(451af) Escherichia Coli Autoinducer 2 Uptake Is Induced by Intracellular Factors Such as Luxs Transcription Rate but Not Necessarily by the Cell Density: Analysis of the Network Using Bifurcation Analysis and Stochastic Approach

Cross-species, quorum-sensing signal AI-2 uptake in E. coli was recently elucidated (J. Bacterio.,187:2066-2076) and it is known to be controlled by the lsr operon in a manner analogous to Salmonella Thyphimurium (Mol. Microbiol. 42:777-793). Previously, it has been found that this signal induces genes related to motility (Mol. Microbiol. 43:809-821), pathogenesis (J. Bacteriol.183: 5187-5197), and biofilm formation (J. Bacteriol.188: 305-316). Nevertheless, the AI-2 synthase, LuxS, is not directly or indirectly induced by the presence of the signal, differing from other prokaryotes such as Pseudomonas aeruginosa whose signal synthases (LasI and RhlI) are positively induced by the autoinducer signal (J. Bacteriol. 179:3127-3132). This difference in the topology of the network raises questions regarding the actual role of the lsr operon and the factors that induce the network. Considering that this kind of cellular response implies a bistable, or hysteretic form of relationship between the signal concentration and the induction of several genes (Biosystems, In press), we modeled the lsr operon in E. coli and carried out a novel method in studying the dynamic quorum sensing network: bifurcation analysis and stochastic modeling. it was found that the concentration of the signal is not the only factor that could switch the network but also luxS translation rate. However, this control is carried out by modifying the ?noise? in gene expression when augmenting the translation rate since we did not find bifurcation points in the network. luxS transcription rate controls the on-off splitting and the dispersion of the population. Catabolic repression, which is known to influence uptake and synthesis of AI-2, also affects the stability of the network primarily through controlling luxS and the stress sigma factor, RpoS, which negatively regulates the lsr operon was found to control the network mainly trough negatively controlling luxS rate. These results corroborate (Microbiol. 148: 909-922) that AI-2 may not play a role as a quorum sensing signal but as an indicator of the physiological state of the cell.