Salmonella exploits host- and bacterial-derived β-alanine for replication inside host macrophages

<i>Salmonella</i> is a major foodborne pathogen that can effectively replicate inside host macrophages to establish life-threatening systemic infections. <i>Salmonella</i> must utilize diverse nutrients for growth in nutrient-poor macrophages, but which nutrients are required for intracellular <i>Salmonella</i> growth is largely unknown. Here, we found that either acquisition from the host or de novo synthesis of a nonprotein amino acid, β-alanine, is critical for <i>Salmonella</i> replication inside macrophages. The concentration of β-alanine is decreased in <i>Salmonella</i>-infected macrophages, while the addition of exogenous β-alanine enhances <i>Salmonella</i> replication in macrophages, suggesting that <i>Salmonella</i> can uptake host-derived β-alanine for intracellular growth. Moreover, the expression of <i>panD,</i> the rate-limiting gene required for β-alanine synthesis in <i>Salmonella,</i> is upregulated when <i>Salmonella</i> enters macrophages. Mutation of <i>panD</i> impaired <i>Salmonella</i> replication in macrophages and colonization in the mouse liver and spleen, indicating that de novo synthesis of β-alanine is essential for intracellular <i>Salmonella</i> growth and systemic infection. Additionally, we revealed that β-alanine influences <i>Salmonella</i> intracellular replication and in vivo virulence partially by increasing expression of the zinc transporter genes <i>znuABC</i>, which in turn facilitates the uptake of the essential micronutrient zinc by <i>Salmonella</i>. Taken together, these findings highlight the important role of β-alanine in the intracellular replication and virulence of <i>Salmonella</i>, and <i>panD</i> is a promising target for controlling systemic <i>Salmonella</i> infection.