The Study of Ionizing Radiation Effects on Escherichia coli by Density Gradient Sedimentation

Density gradient sedimentation of bacterial cells in cesium chloride has been used to separate cells which have been irradiated with ⁶⁰Co gamma rays and have lost an appreciable amount of their DNA by subsequent degradation. Irradiated cells are found to band mainly at two characteristic densities, one corresponding to normal unirradiated cells and the other at a considerably lower density. The region corresponding to normal density cells is the only one that contains cells which will form colonies. Cells capable of synthesizing DNA following irradiation are found mainly at the region of normal density cells with some spreading into the lower density region. Cells in the lower density region contain less DNA than normal density cells. From an analysis of the relative numbers of cells in the two regions, it is suggested that the process of DNA degradation either takes place to a considerable extent in the genome or not at all. Analysis of the data in terms of numbers of cells having intact DNA and those having degraded DNA indicates a strong correlation between DNA degradation and cell death in this strain, JG151, and suggests that DNA degradation is a major but not the only cause of cell death.