Intracellular survival of wild-type Salmonella typhimurium and macrophage-sensitive mutants in diverse populations of macrophages

Salmonella typhimurium survives within macrophages and causes a fatal infection in susceptible strains of mice. A number of S. typhimurium mutants that contain Tn10 insertions in genes which are necessary for survival within the macrophage have been isolated. To demonstrate the importance of each gene in intracellular survival, the mutations were transduced into a smooth-strain background and the ability to survive intracellularly was assayed in five different populations of macrophages. The majority of the original macrophage-sensitive mutants retained the macrophage-sensitive phenotype in the smooth-strain background. The ability to survive or grow within macrophages varied with both the source of macrophages and the individual mutants. S. typhimurium grew best in the macrophage-like cell line J774, survived at moderate levels in splenic and bone marrow-derived macrophages, and was killed most efficiently in peritoneal macrophages. Macrophage-sensitive mutants transduced into a smooth background were also less virulent than the parent, with a 50% lethal dose of 2 to 5 logs greater than that of the parental strain. These experiments demonstrate that survival of S. typhimurium within macrophages varies with the source of cells, with a distinct ability to survive in macrophages from mouse spleens, where S. typhimurium grows rapidly. These experiments also demonstrate the heterogeneity in intracellular survival among the various macrophage-sensitive mutants, which may reflect the relative importance of the individual mutated genes in survival within macrophages.