EFFECTS OF RADIATION ON BACTERIA 379 



tions of the various compounds results in an increasing protective effect up 

 to a certain level, after which the curves reach a plateau. 



Tests for additivity show that compounds within a group are com- 

 pletely additive as long as the total concentration is below the plateau 

 level. There is no increase in protection when increasing amounts of 

 cysteine, for example, are added to optimal or plateau concentrations 

 of BAL. In contrast to this, combinations between groups of compounds 

 do give additive protection when combined at plateau level. Thus 

 all combinations of alcohol, BAL, and sodium hydrosulfite by pairs 

 give additive protection, but combinations of all three give no further 

 increase in protection. This may indicate that the residual killing is the 

 direct effect of the X rays on the cells. 



Stapleton, Billen, and Hollaender (1952) have studied the mechanism 

 of the protective action of those compounds which reciuire preincubation 

 for maximum protection. The necessity of preincubation suggests an 

 enzymatic mechanism, and the evidence indicates that only those com- 

 pounds utilized as oxidative substrates are effective. This was further 

 indicated by tests in which inhibition of respiration was shown to cause a 

 corresponding decrease in protection. In general, respiratory inhibitors 

 had no influence on the degree of protection afforded by sulfhydryl com- 

 pounds and sodium hydrosulfite although the protective efficacy of cys- 

 teine was reduced. Varying degrees of respiratory inhibition with 

 cyanide were accompanied by a corresponding decrease in protection b}^ 

 succinate, suggesting a critical intracellular oxygen concentration. This 

 is also suggested by the sharp increase in protection afforded by an 

 increase in sodium hydrosulfite concentration from about 2 X 10~^ M 

 to 8 X 10~^ M, there being no protection at concentrations lower than 

 2 X 10~^ M. Studies of cells possessing an active hydrogenase system 

 indicated that neither hydrogen donation nor production of a highly 

 reduced state within the cell is a major mechanism in chemical protection. 

 The respiration studies are compatible with the hypothesis that either 

 enzymatic or nonenzymatic removal of oxygen from around or within the 

 cells is a major mode of protection. 



Various hypotheses to explain the mode of action of the different pro- 

 tective substances have been advanced by these workers. In general, 

 these hypotheses involve either a reduction in the yield of toxic radio- 

 decomposition products due to the removal of oxygen from the cellular 

 environment or a competition for highly reactive radiodecomposition 

 products by the protective compound. 



Some similar observations have been reported by Thompson ei al. 

 (1951). They found that the presence of 0.5 per cent pyruvate during 

 irradiation partially protected the bacteria from the lethal and mutagenic 

 effects of X rays and extreme ultraviolet. Addition of pyru\ate after 

 irradiation had no effect. Wyss (1951) reported that X or extreme ultra- 



