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It is shown that the genetic determinants of type-specific antigens I t o V of Shigella flexneri controlling similar functions are localized near the lactosefermentation region on the chromosome; the determinant of antigen VI lies beyond it. The auth,ors support the proposal that Shigella flexneri serotype 6 be excluded from Shigella flexneri and be considered as a subspecies in correspondence with the Soviet scheme of classification or even as a distinct species as Slopek and Mulczyk suggested. Genetic arguments based on the different behavior of S. flexneri strains in recombination with Escherichia coli K-I2 Hfr are presented which support the division of S. flexneri serotype 5 into two subtypes, a and b y in correspondence with a similar division of other serotypes. The advisability of this division is confirmed by receptor analysis demonstrating differences in groupspecific factors in strains of serotype 5. The antigenic formula of subtype 5b is V:7. .. and of 5a, V:4. .. .

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