N-acyl-L-homoserine lactone signal interception by Escherichia coli.

N-acyl-L-homoserine lactone (AHL) mediated quorum sensing is a widespread communication system in gram-negative bacteria which regulates a wide range of target genes in a cell density-dependent manner. Although Escherichia coli is not capable of synthesizing AHL molecules because it lacks an AHL synthase encoding gene, it does produce a predicted AHL receptor of the LuxR family, named SdiA. In this work, we used a promoter trap library to screen for E. coli MG1655 promoters whose expression was affected by synthetic N-hexanoyl-L-homoserine lactone (C6-HSL), and we identified six upregulated and nine downregulated promoters, which also responded to synthetic 3-oxo-N-hexanoyl-L-homoserine lactone (3-oxo-C6-HSL). The AHL responsiveness of these promoters was eliminated by knock-out of sdiA, and was temperature dependent, since the identified promoters showed a response at 30 degrees C but not, or only very weakly at 37 degrees C. In addition, in line with the observed induction of gadA encoding a glutamate decarboxylase, we could demonstrate an increased acid tolerance of E. coli upon exposure to C6-HSL. In conclusion, our work shows that E. coli has the capacity to alter its pattern of gene expression and its phenotypical properties in response to AHLs by means of the AHL responsive transcriptional regulator SdiA.