Abstract
Macrophages are important for host defense against intracellular pathogens like Salmonella and can be differentiated into two major subtypes. M1 macrophages, which are pro-inflammatory and induce antimicrobial immune effector mechanisms, including the expression of inducible nitric oxide synthase (iNOS), and M2 macrophages, which exert anti-inflammatory functions and express arginase 1 (ARG1). Through the process of phagocytosis, macrophages contain, engulf, and eliminate bacteria. Therefore, they are one of the first lines of defense against Salmonella. Infection with Salmonella leads to gastrointestinal disorders and systemic infection, termed typhoid fever. For further characterization of infection pathways, we established an in vitro model where macrophages are infected with the mouse Salmonella typhi correlate Salmonella enterica serovar Typhimurium ( S. tm), which additionally expresses red fluorescent protein (RFP). This allows us to clearly characterize macrophages that phagocytosed the bacteria, using multi-color flow cytometry. In this protocol, we focus on the in vitro characterization of pro- and anti-inflammatory macrophages displaying red fluorescent protein-expressing Salmonella enterica serovar Typhimurium, by multi-color flow cytometry.