Lactate promotes the biofilm-to-invasive-planktonic transition in Salmonella enterica serovar Typhimurium via the de novo purine pathway

Salmonella enterica serovar Typhimurium (S. Typhimurium) infection triggers an inflammatoryresponse that changes the concentration of metabolites in the gut impacting the luminal environment. Some of these environmental adjustments are conducive to S. Typhimurium growth, such as the increased concentrations of nitrate and tetrathionate or the reduced levels of Clostridia-produced butyrate. We recently demonstrated that S. Typhimurium can form biofilmswithin the host environment and respond to nitrate as a signaling molecule, enabling it to transition between sessile and planktonic states. To investigate whether S. Typhimurium utilizes additional metabolites to regulate its behavior, our study delved into the impact of inflammatorymetabolites on biofilmformation. The results revealed that lactate, the most prevalent metabolite in the inflammatoryenvironment, impedes biofilmdevelopment by reducing intracellular c-di-GMP levels, suppressing the expression of curli and cellulose, and increasing the expression of flagellargenes. A transcriptomic analysis determined that the expression of the de novo purine pathway increases during high lactate conditions, and a transposon mutagenesis genetic screen identifiedthat PurA and PurG, in particular, play a significantrole in the inhibition of curli expression and biofilmformation. Lactate also increases the transcription of the type III secretion system genes involved in tissue invasion. Finally, we show that the pyruvate-modulated two-component system BtsSR is activated in the presence of high lactate, which suggests that lactate-derived pyruvate activates BtsSR system after being exported from the cytosol. All these findingspropose that lactate is an important inflammatorymetabolite used by S. Typhimurium to transition from a biofilmto a motile state and fine-tuneits virulence.