oral vaccine. flexneri 2a strain CVD 1203, a prototype live attenuated delta aroA delta virG Shigella Construction and characterization of

acid (10 mg). At 72 h, growthwas measuredby optical densityat 600 nm. In addition to supplementation with tyrosine,tryptophan, and phenylalanine, CVD 1201.1 required p-ami-nobenzoic acid in order to grow.CVD1203, like its wild-type parent, grows on enteric media(which contain sufficient p-aminobenzoic acid and aromaticamino acids) and manifests a typical acid butt-alkaline slantreaction without H2S or gas 18 to 24 h after inoculation oftriple-sugar-iron-agar slants. Lipopolysaccharides (LPS) ex-tracted (43) from strains 2457T and CVD 1203 revealedidentical patterns when examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with silver staining (40) andby