Surface co-expression of Vibrio cholerae and Salmonella typhi O-antigens on Ty21a clone EX210

In an attempt to construct a bivalent, live, oral cholera-typhoid vaccine, genes specifying the biosynthesis of Vibrio cholerae O-antigen have been transferred into a modified version of the attenuated, oral typhoid vaccine strain Salmonella typhi Ty21a. The present report investigates the production of V. cholerae and S. typhi O-antigens by one such clone, EX210. When cultured without galactose supplementation EX210 produces surface O-antigen of V. cholerae type, as detected by haemagglutination-inhibition and bactericidal assays, and by immuno-electron microscopy. However, the protective efficacy of Ty21a depends upon growth in the presence of exogenous galactose and under these conditions only S. typhi O-antigen is detectable on the surface of EX210. Subsequent experiments revealed that the proportion of polysaccharide of S. typhi type is dependent upon the level of galactose supplementation, and identified a limiting sugar concentration which results in surface co-expression of both O-antigens. Visualization of the two polysaccharides on silver-stained polyacrylamide gels indicates that S. typhi O-antigen subunits are polymerized into longer sidechains, suggesting that at higher galactose concentrations their predominance results in a masking of the shorter V. cholerae O-antigen.