ORIGINAL ARTICLE Indole cell signaling occurs primarily at low temperatures in Escherichia coli

We have shown that the quorum-sensing signals acylhomoserine lactones, autoinducer-2 (AI-2) and indole influence the biofilm formation of Escherichia coli. Here, we investigate how the environment, that is, temperature, affects indole and AI-2 signaling in E. coli. We show in biofilms that indole addition leads to more extensive differential gene expression at 30 1C (186 genes) than at 37 1C (59 genes), that indole reduces biofilm formation (without affecting growth) more significantly at 25 and 30 1C than at 37 1C and that the effect is associated with the quorum-sensing protein SdiA. The addition of indole at 30 1C compared to 37 1C most significantly repressed genes involved in uridine monophosphate (UMP) biosynthesis (carAB, pyrLBI, pyrC, pyrD, pyrF and upp) and uracil transport (uraA). These uracil-related genes are also repressed at 30 1C by SdiA, which confirms SdiA is involved in indole signaling. Also, compared to 37 1C, indole more significantly decreased flagella-related qseB, flhD and fliA promoter activity, enhanced antibiotic resistance and inhibited cell division at 30 1C. In contrast to indole and SdiA, the addition of (S)-4,5-dihydroxy-2,3-pentanedione (the AI-2 precursor) leads to more extensive differential gene expression at 37 1C (63 genes) than at 30 1C (11 genes), and, rather than repressing UMP synthesis genes, AI-2 induces them at 37 1C (but not at 30 1C). Also, the addition of AI-2 induces the transcription of virulence genes in enterohemorrhagic E. coli O157:H7 at 37 1C but not at 30 1C. Hence, cell signals cause diverse responses at different temperatures, and indole- and AI-2-based signaling are intertwined