Construction of an auxotrophic Shigella flexneri strain for use as a live vaccine

A virulent Shigella flexneri serotype Y strain, SFL1, was made auxotrophic for aromatic metabolites, including p-aminobenzoic acid, which is not available in mammalian tissues, by transduction of a Tn10-inactivated aroD gene from Escherichia coli K-12 NK5131. One transductant, SFL114, selected for further studies, had the same biochemical and serological characteristics as the parent strain and the O-antigen patterns of the two strains were identical in SDS-PAGE and Western blot experiments. SFL114 was as invasive for cultured epithelial cells as SFL1, and both strains could escape from the phagocytic vacuole into the cytoplasm of the infected cells. However, the ability of SFL114 to multiply intracellularly was considerably reduced. When applied to the conjunctival sac of guinea pigs, the parent strain gave rise to keratoconjunctivitis, i.e. was Serény-positive, in 13 of 16 animals. By contrast, SFL114 was Serény-negative in all 11 guinea pigs tested. These in vitro and in vivo results suggest that the aromatic-dependent transductant S. flexneri SFL114 is attenuated and possesses properties desirable for a live vaccine.