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Quorum sensing (QS) is a phenomenon present in many gram-negative bacteriathat allows bacterial communication and controlled expression of a large suite ofgenes through quorum sensing signals -N-acyl homoserin lactones (AHLs).Generally, QS can occur between diverse species as well as within a singlebacterial species, and can control a host of different processes, essentially servingas a simple communication network. A variety of different molecules can be usedas signals. Common classes of signaling molecules are oligopeptides ingram-positive, AHL in gram-negative bacteria and a family of autoinducers knownas autoinducers-2 in both gram-negative and gram-positive bacteria.
Bacteria that use quorum sensing constantly produce and secrete certainsignaling molecules-autoinducers or pheromones. These bacteria also have areceptor that can specifically detect the signaling molecule (called inducer). Whenthe inducer binds the receptor, it activates transcription of certain genes, includingthose for inducer synthesis. There is a low likelihood of a bacterium detecting itsown secreted inducer. Thus, in order for gene transcription to be activated, the cellmust encounter signaling molecules secreted by other cells in its environment.When only a few other bacteria of the same kind are in the vicinity, diffusionreduces the concentration of the inducer in the surrounding medium to almost zero,so the bacteria produce little inducer. However, as the population grows theconcentration of the inducer passes a threshold, causing more inducer to besynthesized. This forms a positive feedback loop, and the receptor becomes fullyactivated. Activation of the receptor induces the up regulation of other specificgenes, 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 aeruginosaand Serratia marcescens, two reporters E.coli (pHLOl) and E.coli strain (pHL02),were genetically engineered. These utilize a simple, yet effective and inexpensivedetection method based on lasR/lasl and thIR/rhll quorum sensing systems. ForE.coli (pHL01), the biosensor strain responded to the presence of long chainsignals at detection limit 5xl0-8M. The result of detecting AHLs from extractingsof Pseudomonas aeruginosa ATCC9027 showed that this reporter strain wassensitive with AHLs from extractions. For the second reporter strain E.coli(pHL02), this reporter strain responded to the presence of short-chain signals at adetection limit of 5xl0-8M. This was found to be comparable to AHLs extractionfrom Serratia marcescens H30. We examined this ability of this reporter strain fordetection of AHLs from extractions of Pseudomonas aeruginosa ATCC9027, andthe results demonstrated that the reporter strain could be applicable to a wide rangeof 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 reporterstrains were at pH7.20f LB cultures, cultures incubating temperature 370C, andCultures incubating time 18h.
The application of E.coli (pHL02) for studying the effect of some factors onAHL production from P. aeruginosa and S. marcescens showed that AHLproduction was effected not only by strains, but also by surrounding condition suchas medium, temperature, pH, carbon sources. The suitable conditions for producingAHL production from P. aeruginosa and S. marcescens were in SM medium, atincubating temperature 370C for P. aeruginosa and 300C for S. marcescens. Thehighest yields of AHL production and biomass were observed at pH7.0 with theP.aeruginosa strain, while media adjusted with pH 6.5 seemed to be suitable forS.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 notonly for growing but also for producing AHL of two strains S.marcescens andP.aeruginosa, respectively.
Key Words: Serratia marcescens;Pseudomonas aeruginosa quorum sensing;N-acy1 homoserin lactones;detection;biosensor