RESUMEN

After an initial period of growth in medium made up in D2O, most strains of pneumococcus tested dramatically lost viability, the extent of the loss depending on the strain and on the amount of contaminating H2O in the D2O. This was followed by a recovery period. Once a strain was 'adapted', the ability to grow in D2O-medium without cell death was inherited, even after passage through H2O-medium, indicating the selection of mutants. Cultures that had not reached 'full adaptation' also exhibited cell death if transferred into either D2O-medium or H2O-medium, supporting the conclusion that the presence of hydrogen and deuterium together caused the toxicity. 'Adapted' cells exhibited an increased mutation frequency to a variety of antibiotic resistances, the propensity for this appearing in the death phase of 'adaptation'. The specific transforming activity of DNA preparations from cultures undergoing 'adaptation' decreased before DNA synthesis ceased indicating damage to the DNA. The integration efficiency of a low-efficiency marker also dropped during 'adaptation' before returning to the initial value when measured in a Hex- recipient, but remained constant in a Hex+ recipient, suggesting that the Hex system may be involved in repair of the DNA damage. 'Adapted' organisms showed evidence of possessing higher Hex activity and were also able to repair lesions caused by UV-irradiation better than the wild-type.

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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.

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Cultures of a variety of pneumococcal strains maintained competence longer and gave higher yields of transformants when incubated in M1 medium compared with NS medium. This was most probably due to the cells remaining competent for longer in M1 medium. Various parameters controlling the development of competence in M1 medium were investigated. The onset of competence was delayed in M1 medium compared with that in NS medium, probably due to the presence of Casamino acids in the former. Competence developed normally over a pH range of 7.3 to 8.3, with cultures inoculated from the same batch of frozen "precultures" showing consistent characteristics. This was not observed when frozen "sensitization" cultures were revived. The average cell chain length increased with the development of competence in all the strains tested and, with the exception of cultures which had entered the stationary growth phase, declined after the culture had lost its competence. The extent of the increase in chain length was dependent upon the pH of the medium.

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