Salmonella typhi vaccine strain in vitro; low infectivity in human cell line U937

Salmonella typhi strain Ty21a has been used for live oral vaccine. The infectivity of Ty21a, in comparison with S. typhi Ty2, was evaluated using the human monocyte-macrophage cell line U937. Assays were performed by quantitative microscopy and viable count technique. Ty2 infected approximately 100% of the cells, multiplied extensively within these cells and caused cell death. The same dose of Ty21a infected only about 15% of the cells, resulting in a low number of intracellular bacilli and cell survival. The use of gentamicin in the test confirmed intracellular multiplication of Ty2 but not Ty21a. The system described may be suitable as a test system for characterization of the degree of virulence of Ty21a and other live, oral typhoid vaccines.