Functional Studies Of Quorum Sensing Autoinducer Degradation Gene AiiA From Bacillus Thuringiensis

Bacterial cells sense their population density through autoinducers (AIs)synthesized and secreted by bacteria. AIs will trigger the expression of particulargenes when population density reaches a threshold. This type of cell-cellcommunication system is termed quorum sensing (QS). N-acyl-homoserine lactones(AHLs) are one type of the autoinducers, and they are found in many Gram-negativebacterial pathogens. AiiA, an AHL-lactonase encoded by aiiA gene from somebacteria, can degrade the QS autoinducers by hydrolyzing the lactone ring of AHLsand quench the QS system. This thesis is about the expression and functional studiesof the aiiA gene in human pathogenic bacterium (Pseudomonas aeruginosa) andplant (tobacco), and includes two parts:Part one is about the expression of the aiiA gene from Bacillus thuringiensis inPseudomonas aeruginosa strain PAO1 and two clinical strains. The reduced amountof AHLs in PAO1 was also correlated with the production of several virulencefactors such as elastase and pyocyanin. AiiA also influenced bacterial swarmingmotility. Most importantly, our studies indicated that aiiA played significant roles inP. aeruginosa biofilm formation and dispersion, as observed by the differences ofthe biofilm formation on liquid and solid surfaces, and biofilm structures underscanning electron microscope. In addition, the expression of the aiiA gene increasedthe sensitivity to several antibiotics. These results showed that the expression of this gene can not only decrease the expression of virulence factors and the formation ofbiofilm, but also decrease the antibiotics resistance level of P. aeruginosa.Part two is about the expression of the aiiA gene in tobacco and the analysis ofbroad-spectrum disease resistance to different pathogens. Inoculation of transgenictobacco leaves with bacterium (Erwinia carotovora) and fungus (Sclerotiniasclerotiorum) showed enhanced disease resistance markedly. Plants expressing AiiAquenched pathogen quorum-sensing signaling molecule and showed significantlyenhanced resistance to E. carotovora infection. Furthermore, total oxalic acid wasanalyzed in transgenic and control plants by HPLC after inoculation with S.sclerotiorum. Our results showed that the mechanism of transgenic tobacco plantsresistance to this phytopathogenic fungus might due to degradation of oxalic acid, akey factor in the process of infection.