Radiosensitization of microorganisms by radical anions. I. Medium effects

Irradiation of Streptococcus faecalis in a neutral, N2O/Br- system leads to practically complete killing with extraordinarily low doses of irradiation, namely a D10 of 13 Gy compared to 470 Gy in N2, 250 Gy in N2O and 160 Gy in O2. However, irradiation and chemical investigations demonstrated that the apparent radiosensitization in neutral, N2O/Br- is due mainly to bromine, Br2 and HOBr rather than Br-3 or the radical anion, Br-2. For example, addition of unirradiated bacteria to a previously irradiated neutral solution of N2O/Br- reduces survival. The medium effects are eliminated by radiation chemical and/or chemical reactions which destroy bromine, such as occur in basic solutions, in N2/Br- or O2/Br- systems because of back reactions of Br2 with e-aq in the former and of Br2 with H2O2 and O-2 in the latter. Neither are medium effects produced in N2O/Br- systems at pH greater than 9. However, in N2/Br- the D10 = 82 Gy compared to 160 Gy in O2 which indicates that for S. faecalis Br-2 is intrinsically a more effective radiosensitizing agent than oxygen.