Virulence of Rifampicin-Resistant Mutants of Shigella and Enteropathogenic Escherichia coli with Special Reference to Their Cell Invasiveness

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1. Changes in virulence of Shigella and Shigella-like enteropathogenic Escherichia coli, which occurred during their acquiring resistance to rifampicin were examined, Virulence was evaluated in vitro by their cell-invasive activity, infectivity to the eye of guinea pigs, and LD₅₀ values in intraperitoneally infected mice. 2. The mutants obtained in vivo and those obtained in vitro somewhat varied in cell invasiveness. Ability to produce keratoconjunctivitis in guinea pigs was lacking or diminished in all mutants though the parent strains constantly showed high abilities to invade cultured cells or produce keratoconjuctivitis in guinea pigs. 3. Mutations to resistant strains were not accompanied by any changes in LD₅₀ values in mice.