Isolation and characterization of Gal E mutant Ty 21a of Salmonella typhi: a candidate strain for a live, oral typhoid vaccine.

A gal E mutant of Salmonella typhi was isolated; results obtained with Salmonella typhimurium and the mouse as a model for human typhoid fever indicated that this mutant has the potential for use as a live, oral typhoid vaccine. The mutant, Ty 21a, took up galactose from exogenous sources and accumulated sufficient quantities of galactose-1-phosphate and uridine diphosphate galactose to cause lysis of the cells, an event that resulted in the avirulence of the strain. Galactose was incorporated sufficiently into the cell wall of Ty 21a to allow the synthesis of smooth-type lipopolysaccharides, which are necessary for the proper immunogenicity. Cells of strain Ty 21a, when given intraperitoneally, protected mice against lethal challenge with strain Ty 2 of S. typhi.