In Escherichia coli, Autoinducer-2 (AI-2) plays a pivotal role in biofilm formation by coordinating the expression of genes involved in extracellular matrix production and cell adhesion. AI-2, particularly in the form of (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran, functions as a quorum sensing molecule that allows bacteria to sense population density and regulate biofilm-related behaviors. At high concentrations, (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran is spontaneously converted to (4S)-4,5-dihydroxy-2,3-pentanedione (DPD), which is then phosphorylated by L-xylulose/3-keto-L-gulonate kinase to form Phospho-AI-2. Phospho-AI-2 inhibits the transcriptional regulator LsrR, relieving its repression of the lsr operon. This enables the transcription of genes encoding the Autoinducer 2 import ATP-binding protein LsrA, Autoinducer 2 import system permease proteins LsrC and LsrD, and the Autoinducer 2-binding protein LsrB, which collectively facilitate the uptake of more (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran into the cell. Once inside, LsrG and LsrF convert Phospho-AI-2 into 3-hydroxy-2,4-pentanedione 5-phosphate (3H2P5P), which enters the pentose phosphate pathway (PPP) for further metabolism. Simultaneously, under conditions of low glucose concentration, the cAMP-CRP complex is activated, which in turn activates the csg operon. The csg operon produces CsgD, CsgE, CsgF, and CsgG, with CsgD acting as a master regulator that activates the csgBAC operon (responsible for curli fiber production) and the yhjr-bcsqabzc operon (responsible for cellulose biosynthesis). Curli fibers and cellulose are critical components of the biofilm extracellular matrix, providing structural integrity and facilitating cell-surface and cell-cell adhesion. Thus, through the integration of AI-2 signaling, metabolic regulation, and transcriptional control, E. coli coordinates the production of curli fibers and cellulose, ultimately leading to the formation of robust biofilms.

Quorum sensing - Autoinducer-2 Biofilm Regulation References