An oxygen dependent X-ray lesion in Escherichia coli strain B/r detected by penicillin.

Enhancement of lethal damage to E. coli B/r by penicillin was observed after X-irradiation under aerobic conditions but not after exposure to X-rays under anoxia or after U.V. (260 nm). No enhancement of damage occurred when incubation with penicillin was delayed for 2 hours after aerobic X-irradiation. This enhancing effect was only detected in this strain and not in the filamentous strain E. coli B. It is concluded that an X-ray induced lesion, sensitive to the presence of oxygen at the time of irradiation and probably located in the cell envelope, initiates filamentation in E. coli B/r, which results in lethal damage in this strain.