Phase I and Phase II Sh. sonnei exhibit differences in their susceptibility to the T series of bacteriophages. Both microorganisms are lysed by T2 and T6, but only the Phase II bacillus is lysed by T3, T4, and T7.

Lysis of the Phase I or Phase II bacillus by T2 or T6 is not inhibited by the homologous type-specific antigen. In the presence of an appropriate cofactor, however, the lysis of Phase II Sh. sonnei by T3, T4, and T7 is specifically inhibited by the homologous somatic antigen but not by the Phase I antigen.

The significance of these observations is discussed in respect to the nature of the virus receptor of these microorganisms.

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