SUMMARY: Damage to living organisms by X-rays (but not by ultraviolet radiation) is generally enhanced by the presence of oxygen, which probably acts at a physico-chemical level. Sensitivity to both u.v.- and X-radiation may also be related to the presence of oxygen acting as a metabolic factor during the growth of micro-organisms before or after irradiation.

Anaerobic growth of strain B after u.v. or X-rays decreased lethal damage, the treatment being most effective after u.v.-and more effective after X-rays delivered in the absence than in the presence of oxygen. In this respect anaerobic growth corresponded with a variety of inhibitory conditions after irradiation, and rescue by anaerobic growth was about as effective as chloramphenicol treatment; but when these two treatments were used together, the effectiveness of each was decreased.

The effects of u.v.- and X-radiation on B/r were unchanged by anaerobic growth after irradiation; but anaerobic growth beforehand resulted in survival curves which were much more sigmoid in shape than those pertaining to aerobically grown organisms. This tendency was noted with all strains tested. Anaerobic growth before X-irradiation did not alter the effect of oxygen during irradiation.


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