Measurements of the inhibition of DNA synthesis in bacteria and the formation of thymine dimers by ultraviolet irradiation of the cells indicate that one thymine dimer per 350-F strand of DNA acts as a block to further synthesis.In a sensitive strain of Escherichia coli the blocks are permanent. In a resistant strain the blocks are only temporary but splitting dimers.

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Ionic and nonionic detergents have little effect on respiring bacteria, but in cultures poisoned with KCN rapid solubilization of the cell membrane, as indicated by turbidity losses, takes place. Ultraviolet radiations cause Escherichia coli cells grown in minimal medium with glycerol as a carbon source to cease respiring and growing about 1 h after irradiation. We tested the effect of the nonionic detergent Triton X-100 on growth and cell membrane dissolution (both measured by turbidity changes), respiration, and viability of unirradiated and irradiated E. coli B/r cells. When the detergent was added to cells immediately after irradiation, a decrease in turbidity occurred only when respiration was about to cease; when it was added after cessation of respiration, the turbidity loss was immediate. In both cases the turbidity loss was about 60%, and disintegration of the cell walls did not take place. 5-Fluorouracil (FU) and thermal (42 C) treatments cause respiration of irradiated cells to be maintained and also cause viability increases. Irradiated cells treated with FU and detergent show no turbidity loss just prior to the time respiration normally ceases, but a loss does occur in irradiated cells incubated with detergent at 42 C. We conclude that FU maintains respiration for all of the cells, but that thermal treatment maintains respiration for only part of the cells. In all cases the detergent had only a negligible effect on the respiration and viability of unirradiated and irradiated cells. We conclude that Triton X-100 causes solubilization of cell membranes of only nonrespiring cells that are not destined to survive.The structure and behavior of bacterial surfaces have been extensively studied with ionic and nonionic detergents. Electron micrographic and biochemical studies of partially purified Escherichia coli cell envelopes and cell walls treated with the nonionic detergent Triton X-100 in combination with ethylenediaminetetraacetic acid (EDTA) and lysozyme has revealed important details of this complex structure (6,16,17). Based on this work and recent work of others, Schnaitman (17) has summarized the main features of the E. coli cell wall. The outermost and middle layers, lipopolysaccharide and outer membrane, respectively, are in contact with each other. Their orientation is stabilized by divalent ions as evidenced by the dissociation of the two layers in the presence of EDTA (6). The outer membrane is composed of phospholipid and protein and is covalently linked to the underlying peptidoglycan or murein layer (4). This rigid innermost layer of the cell wall is disrupted by lysozyme. The cell wall and the cell membrane-together comprising the cell envelope-are separated by the periplasmic space. The cell membrane, unlike the outer membrane, is selectively solubilized by Triton X-100 when cell envelope preparations are treated with detergent (16). Triton X-100 used in combination with EDTA solubilizes the outer membrane because of its dissociation from the lipoprotein layer (6). An effective method by...

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