Change of non-typable (R) strains of H. influenzae into the specific type of origin or new types (S) has been effected in vitro in a predictable fashion within a single 24 hour growth period, by a desoxyribonucleic acid-containing fraction isolated from type-specific cells of the type desired. Only a small proportion of the population of R cells are susceptible to the change induced in inherited characteristics by the desoxyribonucleic acid fraction. The data suggest that the number of susceptible cells present in any given population size varies with the specific type of origin of the R cells; a lesser degree of variation in different independent cultures of the same strain and population size has been demonstrated. The results suggest that the R H. influenzae cells which are susceptible to transformation arise as the result of mutation. It has been demonstrated that the reaction necessary for transformation takes place virtually immediately if susceptible cells are present. Furthermore it has been shown that the change which enables an R cell to form a colony of type-specific organisms has been completed within 15 minutes in an environment which does not permit cell multiplication.

Resistance to streptomycin, of a degree exceeding 1000 µg. per ml., has been induced in sensitive strains of Hemophilus influenzae by exposure for 10 minutes to desoxyribonucleic acid-containing extracts isolated from a strain of type b Hemophilus influenzae which had emerged resistant to 1000 µg. of streptomycin per ml. DNA is essential for the process which brings out this change; the reaction can be prevented by destruction of the DNA with crystalline desoxyribonuclease.The resistant trait which is created in this way is heritable. The nature of the process which induces resistance is similar in all respects to the reaction which induces heritable changes in type specificity of H. influenzae. These results offer another example of the gene-like action of highly specific DNA's. The pattern of resistance brought out in a bacterial population exposed to the DNA-containing, resistance-inducing extract, is similar to that which occurs when emergence of resistance of H. influenzae to streptomycin follows the selection by streptomycin of spontaneously occurring resistant mutants. The change in a bacterial cell from average susceptibility to streptomycin to resistance to 1000 µg. of streptomycin per ml. can occur in a single step.

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In E. influenzae the highly specific desoxyribonucleic acids (DNA's) which play the role of heredity determinants of type specificity and SM resistance, have induced these traits in only a small proportion of the population exposed to their action. The evidence suggests that this small proportion, "the susceptible cells" possess a property or substance needed by the DNA in order to induce an heritable change. The size of the small proportion of susceptible cells can be influenced significantly by a number of factors; when all the factors now to be listed are operating the frequency has not exceeded 1:1000. The Type of Origin of Recipient Cells.—Type a exhibits the lowest frequency, about 1:10,000,000, and type d the highest, approximately 1 per 1000 cells exposed. This type-specific property which controls the frequency of susceptible cells is an inherited trait; repeated change to a heterologous type shows no influence on the incidence of these cells. Concentration of DNA.—Within certain limits increase in the concentration of DNA controlling streptomycin resistance can increase the size of the proportion of cells in which streptomycin resistance can be induced. However, increases in concentrations greater than 10–1 µg. per ml. have not induced streptomycin resistance in a higher proportion of cells. Phase of Growth Cycle.—Predictable fluctuations in frequency of induced heritable changes have been demonstrated in both Rb and Rd populations during growth. There is no evidence that susceptible cells reproduce their kind; they emerge in all experiments when the population reaches the end of the logarithmic period and a density of 2 to 4 x 108 cells per ml. In the early logarithmic phase it is difficult to demonstrate the presence of susceptible cells. The peak frequency of susceptible cells occurs in the early stationary phase of the growth cycle. Thereafter, the decline in frequency is a gradual one. The data suggest that in a given population the same cells are susceptible to a number of different type-specific DNA's and the DNA controlling SM resistance. Comparison of Frequency of Cells Susceptible to Different DNA's.—In a given population the frequency of cells susceptible to different type-specific DNA's and the DNA controlling streptomycin resistance is not significantly different. Competition between Type-Specific DNA's—The data suggest that DNA's of types a, b, and c compete for the same cells in Rd populations. When Rd populations are exposed simultaneously to 2 of these 3 DNA's in different concentrations the proportion which each type contributes to the total type-specific cells induced is closely correlated with the concentration of the corresponding DNA. Exclusion of DNA's.—Induction of one type specificity or streptomycin resistance can be completely prevented in a population containing susceptible cells by previous exposure for 15 minutes to a 1000-fold higher concentration of another type-specific DNA.