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

Biofilm formation is stimulated by different stress-related physiological and environmental conditions. In Salmonella and Escherichia coli, 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 csgD transcription in both Salmonella and E. coli. Recently, MlrB, a SPI-2-encoded MlrA-like regulator that counteracts MlrA by repressing csgD transcription and biofilm formation inside macrophages was identified. Here, we characterize STM1266, a Salmonella-specific MlrA-like regulator, recently renamed BrfS. In contrast to mlrA, brfS 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 brfS affects extracellular matrix production. Using transcriptomics, we uncovered genes under BrfS control relevant for biofilm formation such as csgB and bapA. Transcriptional analysis of these genes in mutants lacking brfS, csgD or both, indicates that BrfS controls curli biosynthesis both in a CsgD-dependent and independent manner. By contrast, at low temperatures, bapA transcription depends only on BrfS, and neither deletion of csgD nor of mlrA modify its expression. Based on these results, we propose that BrfS contributes to Salmonella persistence in the environment, where the pathogen encounters low temperatures and nutrient limitation.