Flagella help Salmonella typhimurium survive within murine macrophages.

In this study, we evaluated how flagella enhance the pathogenicity of Salmonella typhimurium in strain C57BL/6J mice. When mice were infected orally with flagellated or nonflagellated S. typhimurium, equivalent numbers of bacteria colonized the gastrointestinal tracts of the animals, but the number of flagellated organisms increased faster once colonization began in the spleens and livers. To evaluate this differential rate of Salmonella growth, the rate of blood clearance, and the kinetics of net multiplication of salmonellae in splenic tissue after intravenous challenge, the two groups of mice were compared. We found that clearance of bacteria from the blood was the same for flagellated or nonflagellated strains. However, the number of flagellated bacteria in the spleen increased logarithmically until the death of the animals, whereas the number of nonflagellated salmonellae increased only slightly. In contrast, both flagellated and nonflagellated strains grew exponentially in the spleens of mice pretreated with silica, a macrophage toxic agent. In an in vitro macrophage assay, flagellated salmonellae survived longer than nonflagellated organisms. These results indicate that flagella either protect S. typhimurium from the intracellular killing mechanisms of murine macrophages or that flagella enhance the ability of S. typhimurium to multiply within murine macrophages.