Integrated regulation of acetoin fermentation by quorum sensing and pH in Serratia plymuthica RVH1

During fermentation of sugars, a number of bacterial species are able to switch from mixed acids to acetoin and 2,3-butanediol production in order to avoid lethal acidification of their environment, although the regulation of this switch is only poorly understood. In this study, we report the identification of the budAB structural operon involved in acetoin production in Serratia plymuthica RVH1, and its activation by a LysR-type regulator encoded by budR immediately upstream of this operon. In addition, the regulation of budR transcription was elucidated and found to be subject to negative control by BudR itself, and to positive control by external stimuli such as N-(3-oxohexanoyl)-L-homoserine lactone (OHHL) quorum sensing signalling molecules and acetate. Interestingly, however, we observed that induction of budR transcription by OHHL or acetate did not require BudR, indicating the involvement of additional regulatory factors in relaying these environmental signals to the budR promoter.