AUTOINDUCER-2 (AI-2) MEDIATED QUORUM SENSING IN ESCHERICHIA COLI

Abstract

Bacteria have evolved complex genetic circuits to regulate their physiological activities and behaviors in response to extracellular signals. In a process termed "quorum sensing", or density-dependent gene regulation, bacteria produce, release and respond to certain signaling molecules termed autoinducers. The bacterial autoinducer-2 (AI-2) has received intense interest recently because the gene for its synthesis, luxS, is common in a large number of Gram-negative and Gram-positive bacterial species. In this study, the luxS controlled genes were identified in Escherichia coli K12 strain under two different growth conditions using DNA microarrays. Deletion of the luxS was shown to affect expression of genes involved in AI-2 transport (the lsr operon) and methionine biosynthesis (metE), and to a lesser degree those involved in methyl transfer, iron uptake, resistance to oxidative stress, utilization of various carbon sources, and virulence. The effects of glucose on extracellular AI-2 level were investigated further. It was shown that both AI-2 synthesis and uptake in Escherichia coli are subject to catabolite repression through the cAMP-CRP complex. This complex directly stimulates transcription of the lsr (luxS regulated) operon and indirectly represses luxS expression. Specifically, cAMP-CRP is shown to bind to a CRP binding site located in the upstream region of the lsr promoter and works with LsrR repressor to regulate AI-2 uptake. This study, for the first time, has shown that quorum sensing regulates specific activities in E. coli K12, and has elucidated regulatory mechanisms for AI-2 biosynthesis and transport in this organism. With a better understanding of AI-2/luxS mediated gene regulation, we may be able to develop strategies for harnessing AI-2 quorum sensing for our advantage in bioreactor studies and ultimately in control of the bacterial pathogenicity.