Effect of transforming DNA on growth and frequency of mutation of Streptococcus pneumoniae.

We studied the effect of the presence of homologous transforming DNA on the growth of several transformable strains of Streptococcus pneumoniae and on the frequency of mutation of these strains to various antibiotic resistances. We observed no effect on growth until the strains became competent, when growth was depressed. At the end of the competence period, some strains showed recovery to varying degrees, whereas others showed evidence of cell death. Growth was also depressed by the presence of DNA from Escherichia coli, indicating that recombination was not likely to be the cause of the observed effect. Furthermore, cell death was not caused by the induction of a prophage. Several of the strains showed increased mutation frequencies during the competence period, although treatment with E. coli DNA gave no such effect, indicating that the mutagenesis was due to recombination. We observed no mutagenesis due to UV irradiation of the strains. The possibility that integration of the transforming DNA may produce lesions which induce error-prone repair is discussed. Furthermore, a strain that showed no mutability by transforming DNA, indicating the presence of a more efficient repair system, gave evidence of producing higher amounts of the hex system when competent, and the possible relationship between these properties is discussed.