Lactate promotes Salmonella intracellular replication and systemic infection via driving macrophage M2 polarization.

Abstract

Salmonella is one of the most important enteric pathogens worldwide, which is able to cause lethal systemic infection via survival and replication in host macrophages. Lactate, a byproduct of anaerobic or aerobic glycolysis, can induce macrophage M2 polarization, but the relationship between lactate-mediated macrophage M2 polarization and bacterial infection is poorly understood. Here, we evaluated the role of lactate and lactate-mediated macrophage M2 polarization in the pathogenicity of Salmonella. We found that lactate levels were significantly increased in Salmonella-infected macrophages, and the increased lactate was derived from the host. Macrophage and mouse infection assays showed that the addition of lactate enhanced Salmonella replication within macrophages and the colonization of mouse systemic loci, while pharmacological or genetic inhibition of host lactate production impaired Salmonella intracellular replication and its virulence in mice. Further analysis revealed that lactate promotes M2 polarization of Salmonella-infected macrophages, and the induction of macrophage M2 polarization by lactate is responsible for lactate-mediated Salmonella growth promotion. Moreover, we showed that macrophage-derived lactate induces the Salmonella pathogenicity island (SPI)-2 type III secretion system, leading to increased translocation of the SPI-2 effector SteE, which is responsible for driving M2 polarization. Overall, these findings suggest that lactate promotes Salmonella intracellular replication and systemic infection via driving macrophage M2 polarization and highlight the complex interactions between Salmonella and macrophages. IMPORTANCE The important enteropathogen Salmonella can cause lethal systemic infection via survival and replication in host macrophages. Lactate represents an abundant intracellular metabolite during bacterial infection, which can also induce macrophage M2 polarization. In this study, we found that macrophage-derived lactate promotes the intracellular replication and systemic infection of Salmonella. During Salmonella infection, lactate via the Salmonella type III secretion system effector SteE promotes macrophage M2 polarization, and the induction of macrophage M2 polarization by lactate is responsible for lactate-mediated Salmonella growth promotion. This study highlights the complex interactions between Salmonella and macrophages and provides an additional perspective on host-pathogen crosstalk at the metabolic interface.