Integration of BrfS into the biofilm-controlling cascade promotes sessile growth at low temperatures.

Biofilm formation is stimulated by different stress-related physiological and environmental conditions. In and , curli fibers and phosphoethanolamine-cellulose are the major extracellular components of biofilms. The production of both is under the control of CsgD, a transcriptional regulator whose expression is modulated by a number of factors responding to different signals. The atypical MerR-like regulator MlrA is key in the activation of transcription in both and . Recently, MlrB, a SPI-2-encoded MlrA-like regulator that counteracts MlrA by repressing transcription and biofilm formation inside macrophages was identified. Here, we characterize STM1266, a -specific MlrA-like regulator, recently renamed BrfS. In contrast to , transcription increases in minimal growth media and at 20 °C, a temperature not commonly tested in laboratories. Under these conditions, as well as in salt-limited rich medium, deletion or overexpression of affects extracellular matrix production. Using transcriptomics, we uncovered genes under BrfS control relevant for biofilm formation such as and . Transcriptional analysis of these genes in mutants lacking , or both, indicates that BrfS controls curli biosynthesis both in a CsgD-dependent and independent manner. By contrast, at low temperatures, transcription depends only on BrfS, and neither deletion of nor of modify its expression. Based on these results, we propose that BrfS contributes to persistence in the environment, where the pathogen encounters low temperatures and nutrient limitation.