The relation between a strain of B. coli and its mutant with reference to the production of agglutinins and protective antibodies may be expressed by the statement that the original strain when injected into cows develops antibodies both towards itself and the mutant whereas the mutant produces them only towards itself. The results point to the capsular substance as the material carrying virulence or, expressed somewhat differently, the factor which protects the microorganism in the host.

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