The stressosome is required to transduce low pH signals leading to increased transcription of the amino acid-based acid tolerance mechanisms in .

Increasing proton concentration in the environment represents a potentially lethal stress for single-celled microorganisms. To survive in an acidifying environment, the foodborne pathogen quickly activates the alternative sigma factor B (σ), resulting in upregulation of the general stress response (GSR) regulon. Activation of σ is regulated by the stressosome, a multi-protein sensory complex involved in stress detection and signal transduction. In this study, we used strains harbouring two stressosome mutants to investigate the role of this complex in triggering expression of known amino acid-based resistance mechanisms in response to low pH. We found that expression of glutamate decarboxylase () and arginine and agmatine deiminases ( and , respectively) were upregulated upon acid shock (pH 5 for 15 min) in a stressosome-dependent manner. In contrast, transcription of the operons ( and ), which encode enzymes for the l-arginine biosynthesis pathway, were upregulated upon acid shock in a stressosome-independent manner. Finally, we found that transcription of , which encodes a transcriptional regulator of the and operons, was largely unaffected by acidic shock. Thus, our findings suggest that the stressosome plays a role in activating amino acid-based pH homeostatic mechanisms in . Additionally, we show that genes encoding the l-arginine biosynthesis pathway are highly upregulated under acidic conditions, suggesting that intracellular arginine can help withstand environmental acidification in this pathogen.