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Abstract
Lag, exponential and stationary phase cells of Salmonella typhimurium lt-2 (trpC3) grown in nutrient broth or tryptophan-supplemented minimal medium were irradiated with doses of u.v. up to 570 ergs/mm2. Casein hydrolysate supplementation of the post-irradiation plating medium causes enhanced survival only for lag and exponential phase broth-grown and exponential phase minimal- grown cells. Caffeine invariably leads to decreased survival levels and abolition of any casein hydrolysate enhancement of survival. Cells of all six growth conditions give induced Trp+ reversions, often due to suppressor mutations, on plating media containing casein hydrolysate. Only for minimal-grown lag and exponential phase cells do any induced Trp+ appear on media devoid of casein hydrolysate supplementation. In these cases too, caffeine has a definite antimutagenic action. Mutation frequency decline (MFD) experiments revealed that those cells exhibiting a casein hydrolysate enhancement of survival on plates also show a delayed onset of MFD and a fall of survival, after an initial delay, in liquid minimal medium. MFD experiments in liquid do not give a complete quantitative explanation for the Trp+ revertant yields found on plates. We suggest that intracellular free amino acid pool sizes may be a common factor in the correlations which we have observed.
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