Enteric pathogens such as Salmonella enterica serovar Typhimurium experience spatial and temporal changes to the metabolic landscape throughout infection. Host reactive oxygen and nitrogen species non-enzymatically convert monosaccharides to alpha hydroxy acids, including L-tartrate. Salmonella utilizes L-tartrate early during infection to support fumarate respiration, while L-tartrate utilization ceases at later time points due to the increased availability of exogenous electron acceptors such as tetrathionate, nitrate, and oxygen. It remains unknown how Salmonella regulates its gene expression to metabolically adapt to changing nutritional environments. Here, we investigated how the transcriptional regulation for L-tartrate metabolism in Salmonella is influenced by infection-relevant cues. L-tartrate induces the transcription of ttdBAU, genes involved in L-tartrate utilization. L-tartrate metabolism is negatively regulated by two previously uncharacterized transcriptional regulators TtdV (STM3357) and TtdW (STM3358), and both TtdV and TtdW are required for the sensing of L-tartrate. The electron acceptors nitrate, tetrathionate, and oxygen repress ttdBAU transcription via the two-component system ArcAB. Furthermore, the regulation of L-tartrate metabolism is required for optimal fitness in a mouse model of Salmonella-induced colitis. TtdV, TtdW, and ArcAB allow for the integration of two cues, i.e., substrate availability and availability of exogenous electron acceptors, to control L-tartrate metabolism. Our findings provide novel insights into how Salmonella prioritizes the utilization of different electron acceptors for respiration as it experiences transitional nutrient availability throughout infection.

Importance

Bacterial pathogens must adapt their gene expression profiles to cope with diverse environments encountered during infection. This coordinated process is carried out by the integration of cues that the pathogen senses to fine-tune gene expression in a spatiotemporal manner. Many studies have elucidated the regulatory mechanisms of how Salmonella sense metabolites in the gut to activate or repress its virulence program; however, our understanding of how Salmonella coordinates its gene expression to maximize the utilization of carbon and energy sources found in transitional nutrient niches is not well understood. In this study, we discovered how Salmonella integrates two infection-relevant cues, substrate availability and exogenous electron acceptors, to control L-tartrate metabolism. From our experiments, we propose a model for how L-tartrate metabolism is regulated in response to different metabolic cues in addition to characterizing two previously unknown transcriptional regulators. This study expands our understanding of how microbes combine metabolic cues to enhance fitness during infection.