Effect of cumulative X-irradiation on the development of colour variantsof Serratia marcescens

Abstract

o| (1)|The authors studied the development of spontaneous mutations inSerratia marcescens, and investigated the effect of X-irradiation on the mutation percentage of these cells. (2)|An analysis of variance showed that differences in the composition ofthe nutrient media significantly influenced the formation ofmutants. Differences between the mutation percentages of individual colonies were not significant. There was likewise no relationship between the incidence of spontaneous mutations and the rate of cell division in colonies of different sizes. (3)|The rate of mutant formation in an aging culture at 30°C varied in different clones, but was in general linear. In a study of the effect of different culture temperatures, the relatively highest number of mutations was obtained at lower temperatures. Mutation steps inSerratia marcescens are evidently not related to shifts in one gene,as in that case the percentage of mutations would increase with the temperature. (4)|Inaetivation of Serratia marcescens by X-rays follows an exponential relationship, the course of the curve being influenced by the pigmentation and age of the irradiated cells. The red variant is almost twiceassensitiveas the white variant, and 24-hour-old cells are about twice as resistant as freshly inoculated cells.If the cells are cultured at4°C prior to irradiation,they are about 1ji more sensitive than at 30° C. (5)|Irradiation with single doses of X-rays caused no increase in the number ofvariants. With a cumulative dose, however, the percentage of variants rose almost linearly. This increase in the percentage of variants is caused by an actual change in the pigmentation of surviving cells, and probably not by selection of more resistant variants. It thus follows that successive irradiation of surviving cells of Serratia marcescens gives rise to a labile state, which is manifested in increased mutability. It was assumed that cytoplasmic factors play a decisive role in this case, as well as the nuclear system of the bacterium. (6)|Successive irradiation progressively gives rise to cells which are more sensitive to irradiation, with the exception of pink variants, the sensitivity of which is not changed by a cumulative dose of radiation. Previousirradiation alsosensitizesthe bacteriatothe mutagenic effect ofa further single radiation dose. (7)|The stability of variants obtained by irradiation was studied by ten fold reinoculation. Infurther generations, white and red colonies usually produced types of the same colour as the parent colony. In the case of pink variants, the relationship between red and pink colonies fluctuated somewhat, but the number of pink colonies tended to increase