Study On The Role Of Quorum Sensing In Pollutants Biodegradation

Quorum sensing (QS) is a widely conserved cell-to-cell communication system in micro-organisms especially for bacteria. This QS process enables bacteria to cooperatively control behaviour at population level and, therefore, effectively complete the processes which are unproductive when undertaken by an individual bacterium. Acylated homoserine lactone (AHL) mediated QS is wide-spread in Gram-negative bacteria and plays important roles in many biological processes.Biodegradation by micro-organisms especially bacteria is crucial for pollutant treatment. But, the role of QS in pollutants biodegradation is still unclear. There is only one related report which indicated AHL addition could change the microbial community and improve the stability of activated aludge. But no information about AHL production or QS role in pollutants biodegradation was available. In this work, the role of QS in pollutant biodegradation was investigated in a model system. This work provided us important information to re-visit the regulation process in pollutants biodegradation from a population perspective and, also is useful for effective biodegradation strategy development based on this new perspective.At first, a simple model system based on aromatic pollutants biodegradation and AHL QS system was developed. Pseudomonas aeruginosa CGMCC 1.860 was screened out due to the presence of AHL QS system and its good aromatics biodegradation ability. Further analysis of its aromatics biodegradation was also conducted. In addition, the Rhl QS system (rhlIR cluster) was confirmed in this strain and it could synthesis short-chain AHL in LB medium as confirmed by biosensor assay. This developed model system could be useful for investigation of the regulation on aromatics biodegradation by Rhl QS system. Furthermore, mutants ofâ–³rhlI,â–³rhlIR were constructed by gene in-frame deletion and, therefore facilitate to unveil the RhI regulation at molecular level.Quantification method for AHL is still severely limited. Until now, there is no report about method for AHL quantification in toxic environmental sample. By genetic modification, an isolated aromatic degrader was constructed to be a BHL biosensor. By using the pigment production which responded to exogenous BHL as biosensor output, BHL quantification in samples was simply achieved with spectrophotometric detection. The sensitivity for BHL detection was improved by using this developed biosensor compared with previous methods. In addition, the biosensor output showed nice interferences resistance, good stability and reliability. It was also successfully applied to environmental samples even without extraction. The developed method is useful for quantitative investigation of the role of QS signals in environmental processes.Recently, strains which can synthesis AHL in rich medium were isolated from pollutants treatment processes. However, there is still no evidence to indicate AHL could be synthesized during pollutants biodegradation and no report about the effect of QS system on pollutants biodegradation ability of bacteria. By biosensor detection, TLC and HPLC-APCI-MS/MS analysis, BHL and HHL were identified during aromatics biodegradation. In addition, AHL crude extract addition improved phenol biodegradation by P. aeruginosa CGMCC 1.860. But crude extract without short chain AHL did not significantly affect its biodegradation ability. The results indicated that AHL could enhance phenol biodegradation by P. aeruginosa. In addition, the aromatics degradation ability forâ–³rhlI andâ–³rhlIR was significantly reduced compared with the wild type strain. By BHL addition inâ–³rhlI or plasmid complementation inâ–³rhlIR, their degradation ability could be restored. The results strongly indicated Rhl QS system could improve the aromatics biodegradation ability of P. aeruginosa.Furthermore, the regulation by Rhl QS system on aromatics biodegradation was studied at enzyme and transcription level. Using phenol biodegradation as a typical example, the transcription level of nahR and nahH was up-regulated by Rhl QS system. The activity of catechol 2,3-dioxygense was also positively regulated by Rhl QS system. The results suggested Rhl QS system could activate the catechol meta-cleavage pathway and therefore improved the phenol biodegradation by P. aeruginosa CGMCC 1.860. Interestingly, our results indicated that the transcription level of phenol hydroxylase gene cluster (phcR, phcP) and global regulators (relA, spoT, crc) was down-regulated by Rhl QS system at late logarithmic phase. This finding provided important information for the mechanism investigation.In this work, a biosensing system for BHL quantification in environmental sample was developed; AHL production during aromatics biodegradation was identified; the regulation of Rhl QS system on aromatics was investigated; The information and tool provide a good basis for further investigation of QS regulation on aromatics biodegradation. This study is also considered useful for manipulation aromatics biodegradation at population perspective. The idea may be useful for investigation of regulation on other pollutants biodegradation by other QS systems.