STUDIES ON THE MECHANISM OF RADIATION PROTECTION AND RECOVERY 



cysteamine protects the bacterial cells since increasing the concentration of 

 cysteamine many times over that of phosphate within the maximally pro- 

 tective concentration range of cysteamine (0 • 006 to • 02 M) does not increase 

 protection. A number of interesting aspects of the interference of phosphate 

 with cysteamine protection are under investigation. 



Studies in our laboratory have shown that E. coli B/r grown aerobically in 

 nutrient broth will recover from X-ray damage to a considerable degree if 

 plated out on agar containing either spleen, beef, or yeast extract^. Although 

 time is too limited to discuss all the comparative studies carried out in our 

 laboratory on the nutritional requirements for recovery of E. coli, we wish 

 to point out that a synthetic medium containing — in addition to inorganic 

 salts and glucose — glutamine, uracil, and guanine will support recovery of 

 broth-grown E. coli equivalent to that obtained with natural extracts. This 

 medium is given in Table I. We have found thai cysteamine protection 



Table I. — Composition of medium II 



Component 



L-Glutamic acid or glutamine or aspartic acid . . 



Guanine 



Uracil . . 



Salts A (K2HPO4, 25g ; KH2PO4, 25g ; H2O to make 250ml) 



Salts B (MgSOj . 7H2O, 1 g ; NaCl, FeSOj . 7H2O, and MnSO^ . 4H2O, • 5 g ; 



H2O to make 250 ml) 



Glucose 

 Agar . . 

 Water . . 



Amount 



150i^g 



30[xg 



30 [xg 



1ml 



1ml 



2g 

 3-4g 

 200 CO 



(l-rom Stapleton, Sbarra and Hollaender') 



is limited if X-irradiated, broth-grown bacteria are not supplied, after 

 irradiation, with some nutrient medium. Here again beef, yeast, or 

 spleen extract will give a high level of survival, whereas, on minimal medium, 

 survival is markedly reduced. The difference in survival on the two types 

 of media is about one thousandfold at 90 kr. If cysteamine-protected bac- 

 teria are plated on minimal agar the DRF is changed from 12 to about 2. 

 E. coli protected by mercaptoethanol do not show this striking effect. Here 

 the DRF is reduced only from 8 to 6 if the bacteria are plated on minimal 

 plates instead of nutrient plates. We have shown that marked recovery 

 of cysteamine-protected cells is not dependent on added nutritional factors 

 of growth at suboptimal temperature. Maximum recovery occurs at 18°C. 

 This compares^ with recovery at suboptimal temperatures of unprotected 

 E. coli B/r. 



Our first thought in trying to explain the peculiar protection curve demon- 

 strated with cysteamine, was that radiation must affect the cysteamine 

 either directly or indirectly. This was tested by irradiating the cysteamine 

 before it was added to the bacterial suspension. There was practically no 

 effect of X-radiation up to 120kr in modifying the curve obtained for a given 

 concentration of cysteamine. This, of course, does not rule out some effect 

 of X-rays on the compound in interaction with the bacterial cell to explain 

 the peculiar curves obtained. 



114 



