescens were in SM medium, at incubating temperature 37℃for P. aeruginosa and 30℃for S. marcescens. The highest yields of AHL production and biomass were observed at pH 7.0 with the P.aeruginosa strain, while media adjusted with pH 6.5 seemed to be suitable for S.marcescens to grow as well as produce AHL. In the study the Effect of carbon sources on AHL production, Glucose and Sucrose were suitable carbon sources not only for growing but also for producing AHL of two strains S.marcescens and P.aeruginosa, respectively.
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