After an initial period of growth in medium made up in DO, most strains of pneumococcus tested dramatically lost viability, the extent of the loss depending on the strain and on the amount of contaminating HO in the DO. This was followed by a recovery period. Once a strain was ‘adapted’, the ability to grow in DO-medium without cell death was inherited, even after passage through HO-medium, indicating the selection of mutants. Cultures that had not reached ‘full adaptation’ also exhibited cell death if transferred into either DO-medium or HO-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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