DELAY IN GROWTH AND DIVISION INDUCED BY NEAR ULTRAVIOLET RADIATION IN <i>ESCHERICHIA COLI</i> B AND ITS ROLE IN PHOTOPROTECTION AND LIQUID HOLDING RECOVERY

Abstract

Abstract. Microscopic observations show that growth delay and division delay occur on nutrient agar after Escherichia coli B has been irradiated at 3341 Å. These effects also occur in nutrient broth. A near u.v. action spectrum for growth delay in nutrient broth has been obtained. It shows a single peak at 3380 Å and is indistinguishable from the action spectrum for photo‐protection from far u.v. (2537 Å) killing in the same organism. Furthermore, photoprotected cells show a much greater growth delay than cells that have not been photoprotected. These, as well as kinetic data, suggest that the essential action of a photoprotection treatment consists in the induction of a growth‐division delay. This delay would presumably permit more time for intracellular recovery systems to operate on the far u. v. damage to nucleic acids. Liquid holding recovery (effected by holding cells in phosphate buffer after far u. v. irradiation) shows complete overlap with photoprotection. It is concluded that photoprotection and liquid holding recovery operate on the same far u. v. damage. As with photoprotection, it is probable that the essential action of a liquid holding treatment is the induction of a growth‐division delay. No photoprotection is observed of intracellular T2 bacteriophage or of E. coli B s‐l (Hill).