Salmonella Typhimurium evades from host immunity and antibiotics in monocytes.

Intracellular bacteria can survive in vivo, evading host immunity and antibiotics. Salmonella persists in organs such as the spleen by invading the phagocytic cells. However, it remains controversial which specific cell populations, e.g. macrophages, monocytes, neutrophils, or dendritic cells, harbor the bacteria during persistent infection. To address this question, we employed a persistent infection mouse model using attenuated Salmonella expressing an acid tolerant fluorescent protein. We found that the bacteria predominantly resided in monocytes. Although these cells expressed Ly-6G, a typical marker for neutrophils, they did not exhibit a polymorphonuclear morphology. Furthermore, Salmonella primarily and preferentially invaded monocytes over other phagocytic cell types. Importantly, Salmonella was able to survive in monocytes even in the presence of antibiotics. Our findings demonstrate that monocytes serve as a critical survival niche for Salmonella in vivo, allowing the bacteria to evade both host immunity and antibiotics.